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life
life, living matter and, as such, matter that shows
TABLE OF CONTENTS
certain attributes that include responsiveness,
growth, metabolism, energy transformation, and Introduction
reproduction. Although a noun, as with other Definitions of life
defined entities, the word life might be better cast Life on Earth
as a verb to reflect its essential status as a process. The origin of life
Life comprises individuals, living beings, assignable

Streptococcus pyogenes

Scanning electron micrograph of

Streptococcus pyogenes, associated
with strep throat and scarlet fever.

to groups (taxa). Each individual is composed of one or more minimal living

units, called cells, and is capable of transformation of carbon-based and other
compounds (metabolism), growth, and participation in reproductive acts. Life-
forms present on Earth today have evolved from ancient common ancestors
through the generation of hereditary variation and natural selection. Although
some studies state that life may have begun as early as 4.1 billion years ago, it
can be traced to fossils dated to 3.5–3.7 billion years ago, which is still only
slightly younger than Earth, which gravitationally accreted into a planet about
4.5 billion years ago. But this is life as a whole. More than 99.9 percent of
species that have ever lived are extinct. The several branches of science that
reveal the common historical, functional, and chemical basis of the evolution of
all life include electron microscopy, genetics, paleobiology (including
paleontology), and molecular biology.

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Mangrove roots, Thailand n
Mangroves (Rhizophora apiculata) at o
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non of life can be approached in several ways: life as it is known and studied on
planet Earth; life imaginable in principle; and life, by hypothesis, that might exist
elsewhere in the universe (see extraterrestrial life). As far as is known, life exists
only on Earth. Most life-forms reside in a thin sphere that extends about 23 km
(14 miles) from 3 km (2 miles) beneath the bottom of the ocean to the top of
the troposphere (lower atmosphere); the relative thickness is comparable to a
coat of paint on a rubber ball. An estimated 10–30 million distinguishable
species currently inhabit this sphere of life, or biosphere.
Definitions of life
Much is known about life from points of view
reflected in the various biological, or “life,”
sciences. These include anatomy (the study of
form at the visible level), ultrastructure (the
study of form at the microscopic level),
physiology (the study of function), molecular
reed frog biology and biochemistry (the study of form
Reed frog perched on a lily. and function at chemical levels), ecology (the
study of the relations of organisms with their
environments), taxonomy (the naming, identifying, and classifying of organisms),
ethology (the study of animal behaviour), and sociobiology (the study of social
behaviour). Specific sciences that participate in the study of life focus more
narrowly on certain taxa or levels of observation—e.g., botany (the study of
plants), lichenology (the study of lichens, leafy or crusty individuals composed of
permanent associations between algae or photosynthetic bacteria and fungi),
herpetology (the study of amphibians and reptiles), microbiology (the study of

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bacteria, yeast, and other unicellular fungi, archaea, protists, viruses), zoology
(the study of marine and land animals), and cytology (the study of cells).
Although the scientists, technicians, and others who participate in studies of life
easily distinguish living matter from inert or dead matter, none can give a
completely inclusive, concise definition of life itself. Part of the problem is that
the core properties of life—growth, change, reproduction, active resistance to
external perturbation, and evolution—involve transformation or the capacity for
transformation. Living processes are thus antithetical to a desire for tidy
classification or final definition. To take one example, the number of chemical
elements involved with life has increased with time; an exhaustive list of the
material constituents of life would therefore be premature. Nonetheless, most
scientists implicitly use one or more of the metabolic, physiological, biochemical,
genetic, thermodynamic, and autopoietic definitions given below.

Metabolic

Metabolic definitions are popular with biochemists and some biologists. Living
systems are objects with definite boundaries, continually exchanging some
materials with their surroundings but without altering their general properties,
at least over some period of time. However, there are exceptions. There are
frozen seeds and spores that remain, so far as is known, perfectly dormant. At
low temperatures they lack metabolic activity for hundreds, perhaps thousands,
of years but revive perfectly well upon being subjected to more clement
conditions. A candle flame has a well-defined shape with a fixed boundary and
is maintained by “metabolizing” its organic waxes and the surrounding
molecular oxygen to produce carbon dioxide and water. Similar reactions,
incidentally, occur in animals and plants. Flames also have a well-known capacity
for growth. These facts underscore the inadequacy of this metabolic definition,
even as they suggest the indispensable role of energy transformation to living
systems. (See metabolism.)

Physiological

Physiological definitions of life are popular. Life is defined as any system capable
of performing functions such as eating, metabolizing, excreting, breathing,
moving, growing, reproducing, and responding to external stimuli. But many
such properties are either present in machines that nobody is willing to call alive
or absent from organisms, such as the dormant hard-covered seed of a tree,

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that everybody is willing to call alive. An

automobile, for example, can be said to eat,
metabolize, excrete, breathe, move, and be
responsive to external stimuli. A visitor from
another planet, judging from the enormous
number of automobiles on Earth and the way
in which cities and landscapes have been
Venus flytrap
designed for the special benefit of motorcars,
Active traps of the Venus flytrap
might well believe that automobiles are not
(Dionaea muscipula), a carnivorous
plant. If depressed at least twice, thin only alive but are the dominant life-form on
pressure-sensitive hairs in the trap the planet. (See physiology.)
stimulate the lobes to clamp tightly
over an insect. Biochemical

A biochemical or molecular biological

definition sees living organisms as systems that contain reproducible hereditary
information coded in nucleic acid molecules and that metabolize by controlling
the rate of chemical reactions using the proteinaceous catalysts known as
enzymes. In many respects, this is more satisfying than the physiological or
metabolic definitions of life. However, even here there are counterexamples.
Viruslike agents called prions lack nucleic acids, although the nucleic acids of the
animal cells in which they reside may be involved in their reproduction.
Ribonucleic acid (RNA) molecules may replicate, mutate, and then replicate their
mutations in test tubes, although by themselves they are not alive. Furthermore,
a definition strictly in chemical terms seems peculiarly vulnerable. It implies that,
were a person able to construct a system that had all the functional properties
of life, it would still not be alive if it lacked the molecules that earthly biologists
are fond of—and made of. (See biochemistry.)

Genetic

All organisms on Earth, from the tiniest cell to the loftiest trees, display
extraordinary powers. They effortlessly perform complex transformations of
organic molecules, exhibit elaborate behaviour patterns, and indefinitely
construct from raw materials in the environment more or less identical copies of
themselves. How could systems of such staggering complexity and such
stunning beauty ever arise? A main part of the answer, for which today there is
excellent scientific evidence, was carefully chronicled by the English naturalist
Charles Darwin in the years before the publication in 1859 of his epoch-making
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work On the Origin of Species. A modern

rephrasing of his theory of natural selection
goes something like this: Hereditary
information is carried by large molecules
known as genes, composed of nucleic acids.
Different genes are responsible for the
expression of different characteristics of the
organism. During the reproduction of the
organism, the genes also replicate and
initial proposal of DNA structure thereby pass on the instructions for various
The initial proposal of the structure of characteristics to the next generation.
DNA by James Watson and Francis Occasionally, there are imperfections, called
Crick, which was accompanied by a
suggestion on the means of mutations, in gene replication. A mutation
replication. alters the instructions for one or more
particular characteristics. The mutation also
breeds true, in the sense that its capability for determining a given characteristic
of the organism remains unimpaired for generations until the mutated gene is
itself mutated. Some mutations, when expressed, will produce characteristics
favourable for the organism; organisms with such favourable genes will
reproduce preferentially over those without such genes. Most mutations,
however, turn out to be deleterious and often lead to some impairment or to
death of the organism. (To illustrate, it is unlikely that one can improve the
functioning of a finely crafted watch by dropping it from a tall building. The
watch may perform better, but this is highly improbable.) In this way, organisms
slowly evolve toward greater complexity. This evolution occurs, however, only at
enormous cost: modern humans, complex and reasonably well-adapted, exist
only because of billions of deaths of organisms slightly less adapted and
somewhat less complex. In short, Darwin’s theory of natural selection states that
complex organisms evolved through time because of replication, mutation, and
replication of mutations. A genetic definition of life therefore would be a system
capable of evolution by natural selection. (See Darwinism.)

This definition places great emphasis on the importance of replication.

Replication refers to the capacity of molecules such as deoxyribonucleic acid
(DNA) to precisely copy themselves, whereas reproduction refers to the increase
in number of organisms by acts that make a new individual from its parent or

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parents. In any organism, enormous biological effort is directed toward

reproduction, although it confers no obvious benefit on the reproducing
organism itself. However, if life is defined as an entity capable of reproduction,
then a mule, which is clearly alive yet does not reproduce, would be excluded
from the living under this restrictive definition. Indeed, many organisms, such as
hybrid mammals and plants that are past their prime, do not reproduce even
though the individual cells of which they are composed may.

Life defined as a reproductive system dependent on replicating components

does not rule out synthetic reproduction. For example, it should be possible to
construct a machine that is capable of producing identical copies of itself from
preformed building blocks but that arranges its descendants in a slightly
different manner when a random change occurs in its instructions. Such a
machine would of course reproduce its instructions as well. But the fact that
such a machine would satisfy the genetic definition of life is not an argument
against such a definition; in fact, if the building blocks were simple enough, the
machine would have the capability of evolving into very complex systems that
would probably have all the other properties attributed to living systems. (Some
computer programmers have already claimed, on the basis of running
generations of replicating and mutating computer instructions, to have created
artificial life [“a-life”]; such programs do not, however, show any real freedom or
awareness, and their activities are thus far limited to the insides of computers.)
The genetic definition has the additional advantage of being expressed purely in
functional terms; i.e., it does not depend on any particular choice of constituent
molecules. The improbability of contemporary organisms—dealt with more fully
below—is so great that these organisms could not possibly have arisen by
purely random processes and without historical continuity. Fundamental to the
genetic definition of life then seems to be the notion that a certain level of
complexity cannot be achieved without natural selection.
Thermodynamic

Thermodynamics distinguishes between isolated, closed, and open systems. An

isolated system is separated from the rest of the environment and exchanges
neither light nor heat nor matter with its surroundings. A closed system
exchanges energy but not matter. An open system is one in which both material
and energetic exchanges occur. The second law of thermodynamics states that,
in a closed system, no processes will tend to occur that increase the net
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organization (or decrease the net entropy) of the system. Thus, the universe
taken as a whole is steadily moving toward a state of complete randomness,
lacking any order, pattern, or beauty. This fate was popularized in the 19th
century as the “heat death” of the universe.

Living organisms are manifestly organized and at first sight seem to represent a
contradiction to the second law of thermodynamics. Indeed, living systems
might then be defined as localized regions where there is a continuous
maintenance or increase in organization. Living systems, however, do not really
contradict the second law. They increase their organization in regions of energy
flow, and, indeed, their cycling of materials and their tendency to grow can be
understood only in the context of a more general definition of the second law
that applies to open as well as closed and isolated systems. In nature (except at
cosmic scales, where gravity becomes a crucial factor), energy moves from being
concentrated to being spread out; spontaneously occurring complex systems do
not violate the second law but help energy spread out, thus producing entropy
and reducing gradients.

A general statement of open-system thermodynamics is that nature abhors a

gradient, a difference across a distance. Differences and gradients in nature
represent improbable, preexisting organizations. Many complex systems in
nature spontaneously arise to degrade gradients and persist until the gradients
are nullified. A tornado, for example, is an improbable, matter-cycling system
that appears in the area of a barometric pressure gradient; when the air pressure
gradient is gone, the “improbable” tornado disappears. Life seems to be a
similar system, but one that degrades the solar gradient, the electromagnetic
difference between the extremely hot (5,800 K [5,500 °C, or 10,000 °F]) Sun and
very cold (2.7 K [−270.3 °C, or −454.5 °F]) outer space. (K = kelvin. On the Kelvin
temperature scale, in which 0 K [−273 °C, or −460 °F] is absolute zero, 273 K [0
°C, or 32 °F] is the freezing point of water, and 373 K [100 °C, or 212 °F] is the
boiling point of water at one atmosphere pressure.) Most life on Earth is
dependent on the flow of sunlight, which is utilized by photosynthetic
organisms to construct complex molecules from simpler ones. Some deep-sea
and cave organisms called chemoautotrophs depend on chemical gradients,
such as the natural energy-producing reaction between hydrogen sulfide
bubbling up from vents and oxygen dissolved in water. The organization of life
on Earth can thus be seen as being driven by a natural second-law-based
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reduction between the energy of the hot Sun and the cooler space around it.
Although life has not fully reduced the solar gradient, incorporation of carbon
dioxide into chemoautotrophs and production of clouds by plants help keep
Earth’s surface cooler than it would otherwise be, thereby helping to degrade
the solar energy gradient.

