History of Earth

From Wikipedia, the free encyclopedia

Jump to navigationJump to search
This article is about scientific evidence concerning the history of Earth. For the history of
humans, see History of the world.

The history of Earth concerns the development of planet Earth from its formation to the present
day.[1][2] Nearly all branches of natural science have contributed to understanding of the main
events of Earth's past, characterized by constant geological change and biological evolution.
The geological time scale (GTS), as defined by international convention,[3] depicts the large
spans of time from the beginning of the Earth to the present, and its divisions chronicle some
definitive events of Earth history. (In the graphic: Ga means "billion years ago"; Ma, "million years
ago".) Earth formed around 4.54 billion years ago, approximately one-third the age of the
universe, by accretion from the solar nebula.[4][5][6] Volcanic outgassing probably created the
primordial atmosphere and then the ocean, but the early atmosphere contained almost
no oxygen. Much of the Earth was molten because of frequent collisions with other bodies which
led to extreme volcanism. While Earth was in its earliest stage (Early Earth), a giant impact
collision with a planet-sized body named Theia is thought to have formed the Moon. Over time,
the Earth cooled, causing the formation of a solid crust, and allowing liquid water on the surface.
The Hadean eon represents the time before a reliable (fossil) record of life; it began with the
formation of the planet and ended 4.0 billion years ago. The following Archean and Proterozoic
eons produced the beginnings of life on Earth and its earliest evolution. The succeeding eon is
the Phanerozoic, divided into three eras: the Palaeozoic, an era of arthropods, fishes, and the
first life on land; the Mesozoic, which spanned the rise, reign, and climactic extinction of the non-
 avian dinosaurs; and the Cenozoic, which saw the rise of mammals. Recognizable humans
emerged at most 2 million years ago, a vanishingly small period on the geological scale.
The earliest undisputed evidence of life on Earth dates at least from 3.5 billion years
ago,[7][8][9]during the Eoarchean Era, after a geological crust started to solidify following the earlier
molten Hadean Eon. There are microbial mat fossils such as stromatolites found in 3.48 billion-
year-old sandstone discovered in Western Australia.[10][11][12] Other early physical evidence of
a biogenic substance is graphite in 3.7 billion-year-old metasedimentary rocks discovered in
southwestern Greenland[13] as well as "remains of biotic life" found in 4.1 billion-year-old rocks in
Western Australia.[14][15] According to one of the researchers, "If life arose relatively quickly on
Earth … then it could be common in the universe."[14]
Photosynthetic organisms appeared between 3.2 and 2.4 billion years ago and began enriching
the atmosphere with oxygen. Life remained mostly small and microscopic until about 580 million
years ago, when complex multicellular life arose, developed over time, and culminated in
the Cambrian Explosion about 541 million years ago. This sudden diversification of life forms
produced most of the major phyla known today, and divided the Proterozoic Eon from the
Cambrian Period of the Paleozoic Era. It is estimated that 99 percent of all species that ever lived
on Earth, over five billion,[16] have gone extinct.[17][18] Estimates on the number of Earth's
current species range from 10 million to 14 million,[19] of which about 1.2 million are documented,
but over 86 percent have not been described.[20] However, it was recently claimed that 1 trillion
species currently live on Earth, with only one-thousandth of one percent described.[21]
The Earth's crust has constantly changed since its formation, as has life has since its first
appearance. Species continue to evolve, taking on new forms, splitting into daughter species, or
going extinct in the face of ever-changing physical environments. The process of plate
tectonics continues to shape the Earth's continents and oceans and the life they harbor. Human
activity is now a dominant force affecting global change, harming the biosphere, the Earth's
surface, hydrosphere, and atmosphere with the loss of wild lands, over-exploitation of the
oceans, production of greenhouse gases, degradation of the ozone layer, and general
degradation of soil, air, and water quality.

Contents

 1Eons
 2Geologic time scale
 3Solar System formation
 4Hadean and Archean Eons
o 4.1Formation of the Moon
o 4.2First continents
o 4.3Oceans and atmosphere
o 4.4Origin of life
 5Proterozoic Eon
o 5.1Oxygen revolution
o 5.2Snowball Earth
o 5.3Emergence of eukaryotes
o 5.4Supercontinents in the Proterozoic
o 5.5Late Proterozoic climate and life
 6Phanerozoic Eon
o 6.1Tectonics, paleogeography and climate
o 6.2Cambrian explosion
o 6.3Colonization of land
o 6.4Evolution of tetrapods
o 6.5Extinctions
o 6.6Diversification of mammals
 o 6.7Human evolution
 7See also
 8Notes
 9References
 10Further reading
 11External links

Eons[edit]
In geochronology, time is generally measured in mya (megayears or million years), each unit
representing the period of approximately 1,000,000 years in the past. The history of Earth is
divided into four great eons, starting 4,540 mya with the formation of the planet. Each eon saw
the most significant changes in Earth's composition, climate and life. Each eon is subsequently
divided into eras, which in turn are divided into periods, which are further divided into epochs.

Time
Eon Description
(mya)

The Earth is formed out of debris around the solar protoplanetary disk. There is
no life. Temperatures are extremely hot, with frequent volcanic activity and
4,540–
Hadean hellish environments. The atmosphere is nebular. Possible early oceans or
4,000
bodies of liquid water. The moon is formed around this time, probably due to
a protoplanet's collision into Earth.

Prokaryote life, the first form of life, emerges at the very beginning of this eon,
4,000– in a process known as abiogenesis. The continents
Archean
2,500 of Ur, Vaalbara and Kenorland may have been formed around this time. The
atmosphere is composed of volcanic and greenhouse gases.

