THE ORIGIN AND EARLY EVOLUTION OF LIFE
ON EARTH
What is Evolution?
Evolution mainly deals with the origin of life on earth. The conditions and the forms of life on earth
were entirely different from what we see today. Everything evolved from one form to another for a
better chance of survival.
Origin of earth and its atmosphere:
Earth is believed to have formed about 5 billion years ago. In the first 500 million years
a dense atmosphere emerged from the vapor and gases that were expelled during
degassing of the planet's interior. These gases may have consisted of hydrogen (H2),
water vapor, methane (CH4), and carbon oxides. Prior to 3.5 billion years ago the
atmosphere probably consisted of carbon dioxide (CO2), carbon monoxide (CO), water
(H2O), nitrogen (N2), and hydrogen. The hydrosphere was formed 4 billion years ago
from the condensation of water vapor, resulting in oceans of water in which
sedimentation occurred.
PERSPECTIVES AND SUMMARY
Although during the first decades of the twentieth century Darwin's explanation
of the causes of evolution had fallen out of favor among biologists, his ideas
provided the essential framework for the development of a large body of thought
devoted during that period to the question of the origins of life.
Of all the hypotheses that were suggested, few were as fruitful as those of Oparin
, who proposed a long period of chemical abiotic synthesis of organic compounds
as a necessary precondition for the appearance of the first life forms. The first
forms of life would then have been anaerobic heterotrophic microorganisms.
Following the model of Oparin's reducing primordial terrestrial atmosphere
developed by Urey, Miller showed that a number of protein amino acids and a
diverse assortment of other small organic molecules of biochemical significance
could be made in the laboratory under environmental conditions thought to be
representative of the Hadean or early Archean Earth. Since then, a wide variety of
organic compounds of biochemical significance have been experimentally formed
from simple molecules such as water, methane, ammonia, and HeN.
To reconstruct the possible characteristics of the first living systems, we must rely
on studies of prebiotic chemistry as well as on information in contemporary
biological systems. These include studies of macromolecular sequence data from
proteins and nucleic acids of different organisms, and from comparisons of
metabolic pathways and their phylogenetic distribution.
Oberbeck & Fogleman (1 989) have calculated the maximum time required for life
to originate and evolve to the level of complexity represented by the oldest
microfossils; in short, most available calculations and evidence seem to indicate
that the emergence of life occurred approximately 3.85 Gyr ago.
Scientific hypothesis-
Chemical evolution of life:
Chemical evolution describes chemical changes on the primitive Earth that gave rise to
the first forms of life. The first living things on Earth were prokaryotes with a type
of cell similar to present-day bacteria. Prokaryote fossils have been found in 3.4-million-
year-old rock in the southern part of Africa, and in even older rocks in Australia,
including some that appear to be photosynthetic. All forms of life are theorized to have
evolved from the original prokaryotes, probably 3.5-4.0 billion years ago.
a) Formation of Simple Compounds: The first stage of chemical evolution, molecules in
the primitive environment formed simple organic substances, such as amino acids,
methane, ammonia, water etc.
b) Formation of Organic Compounds: In the second stage of chemical evolution, the
simple organic molecules (such as amino acids) that formed and accumulated joined
together into larger structures (such as proteins). The units linked to each other by the
process of dehydration synthesis to form polymers.
c) Formation of complex organic compound: simple organic molecules combined to
form complex organic compounds like polysaccharides, fats, protein, nucleotides,
nucleosides, nucliec acid etc.
THE ORIGIN OF THE FIRST CELLS
Needless to say, there is no detailed scheme of the mechanisms that led
gradually-but not necessarily slowly-from the prebiotically formed monomers and
polymers to the earliest cells. Attempts to solve this problem have sometimes
relied on the assumption that life arose as a result of the spontaneous formation
of a single substance that embodied within itself all of the fundamental properties
associated with life. But there is no such thing as a living molecule. Biology has
been unable to produce a wholly satisfactory definition of life, but at least on
phenomenological grounds it is clear that one cannot reduce all characteristics of
living systems to a particular substance that arises suddenly by a lucky
combination of atoms.
The Origins of Replication and Translation: Two Major
Unsolved Problems
How the first biological systems capable of replication and translation emerged is
one of the major problems in the study of the origins of life. It is even possible
that the genetic code was established prior to the origin of RNA itself It has been
hypothesized that a replicative system could have resulted from the interaction
between abiotic ally synthesized polynucleotides and simple catalytic peptides.
However, the mere coexistence of small peptides and polynucleotides (or nucleic
acid-like molecules) does not guarantee by itself that protein synthesis will
develop.
The ease with which amino acids and their polymers are formed under plausible
prebiotic conditions has led to the idea that the first replicating systems consisted
of polypeptides, but this possibility appears to us to be extremely unlikely . A
counterview is represented by the hypothesis that early biological systems based
both their reproduction and their metabolism in RNA molecules.
The RNA World
The surprising ability of RNA molecules to catalyze an increasingly large
number of chemical reactions has lent strong support to the possibility of
the so-called RNA world. The idea that RNA preceded DNA as genetic material
has been expressed independently by many authors . This hypothesis is supported
by several lines of evidence:
(a) the central role that different RNA molecules play in protein biosynthesis
(b) the well-known fact that this process can take place in the absence of DNA but
not of RNA.
(c) the fact that the biosynthesis of deoxyribonucleotides always proceeds via the
enzymatic reduction of ribonucleotides, i.e. all dexoyribonucleotides are formed
directly or indirectly from a cellular pool of ribonucleotides .
The Appearance of Membranes and Liposomes
If the origin of life did not depend on the chance appearance of a single molecule
but on the gradual evolution of systems formed by sets of different compounds of
biochemical significance, then it is obvious that a mechanism for keeping them
together was required from the very beginning, i.e. a decisive step toward the
origin of life must have been the early appearance of membrane-bound
polymolecular systems . Membranes are essential to life, not only because they
allow cells to maintain an internal microenvironment different from the exterior,
but also because any self-replicating systems lacking them would be unable to
undergo preferential accumulation and, eventually, differential replication.
Liposomes are spheres with diameters of 5-50 micrometer whose amphiphilic
components self-assemble into vesicles with a double-layered membrane in the
absence of any polymerization process.
LIFE IN THE UNIVERSE
We have no evidence of extraterrestrial life. However, in spite of all the
uncertainties involved in our description of the processes that led to the
emergence of biological systems on our planet, it is clear that life is neither a
miracle nor the result of a chance event but rather the outcome of a long
evolutionary process. Whether or not this process can take place in any other
place in the Universe is a matter for speculation, but the cosmic abundances of
carbon, nitrogen, oxygen, and other biogenic elements, the existence of
extraterrestrial organic compounds, and the processes of stellar and planetary
formation are certainly suggestive that at least some of the requirements for life
are met elsewhere in our Galaxy. It is possible that conditions suitable for the
origin and evolution of life have existed elsewhere in the solar system and may
still persist in some of its minor bodies. Some of us hope to see the establishment
of human colonies on the Moon and Mars by the next century in order for science
to unravel pertinent questions concerning chemical evolution and the origin of
life on our beautiful, blue terrestrial planet. It is difficult to conceive from a
scientific point of view that "intelligent" life on Earth is a singular phenomenon in
the trillions of galaxies that comprise the observable universe.