Some scientists argue on grounds of quite

general open-system thermodynamics that
the organization of a system increases as
energy flows through it. Moreover, energy
flow leads to the development of cycles. An
example of a biological cycle on Earth is the
carbon cycle
carbon cycle. Carbon from atmospheric
Carbon is transported in various
carbon dioxide is incorporated by
forms through the atmosphere, the
hydrosphere, and geologic formations. photosynthetic or chemosynthetic organisms
One of the primary pathways for the
and converted into carbohydrates through the
exchange of carbon dioxide (CO2)
takes place between the atmosphere
process of autotrophy. These carbohydrates
and the oceans; there a fraction of the are ultimately oxidized by heterotrophic
CO2 combines with water, forming
organisms to extract useful energy locked in
carbonic acid (H2CO3) that
their chemical bonds. In the oxidation of
subsequently loses hydrogen ions
(H+) to form bicarbonate (HCO3−) carbohydrates, carbon dioxide is returned to

and carbonate (CO32−) ions. Mollusk

the atmosphere, thus completing the cycle. It
shells or mineral precipitates that has been shown that similar cycles develop
form by the reaction of calcium or spontaneously and in the absence of life by
other metal ions with carbonate may
become buried in geologic strata and the flow of energy through chemical systems.
eventually release CO2 through Biological cycles may represent natural
volcanic outgassing. Carbon dioxide
thermodynamic cycles reinforced by a genetic
also exchanges through
photosynthesis in plants and through apparatus. It seems doubtful that open-
respiration in animals. Dead and system thermodynamic processes in the
decaying organic matter may ferment
and release CO2 or methane (CH4) or absence of replication lead to the sorts of
may be incorporated into sedimentary complexity that characterize biological
rock, where it is converted to fossil systems; replication, however, may be
fuels. Burning of hydrocarbon fuels
returns CO2 and water (H2O) to the interpreted as an especially efficient
atmosphere. The biological and thermodynamic means of gradient breakdown
anthropogenic pathways are much
—a kind of special, slow-burning “fire.” In any
faster than the geochemical pathways
and, consequently, have a greater case, it is clear that much of the complexity of

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impact on the composition and life on Earth has arisen through replication,
temperature of the atmosphere.
with thermodynamically favoured pathways
being used by energy-transforming organisms.
Autopoietic

A newer definition of life revolves around the

idea of autopoiesis. This idea was put forth by
Chilean biologists Humberto Maturana and
Francisco Varela and emphasizes the peculiar
closure of living systems, which are alive and
Molecular view of the cell
membrane maintain themselves metabolically whether
Intrinsic proteins penetrate and bind they succeed in reproduction or not. Unlike
tightly to the lipid bilayer, which is machines, whose governing functions are
made up largely of phospholipids and
cholesterol and which typically is embedded by human designers, organisms
between 4 and 10 nanometers (nm; 1 are self-governing. The autopoietic definition
nm = 10−9 metre) in thickness. of life resembles the physiological definition
Extrinsic proteins are loosely bound
to the hydrophilic (polar) surfaces, but emphasizes life’s maintenance of its own
which face the watery medium both identity, its informational closure, its
inside and outside the cell. Some
intrinsic proteins present sugar side
cybernetic self-relatedness, and its ability to
chains on the cell's outer surface. make more of itself. Autopoiesis refers to self-
producing, self-maintaining, self-repairing,
and self-relational aspects of living systems. Living beings maintain their form by
the continuous interchange and flow of chemical components. Cellular
autopoietic systems are bounded by a dynamic material made by the system
itself. In life on Earth the limiting material is lipid membrane studded with
transport proteins fabricated by the incessantly active cell. A source of usable
energy flows to all living or autopoietic systems—either light in the visible or
near-visible spectrum or specific organic carbon or other chemicals such as
hydrogen, hydrogen sulfide, or ammonia. Energy sources that have never been
adequate to maintain autopoiesis on Earth include heat, sound waves, and
electromagnetic radiation outside the visible or near-visible spectrum.

One of the difficulties in defining life is that the only example is life found on the
third planet from the Sun. On Earth all life’s autopoietic systems require a supply
of water in its liquid state for self-maintenance of their parts. Taken together, all
transformations that underlie autopoiesis require six elements: carbon, nitrogen,

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hydrogen, oxygen, phosphorus, and sulfur. The chemical components of all

living entities are fashioned primarily from these elements.

The smallest autopoietic system on Earth is a living bacterial cell. (Viruses,

plasmids, and other replicating molecules cannot, even in principle, behave as
an autopoietic system; no matter how much food, liquid water, and serviceable
energy they are provided, they still require cells for their continuity and
duplication.) Some cells, such as Carsonella ruddii, have fewer than 200 genes
and proteins, but they, like organelles and viruses, are not autopoietic, since
they must be inside an autopoietic system (living cell) to metabolize and survive.
No self-bounded autopoietic system smaller than a cell with at least 450
proteins and the genes that code for these proteins has ever been described.
Larger than bacteria are other autopoietic systems of intermediate size such as
protists, fungal spores, mules and other individual mammals, and plants such as
oak trees or poppies. Autopoietic entities at even larger levels include
ecosystems such as coral reefs, prairies, or ponds. The maximal or largest single
autopoietic system known is referred to as “Gaia,” named by English
atmospheric scientist James E. Lovelock for Gaea, the ancient Greek
personification of Earth. Gaia is basically a closed thermodynamic system
because there is little interchange of matter with the extraterrestrial
environment. There is evidence that the global, Gaian system of life shows
organism-like properties, such as regulation of atmospheric chemistry, global
mean temperature, and oceanic salinity over multimillion-year time spans. Such
regulation may be understood as part of life’s organization as a complex and
cyclical open thermodynamic system.
Life on Earth
The existence of diverse definitions of life, as
detailed in the previous section, surely means
that life is complex and difficult to briefly
define. A scientific understanding of living
systems has existed since the second half of
the 19th century. But the diversity of
definitions and lack of consensus among
timeline of life on Earth professionals suggest something else as well.
Over hundreds of millions of years,
As detailed in this section, all organisms on
life spread through the seas and over Earth are extremely closely related, despite
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Earth's surface. The first life-forms superficial differences. The fundamental

were small and simple. Later forms
were more complicated and diverse. pattern, both in form and in matter, of all life
on Earth is essentially identical. Also, as noted
in this section, this identity implies that all organisms on Earth are evolved from
a single instance of the origin of life. To generalize from a single example is
difficult, especially when the example itself is changing, growing, and evolving.
In this respect the biologist is fundamentally handicapped, as compared with,
say, the chemist, physicist, geologist, or meteorologist, each of whom can now
study aspects of his discipline beyond Earth. If truly only one sort of life on Earth
exists, then perspective is lacking in a most fundamental way. On the other
hand, the historical continuity of all life-forms means that ancient life, perhaps
even the origins of life, may be glimpsed by studying modern cells.

The biosphere

Reconnaissance missions to the planets of the

inner solar system have revealed stark and
barren landscapes. From the heavily cratered
and atmosphereless surfaces of both Mercury
and the Moon to the hot sulfurous fogs of
Venus and the dusty, windswept surface of
Mars, no sign of life is apparent anywhere. The

Earth
biosphere, by definition the place where all
Earth’s life dwells, is a delight with its green,
The planet Earth.
wet contrast. Austrian geologist Eduard Suess
invented the term biosphere to match the other envelopes of the planet: the
atmosphere of gas; the hydrosphere of oceans, lakes, rivers, springs and other
waters; and the lithosphere, or the solid rock surface of the outer portion of
Earth. Yet it was the great Russian crystallographer and mineralogist Vladimir I.
Vernadsky who brought the term into common parlance with his book of the
same name. In The Biosphere (1926) Vernadsky outlines his view of life as a
major geological force. Living matter, Vernadsky contends, erodes, levels,
transports, and chemically transforms surface rocks, minerals, and other features
of Earth. If the biosphere is the place where life is found, the biota (or the
biomass as a whole) is the sum of all living forms: flora, fauna, and microbiota.

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During the second half of the 20th century, study of the deep sea, the upper
atmosphere, the Antarctic dry deserts, newly opened caves, sulfurous tunnels,
and granitic rocks showed that Earth’s surface is vigorously inhabited in places
that were unknown to Vernadsky and his colleagues. Vernadsky’s international
school of thought ushered in the field of “biogeochemistry,” and chemists and
geologists were recruited to consider life as a planetary phenomenon. But not
until giant, mouthless, red-gilled tube worms were videographed in the late
1970s and ’80s did the extent and the weirdness of Earth’s biota begin to be
fathomed. Entire large ecosystems were recognized on the ocean’s bottom that
live not by the usual plant photosynthesis but rather by chemolithoautotrophy,
a kind of metabolism in which organisms make food from carbon dioxide using
energy from the oxidation of sulfide, methane, or other inorganic compounds.
These discoveries have led to a deeper understanding of life’s varied modes of
nutrition and sources of energy. Bacterial symbionts living in the tissues of some
polychaete worms (alvinellids) or pogonophora (such as Riftia pacytila) provide
the animals with their total nutritional needs. The submarine ecosystems
supported by bacteria thrive along the worldwide rift zones that extend along
the borders of huge continental plates at the Mid-Atlantic Ridge, on the East
Pacific Rise, at 21° north of the Equator off the coast of Baja California, Mex., and
at a dozen other newly studied sites. By the beginning of the 21st century it had
become abundantly clear that many life-forms and ecosystems remained
unknown or under-studied. Those in the Siberian tundra, in the thickly forested
portions of the Amazon River valley and its tributaries, at the tops of remote
mountains and inside granitic rocks in temperate zones, and in the centre of
Africa remain as inaccessible to most naturalists as they have been throughout
history. The easily accessed woodlands and fields of well-lit land surfaces are
another story.

On land, 24 percent of the productivity of organic carbon biomass generated by

plants is directly controlled by burgeoning populations of one species, humans.
As Vernadsky noted, life in general and human life in particular tend to
accelerate the number of materials and the rate of flow of these materials
through the biosphere, the place where all life exists—so far.

Vernadsky anticipated new discoveries of life inside hot springs and granitic
rock. Although he qualified this statement by asserting that it would not hold for

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temporary, abnormal circumstances, such as during a lava flow or a volcanic

eruption, he wrote,

Thus far, we have seen that the biosphere, by structure, composition, and physical
makeup, is completely enclosed by the domain of life, which has so adapted itself to
biospheric conditions that there is no place [on Earth] in which it is unable to manifest
itself in one way or another.

Although much is not known about life in the depths of the rocks and the sea,
determination of the total range and mass (biomass) of the biota, the sum of all
life in the biosphere, is a reasonable scientific goal.
Chemistry of life

Human beings, like mammals in general, are

ambulatory collections of some 1014 cells.
Human cells are in all fundamental respects
the same as those that make up the other
animals. Each cell typically consists of one
central, spherical nucleus and another
heterogeneous region, the cytoplasm. (Only
Animal cell bacterial cells lack nuclei; those of plants,
Principal structures of an animal fungi, and all other organisms contain one or
cellCytoplasm surrounds the cell's
specialized structures, or organelles.
more nuclei.) A living nucleated cell, a marvel
Ribosomes, the sites of protein of detailed and complex architecture, appears
synthesis, are found free in the
frenetic with activity when seen through a
cytoplasm or attached to the
endoplasmic reticulum, through microscope. On a deeper chemical level, it is
which materials are transported
known that life’s large molecules, the proteins
throughout the cell. Energy needed by
the cell is released by the and nucleic acids, are synthesized at a very
mitochondria. The Golgi complex, fast rate. Enzymes, which speed up chemical
stacks of flattened sacs, processes and
packages materials to be released reactions, are all proteins, but by no means
from the cell in secretory vesicles. are all proteins enzymes. An enzyme catalyzes
Digestive enzymes are contained in
lysosomes. Peroxisomes contain
the synthesis of more than 1,000 other
enzymes that detoxify dangerous molecules per second. The total mass of a
substances. The centrosome contains
metabolizing bacterial cell can be synthesized
the centrioles, which play a role in cell
division. The microvilli are fingerlike in 20 minutes. The information content of a
extensions found on certain cells.
Cilia, hairlike structures that extend
small cell has been estimated as about 1010
from the surface of many cells, can bits, comparable to about 106 (or one million)
create movement of surrounding
pages of the print version of the Encyclopædia
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fluid. The nuclear envelope, a double Britannica. Although some feel debased by
membrane surrounding the nucleus,
contains pores that control the the implication that people are “nothing
movement of substances into and out more” than a frenetic collection of interacting
of the nucleoplasm. Chromatin, a
combination of DNA and proteins that
molecules, others are thrilled with the power
coil into chromosomes, makes up of science to reveal the inner workings of the
much of the nucleoplasm. The dense
chemistry of life. The spectacular success of
nucleolus is the site of ribosome
production. biochemistry and molecular biology in the
20th century suggests that laws of biology are
derived from the interaction of atoms, thermodynamic principles, and life’s
chemistry, which has persisted with faithful continuity since its origin some 3.7
billion to 3.5 billion years ago.