Eukaryotes, a more complex form of life, emerge, including some forms

of multicellular organisms. Bacteria begin producing oxygen, shaping the third
and current of Earth's atmospheres. Plants, later animals and possibly earlier
2,500– forms of fungi form around this time. The early and late phases of this eon may
Proterozoic
541 have undergone "Snowball Earth" periods, in which all of the planet suffered
below-zero temperatures. The early continents
of Columbia, Rodinia and Pannotia may have formed around this time, in that
order.

Complex life, including vertebrates, begin to dominate the Earth's ocean in a

process known as the Cambrian explosion. Pangaea forms and later dissolves
into Laurasia and Gondwana. Gradually, life expands to land and all familiar
541–
Phanerozoic forms of plants, animals and fungi begin appearing, including annelids, insects
present
and reptiles. Several mass extinctions occur, among which birds, the
descendants of dinosaurs, and more recently mammals emerge. Modern
animals—including humans—evolve at the most recent phases of this eon.

Geologic time scale[edit]

Main article: Geologic time scale
The history of the Earth can be organized chronologically according to the geologic time scale,
which is split into intervals based on stratigraphic analysis.[2][22] The following four timelines show
the geologic time scale. The first shows the entire time from the formation of the Earth to the
present, but this gives little space for the most recent eon. Therefore, the second timeline shows
an expanded view of the most recent eon. In a similar way, the most recent era is expanded in
the third timeline, and the most recent period is expanded in the fourth timeline.

Millions of Years

Solar System formation[edit]

Main article: Formation and evolution of the Solar System
See also: Planetary differentiation

An artist's rendering of a protoplanetary disk

The standard model for the formation of the Solar System (including the Earth) is the solar
nebula hypothesis.[23] In this model, the Solar System formed from a large, rotating cloud of
interstellar dust and gas called the solar nebula. It was composed
of hydrogen and helium created shortly after the Big Bang 13.8 Ga (billion years ago) and
heavier elementsejected by supernovae. About 4.5 Ga, the nebula began a contraction that may
have been triggered by the shock wavefrom a nearby supernova.[24] A shock wave would have
also made the nebula rotate. As the cloud began to accelerate, its angular momentum, gravity,
and inertia flattened it into a protoplanetary disk perpendicular to its axis of rotation.
Small perturbations due to collisions and the angular momentum of other large debris created the
means by which kilometer-sized protoplanets began to form, orbiting the nebular center.[25]
The center of the nebula, not having much angular momentum, collapsed rapidly, the
compression heating it until nuclear fusion of hydrogen into helium began. After more
contraction, a T Tauri star ignited and evolved into the Sun. Meanwhile, in the outer part of the
nebula gravity caused matter to condense around density perturbations and dust particles, and
the rest of the protoplanetary disk began separating into rings. In a process known as
runaway accretion, successively larger fragments of dust and debris clumped together to form
planets.[25] Earth formed in this manner about 4.54 billion years ago (with an uncertainty of
1%)[26][27][4][28] and was largely completed within 10–20 million years.[29]The solar wind of the newly
formed T Tauri star cleared out most of the material in the disk that had not already condensed
into larger bodies. The same process is expected to produce accretion disks around virtually all
newly forming stars in the universe, some of which yield planets.[30]
The proto-Earth grew by accretion until its interior was hot enough to melt the
heavy, siderophile metals. Having higher densities than the silicates, these metals sank. This so-
called iron catastrophe resulted in the separation of a primitive mantle and a (metallic) core only
10 million years after the Earth began to form, producing the layered structure of Earth and
setting up the formation of Earth's magnetic field.[31] J. A. Jacobs [32] was the first to suggest that
the inner core—a solid center distinct from the liquid outer core—is freezing and growing out of
the liquid outer core due to the gradual cooling of Earth's interior (about 100 degrees Celsius per
billion years[33]).

Hadean and Archean Eons[edit]

Main articles: Hadean and Archean

Artist's conception of Hadean EonEarth, when it was much hotter and inhospitable to all forms of life.

The first eon in Earth's history, the Hadean, begins with the Earth's formation and is followed by
the Archean eon at 3.8 Ga.[2]:145 The oldest rocks found on Earth date to about 4.0 Ga, and the
oldest detrital zircon crystals in rocks to about 4.4 Ga,[34][35][36] soon after the formation of the
Earth's crust and the Earth itself. The giant impact hypothesis for the Moon's formation states
that shortly after formation of an initial crust, the proto-Earth was impacted by a smaller
protoplanet, which ejected part of the mantle and crust into space and created the Moon.[37][38][39]
From crater counts on other celestial bodies, it is inferred that a period of intense meteorite
impacts, called the Late Heavy Bombardment, began about 4.1 Ga, and concluded around
3.8 Ga, at the end of the Hadean.[40] In addition, volcanism was severe due to the large heat
flow and geothermal gradient.[41] Nevertheless, detrital zircon crystals dated to 4.4 Ga show
evidence of having undergone contact with liquid water, suggesting that the Earth already had
oceans or seas at that time.[34]
By the beginning of the Archean, the Earth had cooled significantly. Present life forms could not
have survived at Earth's surface, because the Archean atmosphere lacked oxygen hence had
no ozone layer to block ultraviolet light. Nevertheless, it is believed that primordial life began to
evolve by the early Archean, with candidate fossils dated to around 3.5 Ga.[42] Some scientists
even speculate that life could have begun during the early Hadean, as far back as 4.4 Ga,
surviving the possible Late Heavy Bombardment period in hydrothermal vents below the Earth's
surface.[43]
Formation of the Moon[edit]
Main articles: Moon, Origin of the Moon, and Giant impact hypothesis

Artist's impression of the enormous collision that probably formed the Moon

Earth's only natural satellite, the Moon, is larger relative to its planet than any other satellite in
the solar system.[nb 1] During the Apollo program, rocks from the Moon's surface were brought to
Earth. Radiometric dating of these rocks shows that the Moon is 4.53 ± 0.01 billion years
old,[46] formed at least 30 million years after the solar system.[47] New evidence suggests the Moon
formed even later, 4.48 ± 0.02 Ga, or 70–110 million years after the start of the Solar System.[48]
Theories for the formation of the Moon must explain its late formation as well as the following
facts. First, the Moon has a low density (3.3 times that of water, compared to 5.5 for the earth[49])
and a small metallic core. Second, there is virtually no water or other volatiles on the moon.
Third, the Earth and Moon have the same oxygen isotopic signature (relative abundance of the
oxygen isotopes). Of the theories proposed to account for these phenomena, one is widely
accepted: The giant impact hypothesis proposes that the Moon originated after a body the size
of Mars (sometimes named Theia[47]) struck the proto-Earth a glancing blow.[1]:256[50][51]
The collision released about 100 million times more energy than the more recent Chicxulub
impact that is believed to have caused the extinction of the dinosaurs. It was enough to vaporize
some of the Earth's outer layers and melt both bodies.[50][1]:256 A portion of the mantle material
was ejected into orbit around the Earth. The giant impact hypothesis predicts that the Moon was
depleted of metallic material,[52] explaining its abnormal composition.[53] The ejecta in orbit around
the Earth could have condensed into a single body within a couple of weeks. Under the influence
of its own gravity, the ejected material became a more spherical body: the Moon.[54]
First continents[edit]
Geologic map of North America, color-coded by age. The reds and pinks indicate rock from the Archean.

Mantle convection, the process that drives plate tectonics, is a result of heat flow from the Earth's
interior to the Earth's surface.[55]:2 It involves the creation of rigid tectonic plates at mid-oceanic
ridges. These plates are destroyed by subduction into the mantle at subduction zones. During
the early Archean (about 3.0 Ga) the mantle was much hotter than today, probably around
1,600 °C (2,910 °F),[56]:82 so convection in the mantle was faster. Although a process similar to
present-day plate tectonics did occur, this would have gone faster too. It is likely that during the
Hadean and Archean, subduction zones were more common, and therefore tectonic plates were
smaller.[1]:258[57]
The initial crust, formed when the Earth's surface first solidified, totally disappeared from a
combination of this fast Hadean plate tectonics and the intense impacts of the Late Heavy
Bombardment. However, it is thought that it was basaltic in composition, like today's oceanic
crust, because little crustal differentiation had yet taken place.[1]:258 The first larger pieces
of continental crust, which is a product of differentiation of lighter elements during partial
melting in the lower crust, appeared at the end of the Hadean, about 4.0 Ga. What is left of these
first small continents are called cratons. These pieces of late Hadean and early Archean crust
form the cores around which today's continents grew.[58]
The oldest rocks on Earth are found in the North American craton of Canada. They
are tonalites from about 4.0 Ga. They show traces of metamorphism by high temperature, but
also sedimentary grains that have been rounded by erosion during transport by water, showing
that rivers and seas existed then.[59] Cratons consist primarily of two alternating types of terranes.
The first are so-called greenstone belts, consisting of low-grade metamorphosed sedimentary
rocks. These "greenstones" are similar to the sediments today found in oceanic trenches, above
subduction zones. For this reason, greenstones are sometimes seen as evidence for subduction
during the Archean. The second type is a complex of felsic magmatic rocks. These rocks are
mostly tonalite, trondhjemite or granodiorite, types of rock similar in composition
to granite (hence such terranes are called TTG-terranes). TTG-complexes are seen as
the relicts of the first continental crust, formed by partial melting in basalt.[60]:Chapter 5
Oceans and atmosphere[edit]
See also: Origin of the world's oceans

Graph showing range of estimated partial pressureof atmospheric oxygen through geologic time [61]
Earth is often described as having had three atmospheres. The first atmosphere, captured from
the solar nebula, was composed of light (atmophile) elements from the solar nebula, mostly
hydrogen and helium. A combination of the solar wind and Earth's heat would have driven off this
atmosphere, as a result of which the atmosphere is now depleted of these elements compared to
cosmic abundances.[62] After the impact which created the moon, the molten Earth released
volatile gases; and later more gases were released by volcanoes, completing a second
atmosphere rich in greenhouse gases but poor in oxygen. [1]:256Finally, the third atmosphere, rich
in oxygen, emerged when bacteria began to produce oxygen about 2.8 Ga.[63]:83–84,116–117
In early models for the formation of the atmosphere and ocean, the second atmosphere was
formed by outgassing of volatilesfrom the Earth's interior. Now it is considered likely that many of
the volatiles were delivered during accretion by a process known as impact degassing in which
incoming bodies vaporize on impact. The ocean and atmosphere would, therefore, have started
to form even as the Earth formed.[64] The new atmosphere probably contained water vapor,
carbon dioxide, nitrogen, and smaller amounts of other gases.[65]
Planetesimals at a distance of 1 astronomical unit (AU), the distance of the Earth from the Sun,
probably did not contribute any water to the Earth because the solar nebula was too hot for ice to
form and the hydration of rocks by water vapor would have taken too long.[64][66] The water must
have been supplied by meteorites from the outer asteroid belt and some large planetary embryos
from beyond 2.5 AU.[64][67] Comets may also have contributed. Though most comets are today in
orbits farther away from the Sun than Neptune, computer simulations show that they were
originally far more common in the inner parts of the solar system.[59]:130–132
As the Earth cooled, clouds formed. Rain created the oceans. Recent evidence suggests the
oceans may have begun forming as early as 4.4 Ga.[34] By the start of the Archean eon, they
already covered much of the Earth. This early formation has been difficult to explain because of a
problem known as the faint young Sun paradox. Stars are known to get brighter as they age, and
at the time of its formation the Sun would have been emitting only 70% of its current power.
Thus, the Sun has become 30% brighter in the last 4.5 billion years.[68] Many models indicate that
the Earth would have been covered in ice.[69][64] A likely solution is that there was enough carbon
dioxide and methane to produce a greenhouse effect. The carbon dioxide would have been
produced by volcanoes and the methane by early microbes. Another greenhouse gas, ammonia,
would have been ejected by volcanos but quickly destroyed by ultraviolet radiation.[63]:83
Origin of life[edit]
Life timeline
view • discuss • edit
-4500 —
–
-4000 —
–
-3500 —
–
-3000 —
–
-2500 —
–
-2000 —
–
-1500 —
–
-1000 —
–
-500 —
–
0—
water
Single-celled
life
photosynthesis
Eukaryotes
 Multicellular
life
Arthropods Molluscs
Plants
Dinosaurs
Mammals
Flowers
Birds
Primates
←
Earth (−4540)
←
Earliest water
←
Earliest life
←
Earliest oxygen
←
Atmospheric oxygen
←
Oxygen crisis
←
Sexual reproduction
←
Earliest plants
←
Ediacara biota
←
Cambrian explosion
←
Tetrapoda
←
Earliest apes
P
h
a
n
e
r
o
z
o
i
c