DNA, RNA, and protein

The specific carrier of the genetic information

in all organisms is the nucleic acid known as
DNA, short for deoxyribonucleic acid. DNA is a
double helix, two molecular coils wrapped
around each other and chemically bound one
to another by bonds connecting adjacent
DNA and protein synthesis bases. Each long ladderlike DNA helix has a
DNA in the cell nucleus carries a backbone that consists of a sequence of
genetic code, which consists of
alternating sugars and phosphates. Attached
sequences of adenine (A), thymine
(T), guanine (G), and cytosine (C) to each sugar is a “base” consisting of the
(Figure 1). RNA, which contains uracil
nitrogen-containing compound adenine,
(U) instead of thymine, carries the
code to protein-making sites in the guanine, cytosine, or thymine. Each sugar-
cell. To make RNA, DNA pairs its phosphate-base “rung” is called a nucleotide.
bases with those of the “free”
nucleotides (Figure 2). Messenger
A very significant one-to-one pairing between
RNA (mRNA) then travels to the bases occurs that ensures the connection of
ribosomes in the cell cytoplasm,
where protein synthesis occurs
adjacent helices. Once the sequence of bases
(Figure 3). The base triplets of along one helix (half the ladder) has been
transfer RNA (tRNA) pair with those
specified, the sequence along the other half is
of mRNA and at the same time
deposit their amino acids on the also specified. The specificity of base pairing
growing protein chain. Finally, the plays a key role in the replication of the DNA
synthesized protein is released to
perform its task in the cell or molecule. Each helix makes an identical copy
elsewhere in the body. of the other from molecular building blocks in
the cell. These nucleic acid replication events
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are mediated by enzymes called DNA polymerases. With the aid of enzymes,
DNA can be produced in the laboratory.

The cell, whether bacterial or nucleated, is the minimal unit of life. Many of the
fundamental properties of cells are a function of their nucleic acids, their
proteins, and the interactions among these molecules bounded by active
membranes. Within the nuclear regions of cells is a mélange of twisted and
interwoven fine threads, the chromosomes. Chromosomes by weight are
composed of 50–60 percent protein and 40–50 percent DNA. During cell
division, in all cells but those of bacteria (and some ancestral protists), the
chromosomes display an elegantly choreographed movement, separating so
that each offspring of the original cell receives an equal complement of
chromosomal material. This pattern of segregation corresponds in all details to
the theoretically predicted pattern of segregation of the genetic material
implied by the fundamental genetic laws (see heredity). The chromosome
combination of the DNA and the proteins (histone or protamine) is called
nucleoprotein. The DNA stripped of its protein is known to carry genetic
information and to determine details of proteins produced in the cytoplasm of
cells; the proteins in nucleoprotein regulate the shape, behaviour, and activities
of the chromosomes themselves.

The other major nucleic acid is ribonucleic acid (RNA). Its five-carbon sugar is
slightly different from that of DNA. Thymine, one of the four bases that make up
DNA, is replaced in RNA by the base uracil. RNA appears in a single-stranded
form rather than a double. Proteins (including all enzymes), DNA, and RNA have
a curiously interconnected relation that appears ubiquitous in all organisms on
Earth today. RNA, which can replicate itself as well as code for protein, may be
older than DNA in the history of life.
Chemistry in common

The genetic code was first broken in the 1960s. Three consecutive nucleotides
(base-sugar-phosphate rungs) are the code for one amino acid of a protein
molecule. By controlling the synthesis of enzymes, DNA controls the functioning
of the cell. Of the four different bases taken three at a time, there are 43, or 64,
possible combinations. The meaning of each of these combinations, or codons,
is known. Most of them represent one of the 20 particular amino acids found in
protein. A few of them represent punctuation marks—for example, instructions

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to start or stop protein synthesis. Some of the code is called degenerate. This
term refers to the fact that more than one nucleotide triplet may specify a given
amino acid. This nucleic acid–protein interaction underlies living processes in all
organisms on Earth today. Not only are these processes the same in all cells of
all organisms, but even the particular “dictionary” that is used for the
transcription of DNA information into protein information is essentially the
same. Moreover, this code has various chemical advantages over other
conceivable codes. The complexity, ubiquity, and advantages argue that the
present interactions among proteins and nucleic acids are themselves the
product of a long evolutionary history. They must interact as a single
reproductive, autopoietic system that has not failed since its origin. The
complexity reflects time during which natural selection could accrue variations;
the ubiquity reflects a reproductive diaspora from a common genetic source;
and the advantages, such as the limited number of codons, may reflect an
elegance born of use. DNA’s “staircase” structure allows for easy increases in
length. At the time of the origin of life, this complex replication and transcription
apparatus could not have been in operation. A fundamental problem in the
origin of life is the question of the origin and early evolution of the genetic
code.

Many other commonalities exist among

organisms on Earth. Only one class of
molecules stores energy for biological
processes until the cell has use for it; these
molecules are all nucleotide phosphates. The
most common example is adenosine
triphosphate (ATP). For the very different
function of energy storage, a molecule

Hemoglobin tetramer
identical to one of the building blocks of the
nucleic acids (both DNA and RNA) is
Two αβ dimers combine to form the
complete hemoglobin molecule. Each employed. Metabolically ubiquitous molecules
heme group contains a central iron
—flavin adenine dinucleotide (FAD) and
atom, which is available to bind a
molecule of oxygen. The α1β2 region is coenzyme A—include subunits similar to the
the area where the α1 subunit nucleotide phosphates. Nitrogen-rich ring
interacts with the β2 subunit. compounds, called porphyrins, represent
another category of molecules; they are

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smaller than proteins and nucleic acids and common in cells. Porphyrins are the
chemical bases of the heme in hemoglobin, which carries oxygen molecules
through the bloodstream of animals and the nodules of leguminous plants.
Chlorophyll, the fundamental molecule mediating light absorption during
photosynthesis in plants and bacteria, is also a porphyrin. In all organisms on
Earth, many biological molecules have the same “handedness” (these molecules
can have both “left-” and “right-handed” forms that are mirror images of each
other; see below The earliest living systems). Of the billions of possible organic
compounds, fewer than 1,500 are employed by contemporary life on Earth, and
these are constructed from fewer than 50 simple molecular building blocks.

Besides chemistry, cellular life has certain

supramolecular structures in common.
Organisms as diverse as single-celled
paramecia and multicellular pandas (in their
sperm tails), for example, possess little
whiplike appendages called cilia (or flagella, a
term that is also used for completely
unrelated bacterial structures; the correct
generic term is undulipodia). These “moving
paramecia cell hairs” are used to propel the cells through
Paramecium caudatum (highly liquid. The cross-sectional structure of
magnified).
undulipodia shows nine pairs of peripheral
tubes and one pair of internal tubes made of
proteins called microtubules. These tubules are made of the same protein as
that in the mitotic spindle, the structure to which chromosomes are attached in
cell division. There is no immediately obvious selective advantage of the 9:1
ratio. Rather, these commonalities indicate that a few functional patterns based
on common chemistry are used over and over again by the living cell. The
underlying relations, particularly where no obvious selective advantage exists,
show all organisms on Earth are related and descended from a very few
common cellular ancestors—or perhaps one.
Modes of nutrition and energy generation

Chemical bonds that make up the compounds of living organisms have a certain
probability of spontaneous breakage. Accordingly, mechanisms exist that repair
this damage or replace the broken molecules. Furthermore, the meticulous
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control that cells exercise over their internal activities requires the continued
synthesis of new molecules. Processes of synthesis and breakdown of the
molecular components of cells are collectively termed metabolism. For synthesis
to keep ahead of the thermodynamic tendencies toward breakdown, energy
must be continuously supplied to the living system.

Energy, carbon, and electrons

Organisms acquire energy by two general

methods: by light or by chemical oxidation.
Productive organisms, called autotrophs,
convert light or chemicals into energy-rich
organic compounds beginning with energy-
poor carbon dioxide (CO2). These autotrophs
provide energy for the other organisms, the
food capture: animals
heterotrophs. Heterotrophs are organisms
The process of food capture in
different animal species. that acquire their energy by the controlled
breakdown of preexisting organic molecules,
or food. Human beings, like most other animals, fungi, protists, and bacteria, are
heterotrophs.

Autotrophic organisms are often primary producers in their ecosystems. They

acquire their useful free energy from sources other than food: either from the
energy of sunlight (photoautotrophs) or from oxidative chemical reactions
(chemoautotrophs). The latter mode of metabolism refers to life-forms that use
inorganic materials (ammonia [NH3], methane [CH4], or hydrogen sulfide [H2S])
combined with oxygen to generate their energy. Only some bacteria are capable
of obtaining energy by “burning” inorganic chemicals.

Green plants are typical photoautotrophs.

Plants absorb sunlight to generate ATP and to
disassociate water into oxygen and hydrogen.
To break down the water molecule, H2O, into
hydrogen and oxygen requires much energy.
The hydrogen from water is then combined in
the “dark reactions” with carbon dioxide, CO2.
Calvin cycle
The result is the production of such energy-

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Pathway of carbon dioxide fixation rich organic molecules as sugars, amino acids,
and reduction in photosynthesis, the
Calvin cycle. The diagram represents and nucleotides. The oxygen becomes the gas
one complete turn of the cycle, with O2, which is released as waste back into the
the net production of one molecule of
glyceraldehyde-3-phosphate (Gal3P). atmosphere. Animals, which are strictly
This three-carbon sugar phosphate heterotrophs, cannot live on carbon dioxide,
usually is converted to either sucrose
or starch. sunlight, and water with a few salts like plants
do. They must breathe in the atmospheric
oxygen. Animals combine oxygen chemically with hydrogen atoms that they
remove from their food—that is, from organic materials such as sugar, protein,
and amino acids. Animals release water as a waste product from the oxygen
respiration. Animals, like all heterotrophs, use organic materials as their sole
source of carbon. This conversion of carbon provides an example of an aspect of
an ecological cycle in which a required element flows through different types of
organisms as it changes its oxidation state from CO2 to (CH2O)n and back to
CO2.

Metabolic cycles in general—the extraction by organisms of useful energy and

food molecules from environmental source material—can be described in terms
of oxidation-reduction reactions. In the case of oxygen respiration, oxygen
molecules from the air accept electrons ultimately from glucose or amino acids.
The oxygen, which has a great affinity for electrons, is called an electron
acceptor, whereas the glucose, or other sugar or organic molecules, is an
electron donor. Animal respiration is the prototype of oxidation-reduction
reactions, but certainly not all oxidation-reduction reactions (or “redox
reactions,” as they are often called) involve oxygen. Many other inorganic
compounds are respired, or “breathed,” at the cell level. Biological electron
acceptors besides oxygen include nitrate, nitrite, sulfate, carbonate, elemental
sulfur, and methanol. Biological electron donors (other than sugar and amino
acids) include hydrogen, nitrogen compounds (as ammonia, nitrite), sulfide, and
methane. For acceptor-donor transformations to be available to
chemoautotrophs and heterotrophs over sustained periods of time, ecological
cycles are required. For geologically short periods of time, organisms may live
off a finite supply of material; however, for any long-term continuance of life, a
dynamic cycling of matter involving complementary types of organisms must
prevail. If life exists on other planets, the requisite elements and liquid water

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must cycle. A search for such transformations provides one method of detecting
extraterrestrial life.