P
r
o
t
e
r
o
z
o
i
c

A
r
c
h
e
a
n
H
a
d
e
a
n
Pongola
Huronian
Cryogenian
Andean
Karoo
Quaternary
Ice_Ages
Axis scale: million years

Also see: Human timeline and Nature timeline

Main articles: Abiogenesis, Earliest known life forms, Evolution, and Evolutionary history of life
One of the reasons for interest in the early atmosphere and ocean is that they form the
conditions under which life first arose. There are many models, but little consensus, on how life
emerged from non-living chemicals; chemical systems created in the laboratory fall well short of
the minimum complexity for a living organism.[70][71]
The first step in the emergence of life may have been chemical reactions that produced many of
the simpler organic compounds, including nucleobases and amino acids, that are the building
blocks of life. An experiment in 1953 by Stanley Miller and Harold Urey showed that such
molecules could form in an atmosphere of water, methane, ammonia and hydrogen with the aid
of sparks to mimic the effect of lightning.[72] Although atmospheric composition was probably
different from that used by Miller and Urey, later experiments with more realistic compositions
also managed to synthesize organic molecules.[73] Computer simulations show
that extraterrestrial organic molecules could have formed in the protoplanetary disk before the
formation of the Earth.[74]
Additional complexity could have been reached from at least three possible starting points: self-
replication, an organism's ability to produce offspring that are similar to itself; metabolism, its
ability to feed and repair itself; and external cell membranes, which allow food to enter and waste
products to leave, but exclude unwanted substances.[75]
Replication first: RNA world[edit]
Main article: RNA world
Even the simplest members of the three modern domains of life use DNA to record their
"recipes" and a complex array of RNAand protein molecules to "read" these instructions and use
them for growth, maintenance, and self-replication.
The discovery that a kind of RNA molecule called a ribozyme can catalyze both its own
replication and the construction of proteins led to the hypothesis that earlier life-forms were
based entirely on RNA.[76] They could have formed an RNA world in which there were individuals
but no species, as mutations and horizontal gene transfers would have meant that the offspring
in each generation were quite likely to have different genomes from those that their parents
started with.[77] RNA would later have been replaced by DNA, which is more stable and therefore
can build longer genomes, expanding the range of capabilities a single organism can
have.[78] Ribozymes remain as the main components of ribosomes, the "protein factories" of
modern cells.[79]
Although short, self-replicating RNA molecules have been artificially produced in
laboratories,[80] doubts have been raised about whether natural non-biological synthesis of RNA is
possible.[81][82][83] The earliest ribozymes may have been formed of simpler nucleic acids such
as PNA, TNA or GNA, which would have been replaced later by RNA.[84][85] Other pre-RNA
replicators have been posited, including crystals[86]:150 and even quantum systems.[87]
In 2003 it was proposed that porous metal sulfide precipitates would assist RNA synthesis at
about 100 °C (212 °F) and at ocean-bottom pressures near hydrothermal vents. In this
hypothesis, the proto-cells would be confined in the pores of the metal substrate until the later
development of lipid membranes.[88]
Metabolism first: iron–sulfur world[edit]
The replicator in virtually all known life is deoxyribonucleic acid. DNA is far more complex than the original
replicator and its replication systems are highly elaborate.

Main article: iron–sulfur world theory

Another long-standing hypothesis is that the first life was composed of protein molecules. Amino
acids, the building blocks of proteins, are easily synthesized in plausible prebiotic conditions, as
are small peptides (polymers of amino acids) that make good catalysts.[89]:295–297 A series of
experiments starting in 1997 showed that amino acids and peptides could form in the presence
of carbon monoxide and hydrogen sulfide with iron sulfide and nickel sulfide as catalysts. Most of
the steps in their assembly required temperatures of about 100 °C (212 °F) and moderate
pressures, although one stage required 250 °C (482 °F) and a pressure equivalent to that found
under 7 kilometers (4.3 mi) of rock. Hence, self-sustaining synthesis of proteins could have
occurred near hydrothermal vents.[90]
A difficulty with the metabolism-first scenario is finding a way for organisms to evolve. Without
the ability to replicate as individuals, aggregates of molecules would have "compositional
genomes" (counts of molecular species in the aggregate) as the target of natural selection.
However, a recent model shows that such a system is unable to evolve in response to natural
selection.[91]
Membranes first: Lipid world[edit]
It has been suggested that double-walled "bubbles" of lipids like those that form the external
membranes of cells may have been an essential first step.[92] Experiments that simulated the
conditions of the early Earth have reported the formation of lipids, and these can spontaneously
form liposomes, double-walled "bubbles", and then reproduce themselves. Although they are not
intrinsically information-carriers as nucleic acids are, they would be subject to natural
selection for longevity and reproduction. Nucleic acids such as RNA might then have formed
more easily within the liposomes than they would have outside.[93]
The clay theory[edit]
Further information: Graham Cairns-Smith § Clay hypothesis
Some clays, notably montmorillonite, have properties that make them plausible accelerators for
the emergence of an RNA world: they grow by self-replication of their crystalline pattern, are
subject to an analog of natural selection (as the clay "species" that grows fastest in a particular
environment rapidly becomes dominant), and can catalyze the formation of RNA
molecules.[94] Although this idea has not become the scientific consensus, it still has active
supporters.[95]:150–158[86]

Cross-section through a liposome

Research in 2003 reported that montmorillonite could also accelerate the conversion of fatty
acids into "bubbles", and that the bubbles could encapsulate RNA attached to the clay. Bubbles
can then grow by absorbing additional lipids and dividing. The formation of the earliest cells may
have been aided by similar processes.[96]
A similar hypothesis presents self-replicating iron-rich clays as the progenitors of nucleotides,
lipids and amino acids.[97]
Last universal ancestor[edit]
Main article: Last universal ancestor
It is believed that of this multiplicity of protocells, only one line survived.
Current phylogenetic evidence suggests that the last universal ancestor(LUA) lived during the
early Archean eon, perhaps 3.5 Ga or earlier.[98][99] This LUA cell is the ancestor of all life on Earth
today. It was probably a prokaryote, possessing a cell membrane and probably ribosomes, but
lacking a nucleus or membrane-bound organelles such as mitochondria or chloroplasts. Like
modern cells, it used DNA as its genetic code, RNA for information transfer and protein
synthesis, and enzymes to catalyze reactions. Some scientists believe that instead of a single
organism being the last universal common ancestor, there were populations of organisms
exchanging genes by lateral gene transfer.[98]

Proterozoic Eon[edit]
Main article: Proterozoic
The Proterozoic eon lasted from 2.5 Ga to 542 Ma (million years) ago.[2]:130 In this time
span, cratons grew into continents with modern sizes. The change to an oxygen-rich atmosphere
was a crucial development. Life developed from prokaryotes into eukaryotes and multicellular
forms. The Proterozoic saw a couple of severe ice ages called snowball Earths. After the last
Snowball Earth about 600 Ma, the evolution of life on Earth accelerated. About 580 Ma,
the Ediacaran biota formed the prelude for the Cambrian Explosion.[citation needed]
Oxygen revolution[edit]
Main article: Great Oxygenation Event
See also: Ozone layer
Lithified stromatolites on the shores of Lake Thetis, Western Australia. Archean stromatolites are the first
direct fossil traces of life on Earth.

A banded iron formation from the 3.15 Ga Moories Group, Barberton Greenstone Belt, South Africa. Red
layers represent the times when oxygen was available; gray layers were formed in anoxic circumstances.

The earliest cells absorbed energy and food from the surrounding environment. They
used fermentation, the breakdown of more complex compounds into less complex compounds
with less energy, and used the energy so liberated to grow and reproduce. Fermentation can
only occur in an anaerobic (oxygen-free) environment. The evolution of photosynthesis made it
possible for cells to derive energy from the Sun.[100]:377
Most of the life that covers the surface of the Earth depends directly or indirectly on
photosynthesis. The most common form, oxygenic photosynthesis, turns carbon dioxide, water,
and sunlight into food. It captures the energy of sunlight in energy-rich molecules such as ATP,
which then provide the energy to make sugars. To supply the electrons in the circuit, hydrogen is
stripped from water, leaving oxygen as a waste product.[101] Some organisms, including purple
bacteria and green sulfur bacteria, use an anoxygenic form of photosynthesis that uses
alternatives to hydrogen stripped from water as electron donors; examples are hydrogen sulfide,
sulfur and iron. Such extremophileorganisms are restricted to otherwise inhospitable
environments such as hot springs and hydrothermal vents.[100]:379–382[102]
The simpler anoxygenic form arose about 3.8 Ga, not long after the appearance of life. The
timing of oxygenic photosynthesis is more controversial; it had certainly appeared by about
2.4 Ga, but some researchers put it back as far as 3.2 Ga.[101] The latter "probably increased
global productivity by at least two or three orders of magnitude".[103][104] Among the oldest
remnants of oxygen-producing lifeforms are fossil stromatolites.[103][104][61]
At first, the released oxygen was bound up with limestone, iron, and other minerals. The oxidized
iron appears as red layers in geological strata called banded iron formations that formed in
abundance during the Siderian period (between 2500 Ma and 2300 Ma).[2]:133 When most of the
exposed readily reacting minerals were oxidized, oxygen finally began to accumulate in the
atmosphere. Though each cell only produced a minute amount of oxygen, the combined
metabolism of many cells over a vast time transformed Earth's atmosphere to its current state.
This was Earth's third atmosphere.[105]:50–51[63]:83–84,116–117
Some oxygen was stimulated by solar ultraviolet radiation to form ozone, which collected in a
layer near the upper part of the atmosphere. The ozone layer absorbed, and still absorbs, a
significant amount of the ultraviolet radiation that once had passed through the atmosphere. It
allowed cells to colonize the surface of the ocean and eventually the land: without the ozone
layer, ultraviolet radiation bombarding land and sea would have caused unsustainable levels of
mutation in exposed cells.[106][59]:219–220
Photosynthesis had another major impact. Oxygen was toxic; much life on Earth probably died
out as its levels rose in what is known as the oxygen catastrophe. Resistant forms survived and
thrived, and some developed the ability to use oxygen to increase their metabolism and obtain
more energy from the same food.[106]
Snowball Earth[edit]
Main article: Snowball Earth
The natural evolution of the Sun made it progressively more luminous during the Archean and
Proterozoic eons; the Sun's luminosity increases 6% every billion years.[59]:165 As a result, the
Earth began to receive more heat from the Sun in the Proterozoic eon. However, the Earth did
not get warmer. Instead, the geological record suggests it cooled dramatically during the early
Proterozoic. Glacial deposits found in South Africa date back to 2.2 Ga, at which time, based
on paleomagnetic evidence, they must have been located near the equator. Thus, this glaciation,
known as the Huronian glaciation, may have been global. Some scientists suggest this was so
severe that the Earth was frozen over from the poles to the equator, a hypothesis called
Snowball Earth.[107]
The Huronian ice age might have been caused by the increased oxygen concentration in the
atmosphere, which caused the decrease of methane (CH4) in the atmosphere. Methane is a
strong greenhouse gas, but with oxygen it reacts to form CO2, a less effective greenhouse
gas.[59]:172 When free oxygen became available in the atmosphere, the concentration of methane
could have decreased dramatically, enough to counter the effect of the increasing heat flow from
the Sun.[108]
However, the term Snowball Earth is more commonly used to describe later extreme ice ages
during the Cryogenian period. There were four periods, each lasting about 10 million years,
between 750 and 580 million years ago, when the earth is thought to have been covered with ice
apart from the highest mountains, and average temperatures were about −50 °C (−58 °F).[109] The
snowball may have been partly due to the location of the supercontintent Rodinia straddling
the Equator. Carbon dioxide combines with rain to weather rocks to form carbonic acid, which is
then washed out to sea, thus extracting the greenhouse gas from the atmosphere. When the
continents are near the poles, the advance of ice covers the rocks, slowing the reduction in
carbon dioxide, but in the Cryogienian the weathering of Rodinia was able to continue unchecked
until the ice advanced to the tropics. The process may have finally been reversed by the
emission of carbon dioxide from volcanoes or the destabilization of methane gas hydrates.
According to the alternative Slushball Earththeory, even at the height of the ice ages there was
still open water at the Equator.[110][111]
Emergence of eukaryotes[edit]
Further information: Eukaryote § Origin of eukaryotes