In addition to energy, all forms of life require carbon sources. Autotrophic

organisms (chemosynthetic and photosynthetic bacteria, algae, and plants)
derive this essential element from carbon dioxide. Heterotrophs use preformed
organic compounds as their source of carbon. Among autotrophs many types of
cells do not depend on light to generate ATP; those that do without light are the
chemoautotrophic bacteria, including the methanogens, ammonia oxidizers,
sulfide oxidizers, hydrogen oxidizers, and a few obscure others. Indeed, at least
five metabolic pathways entirely different from each other have evolved to use
carbon dioxide gas. One is the oxygenic pathway described above, which is used
by plants, algae, and cyanobacteria: the Calvin-Benson dark reactions. Other,
more obscure pathways include phosphoenolpyruvate (PEP), succinate, and
methanogen pathways. They all need to bring energy-poor carbon dioxide into
the energy-rich carbon-hydrogen compound metabolism of organisms. All life
on Earth depends on these autotrophic reactions that begin with carbon dioxide
or its equivalent. Equivalents as carbon sources in autotrophic metabolism
include the carbonate ion, bicarbonate ion, and carbon monoxide. As usual, with
respect to metabolic variation and virtuosity, the bacterial repertoire is vastly
more diverse than that of eukaryotes—that is, plants, animals, and other
organisms composed of cells with nuclei. In general, nucleated organisms,
eukaryotes, are either photolithoautotrophs (i.e., algae and plants) that derive
energy from light or minerals or chemo-organoheterotrophs (animals, fungi,
and most protists) that derive energy and carbon from preformed organic
compounds (food).
ATP

All ATP biological electron-transfer reactions

lead to the net production of ATP molecules.
Two of the three phosphates (PO4) of this
molecule are held by energy-rich bonds
sufficiently stable to survive for long periods
of time in the cell but not so strong that the
cell cannot tap these bonds for energy when
basic overview of processes of ATP needed. ATP and similar molecules (such as
production
guanosine triphosphate [GTP]) have a five-
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The three processes of ATP carbon sugar and three phosphates. As far as
production include glycolysis, the
tricarboxylic acid cycle, and oxidative is known, such molecules are the general and
phosphorylation. In eukaryotic cells unique energy currency of living systems on
the latter two processes occur within
mitochondria. Electrons that are
Earth.
passed through the electron transport
chain ultimately generate free energy No metabolic process occurs in a single step.
capable of driving the The ordinary six-carbon sugar, glucose, does
phosphorylation of ADP.
not oxidize to carbon dioxide and water in
living cells in the same way that glucose in air burns. Any release of energy by
burning would be too sudden and too concentrated in a small volume to
happen safely inside the tiny cell. Instead, glucose is broken down at ambient
(i.e., relatively cool) temperatures by a series of successive and coordinated
steps. Each step is mediated by a particular and specific enzyme. In most cells
that metabolize glucose, the sugar first breaks down in a set of steps that occur
in the absence of oxygen. The total number of such steps in plants, animals,
fungi, and protists (see below Hypotheses of origins) is about 11. Other
organisms, primarily bacteria and obscure protists and fungi, are anaerobes:
they do not utilize molecular oxygen in their metabolism. In anaerobes, glucose
metabolism stops at compounds such as ethanol or lactic acid. Aerobic
organisms, including all animals, carry the oxidation of glucose farther. They
rapidly use anaerobic glucose breakdown products such as lactic acid, ethanol,
or acetate with Krebs-cycle intermediates in the mitochondria. Aerobic oxidation
of glucose requires an additional 60 enzyme-catalyzed steps. The anaerobic
breakdown of glucose uses enzymes suspended freely in solution in the cells.
The aerobic steps occur on enzymes localized in mitochondria, the “power
packs” of cells where oxygen gas is used to make the energy compound ATP.
The complete aerobic breakdown of sugar to carbon dioxide and water is about
10 times more efficient than the anaerobic in that 10 times as many ATP
molecules are produced. (See metabolism.)

Energy made available to cells in the form of ATP is used in a variety of ways—
for example, for motility. When an amoeba extends pseudopods or when a
person walks, ATP molecules are tapped for their energy-rich phosphate bonds.
ATP molecules are used for the synthesis of proteins that all cells require in their
growth and division, amino acids, and five-carbon sugars of nucleic acids. Each
synthetic process is controlled and enzymatically mediated. Each starts from an
organic building-block compound available to the cell as food. The amino acid
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L-leucine, for example, is produced from pyruvic acid, which is itself the product
of the anaerobic breakdown of glucose. Synthesis of L-leucine from pyruvic acid
involves eight enzyme-mediated steps with an addition of acetic acid and water.

These exquisitely interlocked and controlled metabolic steps are not performed
in a diffuse manner helter-skelter in the cell. Rather, a marvelously architectured
cellular interior displays specialized regions visible at the electron-microscopic
level. Particular chemical reactions are performed in association with specific
structures. In aerobic eukaryotes the mitochondrion with its intricate cristate
membrane (the folds in the membrane are called cristae) provides the site of
pyruvate, acetate, and lactate metabolism. These molecules are transformed and
passed on from one enzyme to another as through a conveyor belt in a factory.
Similarly, in those eukaryotes capable of oxygenic photosynthesis (algae, plants),
photosynthesis occurs only in an organelle (a cell part) called a chloroplast.
Chlorophyll, carotenoids, and other pigments that absorb visible light, as well as
the detailed enzymatic apparatus for the photosynthetic process, reside there.

Chloroplasts and mitochondria contain DNA. Moreover, this DNA has a

sequence distribution that differs entirely from that of the nucleus and greatly
resembles that of free-living photosynthetic and oxygen-respiring bacteria. The
best explanation for these facts is that the ancestors to the oxygen-releasing
chloroplasts and oxygen-respiring mitochondria were once free-living bacteria.
Diversity
Prokaryotes and eukaryotes

All life is composed of cells of one of two types: prokaryotes (those that lack a
nucleus) or eukaryotes (those with a nucleus). Even in one-celled organisms this
distinction is very clear.

All bacteria are prokaryotic, even though many, probably most, are multicelled
in nature. The only other single-celled organisms that exist are fungi (one-celled
fungi are called yeasts). All nucleated organisms (cells with nuclei and
chromosomes in their cells) that are not animals, fungi, or plants are Protista.
This huge group includes the unicellular or few-celled protists and their
multicellular descendants. The large kingdom of Protista has 250,000 estimated
species alive today. Some are very large, such as red algae and the kelp
Macrocystis. One-celled protists include the familiar amoebas, paramecia, and

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euglenas as well as 50,000 less-familiar types. Scientifically speaking, no such

thing as a one-celled animal exists. All animals and plants are by definition
multicellular, since they all develop from multicellular embryos. Accordingly, all
“protozoans” are now classified as single-celled Protista, not animals. Nor are
there any one-celled plants. Organisms formerly called one-celled plants are
algae and, as such, are now classified with Protista as well. If a mature organism
is determined to be one-celled, then it must be either a bacterium (prokaryotic)
or a fungus or protist (eukaryotic). All animals and plants develop from embryos
that by definition combine two complementary sets of chromosomes (i.e., they
are diploids at some stage in their development). They are all multicellular
eukaryotes. But though there are no one-celled plants or animals, there are
indeed myriad many-celled protists. Multicellularity evolved not only in the
ancestors to the plants and the animals but also in the bacteria, the protists, and
the fungi.

All eukaryotic cells undergo some form of

mitosis, a sequence of cell division events that
occurs after chromosomal DNA protein
replication. Mitosis ensures that chromosomal
DNA and protein are equally distributed to
the offspring cells. Mitosis is the most
stages of mitosis
distinctive activity of eukaryotic cells, which
Stages of mitosisA. Prophase.
have nucleoprotein chromosomes in their
Replicated chromosomes, consisting
of two daughter strands (chromatids) nuclei and a membrane that separates the
attached by a centromere, coil and
nucleus from the cytoplasm. In mitosis,
contract. Two pairs of specialized
organelles (centrioles) begin to move mitochondria, which are usually present in the
apart, forming a bridge of hollow cytoplasm as well as in the chloroplasts of
protein cylinders known as
microtubules (spindle fibers) between algae and plants, are smoothly distributed
them. Microtubules also extend in a with the chromosomes to offspring cells. The
radial array (aster) from the centrioles
to the poles of the cell.B. Late
Golgi apparatus and endoplasmic reticulum
prophase. As the centrioles move (ER), an intricately convoluted structure, serve
apart, the nuclear membrane breaks
to anchor many cytoplasmic enzymes
down and microtubules extend from
each centromere to opposite sides or excluded from mitochondria or chloroplasts.
poles of the cell.C. Metaphase. The
They also divide and are distributed in mitosis.
centromeres align in a plane midway
between the poles known as the
equator, or metaphase plate. During
Nuclei, chromosomes, mitochondria,
late metaphase, each centromere chloroplasts, ER, and nuclear membranes are
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divides into two, freeing sister all absent in prokaryotes. Prokaryotic cells,
chromatids from each other.D.
Anaphase. Sister chromatids are which include all the cyanobacteria (formerly
drawn to opposite ends as called blue-green algae), are bacteria in every
centromeric microtubules shorten and
polar microtubules lengthen, causing
way. Division is nonmitotic in all prokaryotes.
the poles to move farther apart.E. Bacteria lack nucleoprotein and a nuclear
Telophase. Chromosomes uncoil,
membrane, and, when chromosome stain is
microtubules disappear, and the
nuclear membrane re-forms around applied, only fuzz or nothing is seen. Whereas
each set of daughter chromosomes.
all eukaryotic cells have more than one
The cytoplasm begins to pinch in to
create two daughter cells. chromosome and sometimes over a thousand,
the genes of prokaryotic cells are organized
into a single “chromoneme” or “genophore.” (The term bacterial chromosome,
while still in use, is, technically speaking, inaccurate.) The genes may or may not
be concentrated enough to be seen, but in any case bacterial DNA floats freely
in the cytoplasm. Prokaryote cell organization is less complex than that of
eukaryotes. The basic question of the evolution of prokaryotes into eukaryotes
—often rated the second major evolutionary mystery, after the origin of life—is
thought to involve a complex series of partnerships in which distinct strains of
bacteria entered each others’ bodies, merged symbiotically, and traded genes.
Multicellularity

Since multicellularity evolved independently in every major group of

microorganisms, the blurred distinction between single-celled and many-celled
organisms has become obsolete. The protists are divisible into about 35
unambiguous groups called phyla. They provide many examples of biological
principles—including the prevalence of independent trends toward
multicellularity. One illustration involves cellular slime molds. These
heterotrophs undergo an extraordinary sequence of events during their life
history. The story begins with single cells, indistinguishable from common
amoebas. When starved, they begin to swarm. Soon they combine into a slimy
mass of many nucleated amoeba cells called a pseudoplasmodium. The
pseudoplasmodium in turn forms a sluglike multicellular creature resembling a
mollusk that has escaped from its shell. This slug, which is entirely multicellular,
migrates and then stops and develops into a stalk structure called a sorocarp
that bears amoeba cysts on top. The cysts were called “spores.” Some have
cellulose cell walls similar to those of plants. The cysts, which are encased
amoebas (just like other amoeba cysts), germinate in turn—when water and
food again become abundant—into new amoebas. The released amoebas
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extend their pseudopods, and, as individuals again, they migrate to feed. The life
history repeats with swarms of migrating amoebas, slugs, stalks, and finally
clusters of amoeba cysts on top as wet, food-rich conditions are followed by
dryness and scarcity.

Biology is replete with life histories of

comparable or even greater complexity. In
protist life histories—by far the most diverse,
exotic, and unique—one can search for
ancestral modes of life, including missing links
between the prokaryotes and eukaryotes. The
fertilization of human egg
one-celled swimming stage is called a sperm,
(1) The sperm release enzymes that
help disperse the corona radiata and whose imperative it is to find another one-
bind to the zona pellucida. (2) The
celled partner, the ovum. Like all animals,
outer sperm head layer is sloughed
off, exposing enzymes that digest a humans develop from a single fertilized egg
path through the zona pellucida. (3) with its complement of two sets of genetic
The sperm fuses with the egg cell
membrane, causing the zona pellucida
material. These diploid fertilized egg cells then
to become impenetrable to other divide to form many presumably identical
sperm. (4) The tail separates from the
cells. The early embryology of all animals goes
sperm head, and the male pronucleus
enlarges and travels to the female through stages that have 2, 4, 8, 16, and so on
pronucleus in the center of the cell.
cells. Genetic information is theoretically
Chromosomes merge to form a
fertilized egg. identical in each cell. But then how does it
ever happen that, as they mature, the cells
become permanently specialized to form hair, bone, liver, blood, or nerve cells?
How does any given cell “know” what sort of specialized cell it must become,
since all cells seem to contain identical nucleic acids? Despite a century of work
on this process (called differentiation) and the discovery of many facts about
embryos, this basic problem still remains unsolved in animal biology.
Classification and microbiota

Prior to the recognition of microbial life, the

living world was too easily divided into
animals that moved in pursuit of food and
plants that produced food from sunlight. The
futility of this simplistic classification scheme
has been underscored by entire fields of
science. Many bacteria both swim (like
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Whittaker five-kingdom animals) and photosynthesize (like plants), yet

classification
they are best considered neither. Many algae
The Whittaker five-kingdom
classification of life. (e.g., euglenids, dinomastigotes, chlorophytes)
also swim and photosynthesize
simultaneously. Molecular biological measurements of the DNA that codes for
components of the ribosomes (organelles that are universally distributed)
consistently find fungi to be extremely different from plants. Indeed, fungi
genetically resemble animals more than plants.

Modern biology, following the lead of the German biologist Ernst Haeckel and
the American biologists Herbert F. Copeland and Robert H. Whittaker, has now
thoroughly abandoned the two-kingdom plant-versus-animal dichotomy.
Haeckel proposed three kingdoms when he established “Protista” for
microorganisms. Copeland classified the microorganisms into the Monerans
(prokaryotes) and the Protoctista (which included fungi with the rest of the
eukaryotic microorganisms). His four-kingdom scheme (Monera, Protoctista,
Animalia, and Plantae) had the advantage of clearly separating microbes with
nuclei (Protoctista) from those without (Monera: the prokaryotes—that is, the
bacteria and archaea) and of distinguishing the two embryo-forming groups—
plants and animals—from the rest of life. Whittaker, on ecological grounds,
raised the fungi to kingdom status. The modified Whittaker five-kingdom
classification system is perhaps the most comprehensible and biologically based
way to unambiguously organize information about all groups of living beings.
American microbiologist Carl Woese has offered still another classification
scheme, in which all organisms are placed in either the Archaea (prokaryotes
that include some salt lovers, acid lovers, and methane producers), the Bacteria
(all other prokaryotes, including obligate anaerobic bacteria as well as
photosynthetic and chemoautotrophic bacteria), or the Eukarya (all eukaryote
forms of life). Woese’s scheme is based on molecular biological criteria that
focus on the RNA sequence of morphological factors to classify new or disputed
organisms. Although Woese’s three-domain system is very popular, a potential
problem with it is that RNA, one characteristic among thousands, does not
consistently correlate with many others.

Microbes (or microbiota) are simply all those organisms too small to be
visualized without some sort of microscopy. Bacteria, the smaller fungi, and the
smaller protists are undoubtedly microbes. Some scientists classify tiny animals,
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worms, and rotifers as microbes as well. Like weed, a plant not wanted in a
garden, microbe is often a more useful term than one with a precise scientific
meaning.
Sex

The world of microbes, in any case, is more vast, complex, diverse, and
widespread than the visible ordinary world of plants and animals. For example,
microbes have sexual lives that are different from those of the animal and plant
kingdoms. In all organisms composed of prokaryotic cells, DNA that is not
complexed with protein (“naked,” or chromonemal, DNA) transfers from a
source (such as a plasmid, a virus, a second cell, or even DNA molecules
suspended in a solution) to a live prokaryotic cell. The recipient cell at the end of
the sex act contains some quantity of its own DNA and integrates some from
the donor. All prokaryotes can reproduce in the absence of any sex act.

In eukaryotes the sexual act requires the opening of membranes and the fusion
of entire cells or at least of cell nuclei. The contribution from genes to the
recombinant offspring is approximately 50 percent from each parent. From two
to a dozen or so genders (in some species of paramecia) are present in any
given sexual group. Although any given sex act requires at least two individuals,
mating tends to be by pairs. Gender is understood to be those traits that
predispose any organism to enter the sex act with any other. In multigender
species only two genders or mating types enter a sex act at any one time. The
rule is that in multigender species a mating requires any gender other than
one’s own. Individual cells or multicellular organisms of complementary
genders, in principle, produce fertile offspring. The universal rule is that no
offspring result from matings of individuals of the same gender. In protists and
fungi, uniparental reproduction (i.e., reproduction of a single parent) can occur
in the absence of any sexual act, but two-parent sex may prevail seasonally or
under other given environmental conditions in many inclusive taxa (such as
families, classes, or phyla). Members of all species of the plant and animal
kingdoms develop from embryos that form from a sexual act between the
parents, and therefore two-parent (biparental) sex is the rule. Biparental
sexuality of plants and animals has likely preceded its loss in all cases where a
plant or animal species has reverted to uniparental reproduction, as in rotifers,
whiptail lizards, and hundreds of plants that reproduce by runners rather than

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by seed. This suggestion is based on the fact that, at the cell level, aspects of
meiosis (required for two-parent sex) continue to occur..

Ploidy, the concept of the number of complete sets of genes organized into
chromosomes, is inapplicable to prokaryotes. Ploidy in protists, depending on
species, varies so greatly and regularly that it is obvious that sexual cycles
evolved in this diverse group of eukaryotes. Fungal cell nuclei are haploid (one
set of chromosomes) or dikaryotic (two distinct nuclei from two different
parents, each with one set of chromosomes sharing the same cell). Plant cell
nuclei have two sets of chromosomes (diploid, in the sporophyte generation) or
one (haploid, in the gametophyte generation). Animal cell nuclei, except in the
gametes (sperm and egg), tend to have two sets of chromosomes (they are
diploid).
Viruses

Viral and plasmidnucleic acids pass from cell

to cell where the DNA or RNA perform their
replication and coding functions efficiently.
These “small replicons”—viruses and plasmids
—are essentially strands of nucleic acid with a
protein coat that depend entirely on the host
virus invading a cell cell for their continual existence. Pieces of the
Virus invading a cell and reproducing. genetic material, virus-sized, pass from one
cell into another cell of the same kind.
Traveling small replicons of DNA produce genetic and permanently heritable
changes in their new locations. Alternatively, part of the virus nucleic acid may
be permanently bound to the nuclear DNA of the cell in which it resides. Viruses
may be thought of as degenerate forms that are highly specialized in order to
live in specific host cells of free-living organisms. Only cells are capable of
performing the metabolic tasks that viruses and plasmids require. Viruses and
plasmids must use the genetic transcription apparatus of cells. Bacterial viruses,
or bacteriophages, may be extremely efficient in turning the luckless bacterium
from a “factory” for the production of more bacteria into a factory for making
more virus particles. It may take no more than 10 minutes for a bacterium
infected by a single virus to produce 100 new virus particles, bursting forth from
the victim bacterium by destroying it. Plasmids do not burst forth; rather, they
benignly incorporate their DNA into that of their host cell.

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Limits to life

Organisms generated by the same fundamental biochemistry survive, grow, and

reproduce in an extraordinarily wide range of conditions on Earth. For example,
an alga called Cyanidium caldarium, a eukaryotic and photosynthetic organism,
thrives in concentrated solutions of hot sulfuric acid and colours a damp
landscape turquoise after a wet volcanic explosion. A swimming relative,
Cyanophora paradoxa, survives in nearly these extremes. Certain less-colourful
bacteria and fungi can live in extremely acidic environments (pH 0–2.5), such as
that of Rio Tinto near Huelva in Spain. Bright blue-green cyanobacteria of many
kinds can grow vigorously in extremely alkaline environments (pH 10–13).

Temperature and desiccation

Most familiar organisms on Earth are of course sensitive to extreme temperature

in their surroundings. Mammals and birds have evolved internal regulation of
their temperatures. Humans cannot tolerate body temperatures below 30 °C (86
°F) or above 40 °C (104 °F). Cold-climate organisms have special insulating layers
of fat and fur. Other organisms adjust to seasonal temperature drops by
developing dormant propagules such as spores, eggs, or tuns, which are hardy
desiccation- and radiation-resistant forms produced by microscopic animals
called tardigrades, also known as “water bears.” Dormancy is often accompanied
by dehydration.

Most organisms are composed of an estimated 70–80 percent water. The

availability of body water is a biological imperative. Certain halophilic bacteria
live on water adsorbed on a single crystal of salt. Others such as the kangaroo
rat (a mammal) and Tribolium (the flour beetle) imbibe no water at all in the
liquid state. They rely entirely on metabolic water—that is, on water released
from chemical bonds through the metabolism of food. A variety of plants,
including Spanish moss, live without contact with groundwater. They extract
water directly from the air, although they do require relatively high humidity.
Desert plants and other plants in very dry environments, such as the two-leaved
Welwitschia of the Namib Desert, have evolved extensive root systems that
absorb subsurface water from a great volume of adjacent soil.

Life has been detected in the stratosphere and in the ocean’s major depths.
Mud-loving photosynthetic bacteria live in pools at Yellowstone National Park at

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temperatures above 90 °C (194 °F), whereas

some unidentified dark-dwelling marine
bacteria at marine hot vents have been
recorded to grow at a superhot temperature
of 113 °C (235 °F). (Because the water is under
pressure, it is not above the boiling point.)
cyanobacteria
Sulfate-reducing bacteria taken from the
Cyanobacteria in a hot spring at
Yellowstone National Park, Wyoming. ocean grow and reproduce at 104 °C (219 °F)
under high hydrostatic pressure. Many
organisms employ organic or inorganic antifreezes to lower the freezing point
of their internal liquids. Many kinds can live at several tens of degrees below 0
°C (32 °F). Some insects use dimethyl sulfoxide as an antifreeze. Other organisms
live in briny pools in which dissolved salts lower the freezing point. San Juan
Pond in Antarctica, for example, contains about one molecule of calcium
chloride for every two water molecules. Not until –45 °C (–49 °F) does the pond
freeze. A type of cryophilic (cold-loving) bacteria that lives there continues to
metabolize down to at least –23 °C (–9 °F). Biological activity does not cease at
the freezing point of water. In certain sea urchins, some microtubule proteins
form the tubules of mitosis best at –2 °C (28 °F), and some enzymes are actually
more active in ice than in water. Many bacteria are routinely frozen at –80 °C (–
112 °F). They are thawed with no decrease in activity. Freezing temperatures
alone cause no damage. Rather, frozen water removes tissue fluidity and leaves
dangerous salt concentrations in its wake. The combination of expansion and
contraction attendant to freezing and thawing harms membranes. Some
arthropods can be severely dehydrated and then revived simply by the addition
of water. Once dehydrated, these animals can be brought to any temperature
from close to absolute zero (–273 °C, or –460 °F) to above the boiling point of
water (100 °C, or 212 °F) without apparent damage. When encysted in response
to dehydration, these arthropods at first glance are indistinguishable from a
weathered grain of sand.

Bacteria and fungal spores have been discovered near the base of the
stratosphere by balloon searches. Organisms sought at much higher altitudes
(up to 30,000 metres [100,000 feet]) have been detected; they are few in number
and are all propagules. Birds have been observed to fly at maximum altitudes of
8,200 metres (27,000 feet), and on Mount Everest jumping spiders have been

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found at 6,700 metres (22,000 feet). At the opposite extreme, ciliates, pout fish,
crabs, and clams have been recovered from ocean depths where pressures are
hundreds of times those found at sea level. At these depths no light penetrates,
and the organisms, some of which are quite large with bioluminescent organs
that glow in the dark, feed on particles of organic matter raining down from the
upper reaches of the oceans. Others sustain themselves by their
chemoautotrophic bacterial associations.
Radiation and nutrient deprivation

The radiation environment of Earth has provoked evolutionary responses in

many types of organisms. Some bacteria are readily killed by the small amount
of solar ultraviolet light that filters through Earth’s atmosphere at wavelengths
near 300 nanometres. To the continuing annoyance of nuclear physicists, the
bacterium Deinococcus radiodurans thrives in the cooling pool of nuclear
reactors amid radioactivity levels lethal to mammals. Some life avoids radiation
by shielding: algae and some desert plants live under a superficial coating of soil
or rock that is more transparent to visible light than to ultraviolet light. Many
produce protective epithelial coatings. Most telling is the fact that some
microbes and animals have active methods of repairing damage produced by
radiation. Some of these repair mechanisms work in the dark; others require
visible light. Nucleic acids of all organisms absorb ultraviolet light very
effectively at a wavelength near 260 nanometres, which accounts for their
ultraviolet sensitivity. The upper limit to the amount of ionizing radiation (which
includes gamma rays, X-rays, and electrons) that an organism can receive
without being killed is approximately 1,000,000 roentgens. Such an
extraordinarily high dose can be withstood only by Deinococcus. Mammals are
killed by vastly lower doses, probably because so much more can go wrong in a
large and complex animal. For the whole body of a human being, a dose of
some 400 roentgens causes radiation sickness and death in half of those
exposed to this level. A thermonuclear weapon dropped on a populated area
may deliver, through direct radiation and fallout, doses of a few hundred
roentgens or more to people within a radius of some tens of kilometres of the
target. Much smaller doses produce a variety of diseases as well as deleterious
mutations in the hereditary material, the DNA of the chromosomes. The effect of
small doses of radiation is apparently cumulative. Until very recently no human
beings had lived in environments with large fluxes of ionizing radiation (see
radiation: Biologic effects of ionizing radiation).
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Sizes of organisms

The sizes of organisms on Earth vary

2 of 2
greatly and are not always easy to
estimate. On the large end, great stands
of poplar trees entirely connected by
common roots are really a single
organism. A variety of influences place an
upper limit to the size of organisms. One
is the strength of biological materials.
Sequoia redwood trees, some of which
exceed 90 metres (300 feet), are
General Grant tree apparently near the upper limit of height
General Grant tree, a giant sequoia for an organism. The Italian astronomer
(Sequoiadendron giganteum), among
the largest trees in total bulk.
Galileo calculated in 1638 that a tree
taller than roughly 90 metres would
buckle under its own weight when displaced slightly from the vertical (for
example, by a breeze). Because of the buoyancy of water, large animals such as
whales are not presented with such stability problems. Other size-related
difficulties arise. The volume of tissues to be nourished increases as the cube of
the characteristic length of the organism, but the surface of the gut, which
absorbs the ingested food, increases only as the square of the length for a fixed
morphology. As an organism’s length is increased, a point of diminishing returns
is ultimately reached where nutrition is irreversibly impeded in an animal.