Chloroplasts in the cells of a moss

Modern taxonomy classifies life into three domains. The time of their origin is uncertain.
The Bacteria domain probably first split off from the other forms of life (sometimes
called Neomura), but this supposition is controversial. Soon after this, by 2 Ga,[112] the Neomura
split into the Archaea and the Eukarya. Eukaryotic cells (Eukarya) are larger and more complex
than prokaryotic cells (Bacteria and Archaea), and the origin of that complexity is only now
becoming known.[citation needed]
Around this time, the first proto-mitochondrion was formed. A bacterial cell related to
today's Rickettsia,[113] which had evolved to metabolize oxygen, entered a larger prokaryotic cell,
which lacked that capability. Perhaps the large cell attempted to digest the smaller one but failed
(possibly due to the evolution of prey defenses). The smaller cell may have tried to parasitize the
larger one. In any case, the smaller cell survived inside the larger cell. Using oxygen, it
metabolized the larger cell's waste products and derived more energy. Part of this excess energy
was returned to the host. The smaller cell replicated inside the larger one. Soon, a
stable symbiosis developed between the large cell and the smaller cells inside it. Over time, the
host cell acquired some genes from the smaller cells, and the two kinds became dependent on
each other: the larger cell could not survive without the energy produced by the smaller ones,
and these, in turn, could not survive without the raw materials provided by the larger cell. The
whole cell is now considered a single organism, and the smaller cells are classified
as organelles called mitochondria.[114]
A similar event occurred with photosynthetic cyanobacteria[115] entering large heterotrophic cells
and becoming chloroplasts.[105]:60–61[116]:536–539 Probably as a result of these changes, a line of cells
capable of photosynthesis split off from the other eukaryotes more than 1 billion years ago. There
were probably several such inclusion events. Besides the well-established endosymbiotic
theory of the cellular origin of mitochondria and chloroplasts, there are theories that cells led
to peroxisomes, spirochetes led to cilia and flagella, and that perhaps a DNA virus led to the cell
nucleus,[117][118] though none of them are widely accepted.[119]
Archaeans, bacteria, and eukaryotes continued to diversify and to become more complex and
better adapted to their environments. Each domain repeatedly split into multiple lineages,
although little is known about the history of the archaea and bacteria. Around 1.1 Ga,
the supercontinent Rodinia was assembling.[120][121] The plant, animal, and fungi lines had split,
though they still existed as solitary cells. Some of these lived in colonies, and gradually a division
of labor began to take place; for instance, cells on the periphery might have started to assume
different roles from those in the interior. Although the division between a colony with specialized
cells and a multicellular organism is not always clear, around 1 billion years ago[122], the first
multicellular plants emerged, probably green algae.[123] Possibly by around 900 Ma[116]:488 true
multicellularity had also evolved in animals.[citation needed]
At first, it probably resembled today's sponges, which have totipotent cells that allow a disrupted
organism to reassemble itself.[116]:483–487 As the division of labor was completed in all lines of
multicellular organisms, cells became more specialized and more dependent on each other;
isolated cells would die.[citation needed]
Supercontinents in the Proterozoic[edit]
Main article: Supercontinent cycle

A reconstruction of Pannotia (550 Ma).