New work on genome sequences, the total amount and quality of all of the
genes that make up a live being, permits more accurate assessment of the
material basis of the theoretically smallest and simplest extant free-living
organisms. The complete DNA sequences of a few extremely small free-living
organisms are now known—e.g., Mycoplasma genitalium with its 480 genes. All
the molecules necessary for metabolism must be present. The smallest free-
living cells include the pleuropneumonia-like organisms (PPLOs). Whereas an
amoeba has a mass of 5 × 10−7 gram (2 × 10−8 ounce), a PPLO, which cannot
be seen without a high-powered electron microscope, weighs 5 × 10−16 gram (2
× 10−15 ounce) and is only about 100 nanometres across. PPLOs grow very
slowly. Other, even smaller organisms that grow even more slowly would be
extremely difficult to detect. An organism the size of a PPLO that has room for
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only about a hundred enzymes depends entirely upon the animal tissue in which
it lives. A much smaller organism would have room for many fewer enzymes. Its
ability to accomplish the functions required for autopoiesis in living systems
would be severely compromised. Were there, however, an environment in which
all the necessary organic building blocks and such energy sources as ATP were
provided “free,” then there might be a functioning organism substantially
smaller than a PPLO. The inside of cells provides just such an environment,
which explains why infectious agents, such as prions, plasmids, and viruses, may
be substantially smaller than PPLOs. But it must be emphasized that viruses and
their kin are not, even in principle, autopoietic.
Metabolites and water

The range of organic molecules that organisms, especially microbes, can

metabolize is very wide and occasionally includes foods such as formaldehyde
or petroleum that seem unlikely from a human point of view. Pseudomonas
bacteria are capable of using almost any organic molecule as a source of carbon
and energy, provided only that the molecule is at least slightly soluble in water.
Microorganisms cannot metabolize plastics, not because of any fundamental
chemical prohibitions but probably because plastics have not been part of the
environment of microorganisms for very long. A lack of oxygen is thought of as
extremely deleterious to life, but this view is anthropocentric. Many bacteria are
facultative anaerobes that can take their oxygen or leave it. Many other bacteria
and protists are obligate anaerobes that are actually poisoned by oxygen.

Water, which is crucial for life, is the major molecule in all organisms. Unless a
massive mineral skeleton is present, the dry matter of most organisms is about
one-half carbon by weight. This reflects the fact that all organic molecules are
composed of carbon bound at least to hydrogen. Metabolism uses a wide
variety of other chemical elements. Amino acids are made of nitrogen and sulfur
in addition to carbon, hydrogen, and oxygen. Nucleic acids are made of
phosphorus in addition to hydrogen, nitrogen, oxygen, and carbon. Sodium,
potassium, and calcium are used to maintain electrolyte balance and to signal
cells. Silicon is used as a structural material in the diatom shell, the radiolarian
and heliozoan spicule, and the chrysophyte exoskeleton. Iron plays a
fundamental role in the transport of molecular oxygen as part of the
hemoglobin molecule. In some ascidians (sea squirts), however, vanadium
replaces iron. Ascidian blood also contains unusually large amounts of niobium,
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titanium, chromium, manganese, molybdenum, and tungsten. The vanadium

and niobium compounds in ascidian blood may be adaptations to low oxygen
levels. Some bacteria use selenium, tellurium, or even arsenic as electron
acceptors. Others produce the fully saturated gas hydrides of carbon, arsenic,
phosphorus, or silicon as a metabolic waste. Still others form compounds of
carbon with such halogens as chlorine or iodine. Not only the foregoing
elements but also copper, zinc, cobalt, and possibly gallium, boron, and
scandium perform particular functions in the enzymatic apparatus of particular
cells. These elements, both the uncommon ones and those as common as
phosphorus, are much more concentrated in living matter than in the
environment where the living matter resides. This concentration suggests that
such rare chemicals play unique functional roles that other, more abundant
elements cannot serve.
Sensory capabilities and awareness

Although any given organism is severely

limited in its range of behaviour patterns and
sensory capabilities, life as a whole is
remarkably sensitive to aspects of its local
social and physical environment. A bird raised
from the egg in the absence of other
members of its species migrates when the
Arctic tern season beckons, builds the proper nest, and
Arctic tern (Sterna paradisaea). It engages in elaborate courtship rituals. Those
breeds in the southerly reaches of the
birds that fail to perpetuate the behaviour
Arctic and winters in Antarctic
regions, thus making the longest pattern do not leave descendants. Such
annual migration of any bird. behavioural accuracy itself must have evolved.
Rats that pass through mazes easily
interbreed, as do rats that pass through with difficulty; eventually two
populations with inherited characteristics called “maze-smart” and “maze-dumb”
are produced. Fruit fly populations attracted to light can be separated from
those that avoid light. Classical genetic-crossing experiments reveal that the two
populations differ largely in a small number of genes for phototropism. Similar
genetic determinants of behaviour exist in humans. For example, possession of a
supernumerary Y-chromosome in males is strikingly correlated with aggressive
tendencies.

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Photosensitivity, audiosensitivity, thermosensitivity,

chemosensitivity, and magnetosensitivity

Humans use only a limited region of the

electromagnetic spectrum, the part called
visible light, which extends from 400 to 700
nanometres in wavelength. While plants,
algae, photosynthetic bacteria, and most
animals are sensitive to this same range of
wavelengths, many are sensitive to other
evening primrose
wavelengths as well. Many plants present
An evening primrose (Oenothera flower patterns visible only in the ultraviolet
biennis) seen (top) in visible light and
(bottom) in ultraviolet light; the latter range at wavelengths below 400 nanometres,
reveals nectar-guide patterns that are where pollinating insects are sensitive.
discernible to the moth pollinating
this flower but not to the human eye. Honeybees use polarized light—which the
unaided human eye is unable to detect—for
direction finding on partly cloudy days. The “pit” of such pit vipers as the
rattlesnake is an infrared (heat) receptor that serves as a direction finder. These
reptiles sense the thermal radiation emitted by mammals and birds, their warm-
blooded prey. Humans are entirely insensitive to this thermal radiation.

That some animals such as dogs are sensitive

to sounds that the human ear cannot detect is
obvious to those who use dog whistles. Bats
emit and detect sound waves at ultrahigh
frequencies, in the vicinity of 100,000 cycles
per second, about five times the highest
frequency to which the human ear is sensitive.
Bats have echolocated their prey by use of
these sounds for millions of years before
humans invented radar and sonar. The audio
receptors of many moths that are prey to bats
epauletted fruit bat respond only to the frequencies emitted by
Epauletted fruit bat (Epomophorus the bats. When the bat sounds are heard, the
wahlbergi).
moths take evasive action. Dolphins
communicate via a very wide frequency range.
They employ a “click” echolocator.

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Some species of animals enjoy highly specialized and exotic organs for the
detection or transmission of sound. Dolphins and whales use their blowholes
rather than their mouths to utter their sounds.

Smell and taste, or some form of detection of

specific chemical molecules, are universal. The
ultimate in olfactory specialization may be
male moths, whose feathery antennae are
underlain by splayed microtubules, each of
which is covered by a membrane at the distal
silkworm moth end. They smell essentially nothing except the
Silkworm moths (Bombyx mori) epoxide compound called disparlure, the
mating on cocoons.
chemical sex attractant discharged by the
female. Only 40 molecules per second need
impact on the antennae to produce a marked response. One female silkworm
moth need release only 10–8 gram (4 × 10–10 ounce) of sex attractant per
second in order to attract every male silkworm moth within a few kilometres.

Magnetotactic bacteria sense Earth’s magnetic field. North Pole-seeking bacteria

swim toward the sediment-water interface as they follow the magnetic lines of
force. South Pole-seeking flagellated magnetotactic bacteria do the same in the
Southern Hemisphere. Since those studied are microaerophiles—i.e., they
require oxygen in lower than ambient concentrations—pole seekers tend to
arrive at oxygen-depleted sediment adequate for their continued growth and
reproduction. Ultrastructural studies reveal magnetosomes, tiny single-domain
crystals of magnetite, an iron oxide mineral sensitive to magnetic fields, or
greigite, an iron sulfide mineral, in their cells. The magnetosomes are aligned
along the axis of the cell and serve to orient the sensitive bacteria. All the
different kinds of magnetotactic bacteria bear magnetosomes in their cells.
Whether magnetotaxis is causal in the orientation of homing pigeons, dancing
bees on cloudy days, or other instinctively orienting animals is under
investigation.

Besides the familiar senses of sight, hearing, smell, taste, and touch, organisms
have a wide variety of other senses (see above Sensory capabilities and
awareness). People have inertial orientation systems and accelerometers in the
cochlear canal of the ear. The water scorpion (Nepa) has a fathometer sensitive

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to hydrostatic pressure gradients. Many plants have chemically amplified gravity

sensors made of modified chloroplasts. Some green algae use barium sulfate
and calcium ion detection systems to sense gravity. Fireflies and squids
communicate with their own kind by producing changing patterns of light on
their bodies. The nocturnal African freshwater fish Gymnarchus niloticus
operates a dipole electrostatic field generator and a sensor to detect the
amplitude and frequency of disturbances in turbulent waters.
Sensing with technology

From the foregoing sample of sensibilities, it is clear that a vast repertoire of

sensors in living beings confers upon their possessors an awareness of the
environment that differs from humanity’s. Humans, however, have an enhanced
ability to extend their sensory and intellectual capabilities through the use of
instrumentation far beyond those with which they are born.

The sensory system of Earth is expanded by the capabilities of human machines.

From those that detect ionizing radiation, wind velocities, the taste of wine, the
concentration of salt in solution, a few photons of light in a dark corridor, or the
blood temperature of an infant to those that record microearthquakes, a lying
smile, or the heat of a furnace, the sensory systems of the biosphere—
nonhuman, human, and human-mediated—have augmented over time. Indeed,
just as seeing-eye dogs transmit visual information to their blind owners, the
sensory system of life extends far beyond any given species of animals and its
machines to the entire sensitive biota in this pulsating biosphere. Sensitivity to
sound, chemicals, heat, light, mechanical movement, magnetism, and charged
particles has been tallied by many a hardworking scientist. Whether entire
categories of sensory information are missing from that list is not entirely clear.
Great sensitivity to the environment abounds even in those smallest life-forms,
the bacteria. Life has been sensing and responding to its environment since its
inception more than 3.7 billion to 3.5 billion years ago. Moreover, it is not clear
at what point in evolutionary history, or where precisely among organisms,
consciousness comes in. Humans are conscious and self-conscious. But are
protists that choose certain shapes and sizes of glass beads over others
conscious of their decision making? Charles Darwin recognized selection among
various male suitors by females as instrumental in the evolution of sexual
species, including birds and insects. The extent to which consciousness and
choice making are important in evolution remains a matter for debate.
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Evolution and the history of life on Earth

Heritability

The evidence is overwhelming that all life on

Earth has evolved from common ancestors in
an unbroken chain since its origin. Darwin’s
principle of evolution is summarized by the
following facts. All life tends to increase: more
organisms are conceived, born, hatched,
germinated from seed, sprouted from spores,
or produced by cell division (or other means)
than can possibly survive. Each organism so
produced varies, however little, in some
measurable way from its relatives. In any given
environment at any given time, those variants
best suited to that environment will tend to
leave more offspring than the others.
Offspring resemble their ancestors. Variant
organisms will leave offspring like themselves.
major evolutionary events Therefore, organisms will diverge from their
The geologic time scale from 650 ancestors with time. The term natural selection
million years ago to the present,
showing major evolutionary events.
is shorthand for saying that all organisms do
not survive to leave offspring with the same
probability. Those alive today have been selected relative to similar ones that
never survived or procreated. All organisms on Earth today are equally evolved
since all share the same ancient original ancestors who faced myriad threats to
their survival. All have persisted since roughly 3.7 billion to 3.5 billion years ago
during the Archean Eon (4 billion to 2.5 billion years ago), products of the great
evolutionary process with its identical molecular biological bases. Because the
environment of Earth is so varied, the particular details of any organism’s
evolutionary history differ from those of another species in spite of chemical
similarities.