Reconstructions of tectonic plate movement in the past 250 million years (the Cenozoic and
Mesozoic eras) can be made reliably using fitting of continental margins, ocean floor magnetic
anomalies and paleomagnetic poles. No ocean crust dates back further than that, so earlier
reconstructions are more difficult. Paleomagnetic poles are supplemented by geologic evidence
such as orogenic belts, which mark the edges of ancient plates, and past distributions of flora
and fauna. The further back in time, the scarcer and harder to interpret the data get and the more
uncertain the reconstructions.[124]:370
Throughout the history of the Earth, there have been times when continents collided and formed
a supercontinent, which later broke up into new continents. About 1000 to 830 Ma, most
continental mass was united in the supercontinent Rodinia.[124]:370[125] Rodinia may have been
preceded by Early-Middle Proterozoic continents called Nuna and Columbia.[124]:374[126][127]
After the break-up of Rodinia about 800 Ma, the continents may have formed another short-lived
supercontinent around 550 Ma. The hypothetical supercontinent is sometimes referred to
as Pannotia or Vendia.[128]:321–322 The evidence for it is a phase of continental collisionknown as
the Pan-African orogeny, which joined the continental masses of current-day Africa, South
America, Antarctica and Australia. The existence of Pannotia depends on the timing of the rifting
between Gondwana (which included most of the landmass now in the Southern Hemisphere, as
well as the Arabian Peninsula and the Indian subcontinent) and Laurentia (roughly equivalent to
current-day North America).[124]:374 It is at least certain that by the end of the Proterozoic eon, most
of the continental mass lay united in a position around the south pole.[129]
Late Proterozoic climate and life[edit]

A 580 million year old fossil of Spriggina floundensi, an animal from the Ediacaran period. Such life forms
could have been ancestors to the many new forms that originated in the Cambrian Explosion.

The end of the Proterozoic saw at least two Snowball Earths, so severe that the surface of the
oceans may have been completely frozen. This happened about 716.5 and 635 Ma, in
the Cryogenian period.[130] The intensity and mechanism of both glaciations are still under
investigation and harder to explain than the early Proterozoic Snowball Earth.[131] Most
paleoclimatologists think the cold episodes were linked to the formation of the supercontinent
Rodinia.[132] Because Rodinia was centered on the equator, rates of chemical
weatheringincreased and carbon dioxide (CO2) was taken from the atmosphere. Because CO2 is
an important greenhouse gas, climates cooled globally.[citation needed] In the same way, during the
Snowball Earths most of the continental surface was covered with permafrost, which decreased
chemical weathering again, leading to the end of the glaciations. An alternative hypothesis is that
enough carbon dioxide escaped through volcanic outgassing that the resulting greenhouse effect
raised global temperatures.[132] Increased volcanic activity resulted from the break-up of Rodinia
at about the same time.[citation needed]
The Cryogenian period was followed by the Ediacaran period, which was characterized by a
rapid development of new multicellular lifeforms.[133] Whether there is a connection between the
end of the severe ice ages and the increase in diversity of life is not clear, but it does not seem
coincidental. The new forms of life, called Ediacara biota, were larger and more diverse than
ever. Though the taxonomy of most Ediacaran life forms is unclear, some were ancestors of
groups of modern life.[134] Important developments were the origin of muscular and neural cells.
None of the Ediacaran fossils had hard body parts like skeletons. These first appear after the
boundary between the Proterozoic and Phanerozoic eons or Ediacaran and Cambrian
periods.[citation needed]

Phanerozoic Eon[edit]
Main article: Phanerozoic
The Phanerozoic is the current eon on Earth, which started approximately 542 million years ago.
It consists of three eras: The Paleozoic, Mesozoic, and Cenozoic,[22] and is the time when multi-
cellular life greatly diversified into almost all the organisms known today.[135]
The Paleozoic ("old life") era was the first and longest era of the Phanerozoic eon, lasting from
542 to 251 Ma.[22] During the Paleozoic, many modern groups of life came into existence. Life
colonized the land, first plants, then animals. Two major extinctions occurred. The continents
formed at the break-up of Pannotia and Rodinia at the end of the Proterozoic slowly moved
together again, forming the supercontinent Pangaea in the late Paleozoic.[citation needed]
The Mesozoic ("middle life") era lasted from 251 Ma to 66 Ma.[22] It is subdivided into
the Triassic, Jurassic, and Cretaceous periods. The era began with the Permian–Triassic
extinction event, the most severe extinction event in the fossil record; 95% of the species on
Earth died out.[136] It ended with the Cretaceous–Paleogene extinction event that wiped out
the dinosaurs.[citation needed].
The Cenozoic ("new life") era began at 66 Ma,[22] and is subdivided into the Paleogene, Neogene,
and Quaternary periods. These three periods are further split into seven sub-divisions, with the
Paleogene composed of The Paleocene, Eocene, and Oligocene, the Neocene divided into
the Miocene, Pliocene, and the Quaternary composed of the Pleistocene, and
Holocene.[137] Mammals, birds, amphibians, crocodilians, turtles, and lepidosaurs survived the
Cretaceous–Paleogene extinction event that killed off the non-avian dinosaurs and many other
forms of life, and this is the era during which they diversified into their modern forms.[citation needed]
Tectonics, paleogeography and climate[edit]

Pangaea was a supercontinent that existed from about 300 to 180 Ma. The outlines of the modern
continents and other landmasses are indicated on this map.