Convergence

Everywhere the environment of Earth is heterogeneous. Mountains, oceans, and

deserts suffer extremes of temperature, humidity, and water availability. All

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ecosystems contain diverse

microenvironments: oxygen-depleted oceanic
oozes, sulfide- or ammonia-rich soils, mineral
outcrops with a high radioactivity content, or
boiling organic-rich springs, for example.
parallel evolution of marsupial and
placental mammals Besides these physical factors, the
Parallel evolution of marsupial environment of any organism involves the
mammals in Australia and placental other organisms in its surroundings. For each
mammals on other continents.
environmental condition, there is a
corresponding ecological niche. The variety of
ecological niches populated on Earth is quite remarkable. Even wet cracks in
granite are replete with “rock eating” bacteria. Ecological niches in the history of
life have been filled independently several times. For example, quite analogous
to the ordinary placental mammalian wolf was the marsupial wolf, the thylacine
(extinct since 1936) that lived in Australia; the two predatory mammals have
striking similarities in physical appearance and behaviour. The same streamlined
shape for high-speed marine motion evolved independently at least four times:
in Stenopterygius and other Mesozoic reptiles; in tuna, which are fish; and in
dolphins and seals, which are mammals. Convergent evolution in hydrodynamic
form arises from the fact that only a narrow range of solutions to the problem of
high-speed marine motion by large animals exists. The eye, a light receptor that
makes an image, has evolved independently more than two dozen times not
only in animals on Earth but in protists such as the dinomastigote
Erythropsodinium. Apparently eyelike structures best solve the problem of visual
recording. Where physics or chemistry establishes one most efficient solution to
a given ecological problem, evolution in distinct lineages will often tend toward
similar, nearly identical solutions. This phenomenon is known as convergent
evolution.
Spontaneous generation

Life ultimately is a material process that arose from a nonliving material system
spontaneously—and at least once in the remote past. How life originated is
discussed below. Yet no evidence for spontaneous generation now can be cited.
Spontaneous generation, also called abiogenesis, the hypothetical process by
which living organisms develop from nonliving matter, must be rejected.
According to this theory, pieces of cheese and bread wrapped in rags and left in
a dark corner were thought to produce mice, because after several weeks mice
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appeared in the rags. Many believed in spontaneous generation because it

explained such occurrences as maggots swarming on decaying meat.

By the 18th century it had become obvious that plants and animals could not be
produced by nonliving material. The origin of microorganisms such as yeast and
bacteria, however, was not fully determined until French chemist Louis Pasteur
proved in the 19th century that microorganisms reproduce, that all organisms
come from preexisting organisms, and that all cells come from preexisting cells.
Then what evidence is there for the earliest life on Earth?
Geologic record

Past time on Earth, as inferred from the rock

record, is divided into four immense periods
of time called eons. These are the Hadean (4.6
billion to 4 billion years ago), the Archean (4
billion to 2.5 billion years ago), the
Proterozoic (2.5 billion to 541 million years
ago), and the Phanerozoic (541 million years
geologic time
ago to the present). For the Hadean Eon, the
The stratigraphic chart of geologic
only record comes from meteorites and lunar
time.
rocks. No rocks of Hadean age survive on
Earth. In the figure, eons are divided into eras, periods, and epochs. Such entries
in the geologic time scale are often called “geologic time intervals.”

Among the oldest known fossils are those

found in the Fig Tree Chert from the Transvaal,
dated over three billion years ago. These
organisms have been identified as bacteria,
including oxygenic photosynthetic bacteria
(cyanobacteria)—i.e., prokaryotes rather than
stromatolites eukaryotes. Even prokaryotes, however, are
Living stromatolites in Hamelin Pool exceedingly complicated organisms that grow
of Shark Bay, Western Australia.
and reproduce efficiently. Structures of
communities of microorganisms, layered rocks
called stromatolites, are found from more than three billion years ago. Since
Earth is about 4.6 billion years old, these finds suggest that the origin of life
must have occurred within a few hundred million years of that time.

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Chemical analyses on organic matter extracted from the oldest sediments show
what sorts of organic molecules are preserved in the rock record. Porphyrins
have been identified in the oldest sediments, as have the isoprenoid derivatives
pristane and phytane, breakdown products of cell lipids. Indications that these
organic molecules dating from 3.1 billion to 2 billion years ago are of biological
origin include the fact that their long-chain hydrocarbons show a preference for
a straight-chain geometry. Chemical and physical processes alone tend to
produce a much larger proportion of branched-chain and cyclic hydrocarbon
molecular geometries than those found in ancient sediments. Nonbiological
processes tend to form equal amounts of long-chain carbon compounds with
odd and even numbers of carbon atoms. But products of undoubted biological
origin, including the oldest sediments, show a distinct preference for odd
numbers of carbon atoms per molecule. Another chemical sign of life is an
enrichment in the carbon isotope C12, which is difficult to account for by
nonbiological processes and which has been documented in some of the oldest
sediments. This evidence suggests that bacterial photosynthesis or
methanogenesis, processes that concentrate C12 preferentially to C13, were
present in the early Archean Eon.

The Proterozoic Eon, once thought to be

devoid of fossil evidence for life, is now known
to be populated by overwhelming numbers of
various kinds of bacteria and protist fossils—
including acritarchs (spherical, robust
unidentified fossils) and the entire range of
Ediacaran fauna. The Ediacarans—large,
enigmatic, and in some cases animal-like
extinct life-forms—are probably related to
extant protists. Almost 100 species are known
from some 30 locations worldwide, primarily
sandstone formations. Most Ediacarans,
presumed to have languished in sandy seaside
locales, probably depended on their internal
Spriggina fossil
microbial symbionts (photo- or
Spriggina fossil from the Ediacaran
Period, found in the Ediacara Hills of chemoautotrophs) for nourishment. No
Australia. evidence that they were animals exists. In

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addition to the Ediacarans, acritarchs, and other abundant microfossils, clear

evidence for pre-Phanerozoic, or Precambrian, life includes the massive banded-
iron formations (BIFs). Most BIFs date from 2.5 billion to 1.8 billion years ago.
They are taken as indirect evidence for oxygen-producing, metal-depositing
microscopic Proterozoic life. Investigations that use the electron microprobe (an
instrument for visualizing structure and chemical composition simultaneously)
and other micropaleontological techniques unfamiliar to classical geology have
been employed to put together a much more complete picture of pre-
Phanerozoic life.

The earliest fossils are all of aquatic forms. Not until about two billion years ago
are cyanobacterial filaments seen that colonized wet soil. By the dawn of the
Phanerozoic Eon, life had insinuated itself between the Sun and Earth, both on
land and in the waters of the world. For example, the major groups of marine
animals such as mollusks and arthropods appeared for the first time about 541
million years ago at the base of the Cambrian Period of the Phanerozoic Eon.
Plants and fungi appeared together in the exceptionally well-preserved Rhynie
Chert of Scotland, dated about 408 million–360 million years ago in the
Devonian Period. Solar energy was diverted to life’s own uses. The biota
contrived more and more ways of exploiting more and more environments.
Many lineages became extinct. Others persisted and changed. The biosphere’s
height and depth increased, as did, by implication, the density of living matter.
The proliferation and extinctions of a growing array of life-forms left indelible
marks in the sedimentary rocks of the biosphere (see evolution: The concept of
natural selection).
The origin of life
Hypotheses of origins

Perhaps the most fundamental and at the same time the least understood
biological problem is the origin of life. It is central to many scientific and
philosophical problems and to any consideration of extraterrestrial life. Most of
the hypotheses of the origin of life will fall into one of four categories:

1. The origin of life is a result of a supernatural event—that is, one irretrievably beyond
the descriptive powers of physics, chemistry, and other science.
2. Life, particularly simple forms, spontaneously and readily arises from nonliving matter
in short periods of time, today as in the past.

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3. Life is coeternal with matter and has no beginning; life arrived on Earth at the time of
Earth’s origin or shortly thereafter.
4. Life arose on the early Earth by a series of progressive chemical reactions. Such
reactions may have been likely or may have required one or more highly improbable
chemical events.
Hypothesis 1, the traditional contention of
theology and some philosophy, is in its most
general form not inconsistent with
contemporary scientific knowledge, although
Michelangelo: The Creation of scientific knowledge is inconsistent with a
Adam literal interpretation of the biblical accounts
The Creation of Adam, detail of the given in chapters 1 and 2 of Genesis and in
ceiling fresco by Michelangelo, 1508–
other religious writings. Hypothesis 2 (not of
12; in the Sistine Chapel, Vatican City.
course inconsistent with 1) was the prevailing
opinion for centuries. A typical 17th-century view follows:

[May one] doubt whether, in cheese and timber, worms are generated, or, if beetles and
wasps, in cow’s dung, or if butterflies, locusts, shellfish, snails, eels, and suchlike be
procreated of putrefied matter, which is apt to receive the form of that creature to which it
is by the formative power disposed. To question this is to question reason, sense, and
experience. If he doubts of this, let him go to Egypt, and there he will find the fields
swarming with mice begot of the mud of the Nylus [Nile], to the great calamity of the
inhabitants.

(Alexander Ross, Arcana Microcosmi, 1652.)

It was not until the Renaissance, with its burgeoning interest in anatomy, that
such spontaneous generation of animals from putrefying matter was deemed
impossible. During the mid-17th century the British physiologist William Harvey,
in the course of his studies on the reproduction and development of the king’s
deer, discovered that every animal comes from an egg. An Italian biologist,
Francesco Redi, established in the latter part of the 17th century that the
maggots in meat came from flies’ eggs, deposited on the meat. In the 18th
century an Italian priest, Lazzaro Spallanzani, showed that fertilization of eggs by
sperm was necessary for the reproduction of mammals. Yet the idea of
spontaneous generation died hard. Even though it was clear that large animals
developed from fertile eggs, there was still hope that smaller beings,
microorganisms, spontaneously generated from debris. Many felt it was obvious

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that the ubiquitous microscopic creatures generated continually from inorganic

matter.

Maggots were prevented from developing on meat by covering it with a

flyproof screen. Yet grape juice could not be kept from fermenting by putting
over it any netting whatever. Spontaneous generation was the subject of a great
controversy between the famous French bacteriologists Louis Pasteur and Félix-
Archimède Pouchet in the 1850s. Pasteur triumphantly showed that even the
most minute creatures came from “germs” that floated downward in the air, but
that they could be impeded from access to foodstuffs by suitable filtration.
Pouchet argued, defensibly, that life must somehow arise from nonliving matter;
if not, how had life come about in the first place?

Pasteur’s experimental results were definitive: life does not spontaneously

appear from nonliving matter. American historian James Strick reviewed the
controversies of the late 19th century between evolutionists who supported the
idea of “life from non-life” and their responses to Pasteur’s religious view that
only the Deity can make life. The microbiological certainty that life always comes
from preexisting life in the form of cells inhibited many post-Pasteur scientists
from discussions of the origin of life at all. Many were, and still are, reluctant to
offend religious sentiment by probing this provocative subject. But the
legitimate issues of life’s origin and its relation to religious and scientific thought
raised by Strick and other authors, such as the Australian Reg Morrison, persist
today and will continue to engender debate.

Toward the end of the 19th century, hypothesis 3 gained currency. Swedish
chemist Svante A. Arrhenius suggested that life on Earth arose from
“panspermia,” microscopic spores that wafted through space from planet to
planet or solar system to solar system by radiation pressure. This idea, of course,
avoids rather than solves the problem of the origin of life. It seems extremely
unlikely that any live organism could be transported to Earth over interplanetary
or, worse yet, interstellar distances without being killed by the combined effects
of cold, desiccation in a vacuum, and radiation.

Although English naturalist Charles Darwin did not commit himself on the origin
of life, others subscribed to hypothesis 4 more resolutely. The famous British
biologist T.H. Huxley in his book Protoplasm: The Physical Basis of Life (1869) and
the British physicist John Tyndall in his “Belfast Address” of 1874 both asserted
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that life could be generated from inorganic chemicals. However, they had
extremely vague ideas about how this might be accomplished. The very phrase
“organic molecule” implied, especially then, a class of chemicals uniquely of
biological origin. Despite the fact that urea and other organic (carbon-
hydrogen) molecules had been routinely produced from inorganic chemicals
since 1828, the term organic meant “from life” to many scientists and still does.
In the following discussion the word organic implies no necessary biological
origin. The origin-of-life problem largely reduces to determination of an organic,
nonbiological source of certain processes such as the identity maintained by
metabolism, growth, and reproduction (i.e., autopoiesis).

Darwin’s attitude was: “It is mere rubbish thinking at present of the origin of life;
one might as well think of the origin of matter.” The two problems are in fact
curiously connected. Indeed, modern astrophysicists do think about the origin
of matter. The evidence is convincing that thermonuclear reactions, either in
stellar interiors or in supernova explosions, generate all the chemical elements
of the periodic table more massive than hydrogen and helium. Supernova
explosions and stellar winds then distribute the elements into the interstellar
medium, from which subsequent generations of stars and planets form. These
thermonuclear processes are frequent and well-documented. Some
thermonuclear reactions are more probable than others. These facts lead to the
idea that a certain cosmic distribution of the major elements occurs throughout
the universe. Some atoms of biological interest, their relative numerical
abundances in the universe as a whole, on Earth, and in living organisms are
listed in the table. Even though elemental composition varies from star to star,
from place to place on Earth, and from organism to organism, these
comparisons are instructive: the composition of life is intermediate between the
average composition of the universe and the average composition of Earth.
Ninety-nine percent of the mass both of the universe and of life is made of six
atoms: hydrogen (H), helium (He), carbon (C), nitrogen (N), oxygen (O), and
neon (Ne). Might not life on Earth have arisen when Earth’s chemical
composition was closer to the average cosmic composition and before
subsequent events changed Earth’s gross chemical composition?

Relative abundances of the elements

(percent)
atom universe life (terrestrial vegetation) Earth (crust)

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hydrogen 87 16 3
helium 12 0* 0
carbon 0.03 21 0.1
nitrogen 0.008 3 0.0001
oxygen 0.06 59 49
neon 0.02 0 0
sodium 0.0001 0.01 0.7
magnesium 0.0003 0.04 8
aluminum 0.0002 0.001 2
silicon 0.003 0.1 14
sulfur 0.002 0.02 0.7
phosphorus 0.00003 0.03 0.07
potassium 0.000007 0.1 0.1
argon 0.0004 0 0
calcium 0.0001 0.1 2
iron 0.002 0.005 18

*0 percent here stands for any quantity less than 10–6 percent.
The Jovian planets (Jupiter, Saturn, Uranus,
and Neptune) are much closer to cosmic
composition than is Earth. They are largely
gaseous, with atmospheres composed
principally of hydrogen and helium. Methane,
ammonia, neon, and water have been
detected in smaller quantities. This
circumstance very strongly suggests that the
massive Jovian planets formed from material
of typical cosmic composition. Because they
photo of Jupiter taken by Voyager
1 are so far from the Sun, their upper
Photograph of Jupiter taken by atmospheres are very cold. Atoms in the
Voyager 1 on February 1, 1979, at a upper atmospheres of the massive, cold
range of 32.7 million km (20.3 million
miles). Prominent are the planet's
Jovian planets cannot now escape from their
pastel-shaded cloud bands and Great gravitational fields, and escape was probably
Red Spot (lower centre).
difficult even during planetary formation.

Earth and the other planets of the inner solar system, however, are much less
massive, and most have hotter upper atmospheres. Hydrogen and helium
escape from Earth today; it may well have been possible for much heavier gases

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to have escaped during Earth’s formation. Very early in Earth’s history, there was
a much larger abundance of hydrogen, which has subsequently been lost to
space. Most likely the atoms carbon, nitrogen, and oxygen were present on the
early Earth, not in the forms of CO2 (carbon dioxide), N2, and O2 as they are
today but rather as their fully saturated hydrides: methane, ammonia, and water.
The presence of large quantities of reduced (hydrogen-rich) minerals, such as
uraninite and pyrite, that were exposed to the ancient atmosphere in sediments
formed over two billion years ago implies that atmospheric conditions then
were considerably less oxidizing than they are today.

In the 1920s British geneticist J.B.S. Haldane and Russian biochemist Aleksandr
Oparin recognized that the nonbiological production of organic molecules in
the present oxygen-rich atmosphere of Earth is highly unlikely but that, if Earth
once had more hydrogen-rich conditions, the abiogenic production of organic
molecules would have been much more likely. If large quantities of organic
matter were somehow synthesized on early Earth, they would not necessarily
have left much of a trace today. In the present atmosphere—with 21 percent of
oxygen produced by cyanobacterial, algal, and plant photosynthesis—organic
molecules would tend, over geological time, to be broken down and oxidized to
carbon dioxide, nitrogen, and water. As Darwin recognized, the earliest
organisms would have tended to consume any organic matter spontaneously
produced prior to the origin of life.

The first experimental simulation of early Earth conditions was carried out in
1953 by a graduate student, Stanley L. Miller, under the guidance of his
professor at the University of Chicago, chemist Harold C. Urey. A mixture of
methane, ammonia, water vapour, and hydrogen was circulated through a liquid
solution and continuously sparked by a corona discharge mounted higher in the
apparatus. The discharge was thought to represent lightning flashes. After
several days of exposure to sparking, the solution changed colour. Several
amino and hydroxy acids, familiar chemicals in contemporary Earth life, were
produced by this simple procedure. The experiment is simple enough that the
amino acids can readily be detected by paper chromatography by high school
students. Ultraviolet light or heat was substituted as an energy source in
subsequent experiments. The initial abundances of gases were altered. In many
other experiments like this, amino acids were formed in large quantities. On the
early Earth much more energy was available in ultraviolet light than from
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lightning discharges. At long ultraviolet wavelengths, methane, ammonia, water,

and hydrogen are all transparent, and much of the solar ultraviolet energy lies in
this region of the spectrum. The gas hydrogen sulfide was suggested to be a
likely compound relevant to ultraviolet absorption in Earth’s early atmosphere.
Amino acids were also produced by long-wavelength ultraviolet irradiation of a
mixture of methane, ammonia, water, and hydrogen sulfide. At least some of
these amino acid syntheses involved hydrogen cyanide and aldehydes (e.g.,
formaldehyde) as gaseous intermediates formed from the initial gases. That
amino acids, particularly biologically abundant amino acids, are made readily
under simulated early Earth conditions is quite remarkable. If oxygen is
permitted in these kinds of experiments, no amino acids are formed. This has led
to a consensus that hydrogen-rich (or at least oxygen-poor) conditions were
necessary for natural organic syntheses prior to the appearance of life.

Under alkaline conditions, and in the presence of inorganic catalysts,

formaldehyde spontaneously reacts to form a variety of sugars. The five-carbon
sugars fundamental to the formation of nucleic acids, as well as six-carbon
sugars such as glucose and fructose, are easily produced. These are common
metabolites and structural building blocks in life today. Furthermore, the
nucleotide bases and even the biological pigments called porphyrins have been
produced in the laboratory under simulated early Earth conditions. Both the
details of the experimental synthetic pathways and the question of stability of
the small organic molecules produced are vigorously debated. Nevertheless,
most, if not all, of the essential building blocks of proteins (amino acids),
carbohydrates (sugars), and nucleic acids (nucleotide bases)—that is, the
monomers—can be readily produced under conditions thought to have
prevailed on Earth in the Archean Eon. The search for the first steps in the origin
of life has been transformed from a religious/philosophical exercise to an
experimental science.
Production of polymers

The formation of polymers, long-chain molecules made of repeating units of

monomers (the essential building blocks mentioned above), is a far more
difficult experimental problem than the formation of monomers. Polymerization
reactions tend to be dehydrations. A molecule of water is lost in the formation
of a peptide from two amino acids or of a disaccharide sugar from two
monomers. Dehydrating agents are used to initiate polymerization. The
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polymerization of amino acids to form long proteinlike molecules (“proteinoids”)

was accomplished through dry heating by American biochemist Sidney Fox and
his colleagues. The polyamino acids that he formed are not random molecules
unrelated to life. They have distinct catalytic activities. Long polymers of amino
acids were also produced from hydrogen cyanide and anhydrous liquid
ammonia by American chemist Clifford Matthews in simulations of the early
upper atmosphere. Some evidence exists that ultraviolet irradiation induces
combinations of nucleotide bases and sugars in the presence of phosphates or
cyanides. Some condensing agents such as cyanamide are efficiently made
under simulated primitive conditions. Despite the breakdown by water of
molecular intermediates, condensing agents may quite effectively induce
polymerization, and polymers of amino acids, sugars, and nucleotides have all
been made this way.

That adsorption of relevant small carbon compounds on clays or other minerals

may have concentrated these intermediates was suggested by the British
scientist John Desmond Bernal. Concentration of some kind is required to offset
the tendency for water to break down polymers of biological significance.
Phosphorus, which with deoxyribose sugar forms the “backbone” of DNA and is
integrally involved in cell energy transformation and membrane formation, is
preferentially incorporated into prebiological organic molecules. It is hard to
explain how such a preference could have happened without the concentration
of organic molecules.

The early ocean and lakes themselves may have been a dilute solution of
organic molecules. If all the surface carbon on Earth were present as organic
molecules, or if many known ultraviolet synthetic reactions that produce organic
molecules were permitted to continue for a billion years with their products
dissolved in the oceans, a 1 percent solution of organic molecules would result.
Haldane suggested that the origin of life occurred in a “hot dilute soup.”
Concentration through either evaporation or freezing of pools, adsorption on
clay interfaces, or the generation of colloidal enclosures called coacervates may
have served to bring the organic molecules in question in contact with each
other.

The essential building blocks for life (the monomers) were probably produced in
relatively abundant concentrations, given conditions on the early Earth.

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Although relevant, this is more akin to the origin of food than to the origin of
life. If life is defined as a self-maintaining, self-producing, and mutable
molecular system that derives energy and supplies from the environment, then
food is certainly required for life. Polynucleotides (polymers of RNA and DNA)
can be produced in laboratory experiments from nucleotide phosphates in the
presence of enzymes of biological origin (polymerases) and a preexisting
“primer” nucleic acid molecule. If the primer is absent, polynucleotides are still
formed, but they lack specific genetic information. Once such a polynucleotide
forms, it can act as a primer for subsequent syntheses.

Even if such a molecular population could replicate polynucleotides, it would not

be considered alive. The polynucleotides tend to hydrolyze (break down) in
water. In the early 1980s American biochemist Thomas Cech and Canadian
American molecular biologist Sidney Altman discovered that certain RNA
molecules have catalytic properties. They catalyze their own splicing, which
suggests an early role for RNA in life or even in life’s origins. Only the
partnership of the two kinds of molecules (proteins and nucleic acids)
segregated from the rest of the world by an oily membrane makes the growth
process of life on Earth possible. The molecular apparatus ancillary to the
operation of the genetic code—the rules that determine the linear order of
amino acids in proteins from nucleotide base pairs in nucleic acids (i.e., the
activating enzymes, transfer RNAs, messenger RNAs, ribosomes, and so on)—
may be the product of a long evolutionary history among natural,
thermodynamically favoured, gradient-reducing complex systems. These rules
are produced according to instructions contained within the code. American
biophysicist Harold J. Morowitz argued cogently that the origin of the genetic
system, the code with its elaborate molecular apparatus, occurred inside cells
only after the origin of life as a cyclic metabolic system. American theoretical
biologist Jeffrey Wicken pointed out that replicating molecules, if they appeared
first, would have had no impetus to develop a complex cellular package or
associated protein machinery and that life thus probably arose as a metabolic
system that was stabilized by the genetic code, which allowed life’s second law-
favoured process to continue ad infinitum.

Many separate and rather diverse instances of the origin of living cells may have
occurred in the Archean Earth, but obviously only one prevailed. Interactions
eventually eliminated all but our lineage. From the common composition,
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metabolism, chemical behaviour, and other properties of life, it seems clear that
every organism on Earth today is a member of the same lineage.
The earliest living systems

Most organic molecules made by living systems inside cells display the same
optical activity: when exposed to a beam of plane-polarized light, they rotate
the plane of the beam. Amino acids rotate light to the left, whereas sugars,
called dextrorotatory, rotate it to the right. Organic molecules produced
artificially lack optical activity because both “left-handed” and “right-handed”
molecules are present in equal quantity. Molecules of the same optical activity
can be assembled in complementary ways like the stacking of right-handed
gloves. The same monomers can be used to produce longer chain molecules
that are three-dimensional mirror images of each other; mixtures of monomers
of different handedness cannot. Cumulative symmetry is responsible for optical
activity. At the time of the origin of life, organic molecules, corresponding both
to left- and right-handed forms, were no doubt formed as they are in laboratory
simulation experiments today: both types were produced. But the first living
systems must have employed one type of component, for the same reason that
carpenters cannot use random mixtures of screws with left- and right-handed
threads in the same project with the same tools. Whether left- or right-handed
activity was adopted was probably a matter of chance, but, once a particular
asymmetry was established, it maintained itself. Optical activity accordingly is
likely to be a feature of life on any planet. The chances may be equal of finding a
given organic molecule or its mirror image in extraterrestrial life-forms if, as
Morowitz suspects, the incorporation of nitrogen into the first living system
involved glutamine, the simplest of the required amino acid precursors with
optical activity.

The first living cells probably resided in a molecular Garden of Eden, where the
prebiological origin of food had guaranteed monomers that were available. The
cells, the first single-celled organisms, would have increased rapidly. But such an
increase was eventually limited by the supply of molecular building blocks.
Those organisms with an ability to synthesize scarce monomers, say A, from
more abundant ones, say B, would have persisted. The secondary source of
supply, B, would in time also become depleted. Those organisms that could
produce B from a third monomer, C, would have preferentially persisted. The

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American biochemist Norman H. Horowitz has proposed that the multienzyme

catalyzed reaction chains of contemporary cells originally evolved in this way.

Carl SaganLynn MargulisDorion Sagan

CITATION INFORMATION
ARTICLE TITLE: life
WEBSITE NAME: Encyclopaedia Britannica
PUBLISHER: Encyclopaedia Britannica, Inc.
DATE PUBLISHED: 14 June 2025
URL: https://www.britannica.comhttps://www.britannica.com/science/life
ACCESS DATE: August 11, 2025

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