At the end of the Proterozoic, the supercontinent Pannotia had broken apart into the smaller
continents Laurentia, Baltica, Siberia and Gondwana.[138] During periods when continents move
apart, more oceanic crust is formed by volcanic activity. Because young volcanic crust is
relatively hotter and less dense than old oceanic crust, the ocean floors rise during such periods.
This causes the sea level to rise. Therefore, in the first half of the Paleozoic, large areas of the
continents were below sea level.[citation needed]
Early Paleozoic climates were warmer than today, but the end of the Ordovician saw a short ice
age during which glaciers covered the south pole, where the huge continent Gondwana was
situated. Traces of glaciation from this period are only found on former Gondwana. During the
Late Ordovician ice age, a few mass extinctions took place, in which many brachiopods,
trilobites, Bryozoa and corals disappeared. These marine species could probably not contend
with the decreasing temperature of the sea water.[139]
The continents Laurentia and Baltica collided between 450 and 400 Ma, during the Caledonian
Orogeny, to form Laurussia (also known as Euramerica).[140] Traces of the mountain belt this
collision caused can be found in Scandinavia, Scotland, and the northern Appalachians. In
the Devonian period (416–359 Ma)[22] Gondwana and Siberia began to move towards Laurussia.
The collision of Siberia with Laurussia caused the Uralian Orogeny, the collision of Gondwana
with Laurussia is called the Variscan or Hercynian Orogeny in Europe or the Alleghenian
Orogeny in North America. The latter phase took place during the Carboniferous period (359–
299 Ma)[22] and resulted in the formation of the last supercontinent, Pangaea.[60]
By 180 Ma, Pangaea broke up into Laurasia and Gondwana.[citation needed]
Cambrian explosion[edit]
Main article: Cambrian explosion

Trilobites first appeared during the Cambrian period and were among the most widespread and diverse
groups of Paleozoic organisms.

The rate of the evolution of life as recorded by fossils accelerated in the Cambrian period (542–
488 Ma).[22] The sudden emergence of many new species, phyla, and forms in this period is
called the Cambrian Explosion. The biological fomenting in the Cambrian Explosion was
unpreceded before and since that time.[59]:229 Whereas the Ediacaran life forms appear yet
primitive and not easy to put in any modern group, at the end of the Cambrian most modern
phyla were already present. The development of hard body parts such as
shells, skeletonsor exoskeletons in animals
like molluscs, echinoderms, crinoids and arthropods (a well-known group of arthropods from the
lower Paleozoic are the trilobites) made the preservation and fossilization of such life forms
easier than those of their Proterozoic ancestors. For this reason, much more is known about life
in and after the Cambrian than about that of older periods. Some of these Cambrian groups
appear complex but are seemingly quite different from modern life; examples
are Anomalocaris and Haikouichthys. More recently, however, these seem to have found a place
in modern classification.[citation needed]
During the Cambrian, the first vertebrate animals, among them the first fishes, had
appeared.[116]:357 A creature that could have been the ancestor of the fishes, or was probably
closely related to it, was Pikaia. It had a primitive notochord, a structure that could have
developed into a vertebral column later. The first fishes with jaws (Gnathostomata) appeared
during the next geological period, the Ordovician. The colonisation of new niches resulted in
massive body sizes. In this way, fishes with increasing sizes evolved during the early Paleozoic,
such as the titanic placoderm Dunkleosteus, which could grow 7 meters (23 ft) long.[citation needed]
The diversity of life forms did not increase greatly because of a series of mass extinctions that
define widespread biostratigraphic units called biomeres.[141] After each extinction pulse,
the continental shelf regions were repopulated by similar life forms that may have been evolving
slowly elsewhere.[142] By the late Cambrian, the trilobites had reached their greatest diversity and
dominated nearly all fossil assemblages.[143]:34
Colonization of land[edit]

Artist's conception of Devonian flora

Oxygen accumulation from photosynthesis resulted in the formation of an ozone layer that
absorbed much of the Sun's ultraviolet radiation, meaning unicellular organisms that reached
land were less likely to die, and prokaryotes began to multiply and become better adapted to
survival out of the water. Prokaryote lineages[144] had probably colonized the land as early as 2.6
Ga[145] even before the origin of the eukaryotes. For a long time, the land remained barren of
multicellular organisms. The supercontinent Pannotia formed around 600 Ma and then broke
apart a short 50 million years later.[146] Fish, the earliest vertebrates, evolved in the oceans
around 530 Ma.[116]:354 A major extinction event occurred near the end of the Cambrian
period,[147] which ended 488 Ma.[148]
Several hundred million years ago, plants (probably resembling algae) and fungi started growing
at the edges of the water, and then out of it.[149]:138–140 The oldest fossils of land fungi and plants
date to 480–460 Ma, though molecular evidence suggests the fungi may have colonized the land
as early as 1000 Ma and the plants 700 Ma.[150] Initially remaining close to the water's edge,
mutations and variations resulted in further colonization of this new environment. The timing of
the first animals to leave the oceans is not precisely known: the oldest clear evidence is of
arthropods on land around 450 Ma,[151] perhaps thriving and becoming better adapted due to the
vast food source provided by the terrestrial plants. There is also unconfirmed evidence that
arthropods may have appeared on land as early as 530 Ma.[152]
Evolution of tetrapods[edit]
Further information: Evolution of tetrapods

Tiktaalik, a fish with limb-like fins and a predecessor of tetrapods. Reconstruction from fossils about
375 million years old.

At the end of the Ordovician period, 443 Ma,[22] additional extinction events occurred, perhaps
due to a concurrent ice age.[139] Around 380 to 375 Ma, the first tetrapods evolved from
fish.[153] Fins evolved to become limbs that the first tetrapods used to lift their heads out of the
water to breathe air. This would let them live in oxygen-poor water, or pursue small prey in
shallow water.[153] They may have later ventured on land for brief periods. Eventually, some of
them became so well adapted to terrestrial life that they spent their adult lives on land, although
they hatched in the water and returned to lay their eggs. This was the origin of the amphibians.
About 365 Ma, another period of extinctionoccurred, perhaps as a result of global
cooling.[154] Plants evolved seeds, which dramatically accelerated their spread on land, around
this time (by approximately 360 Ma).[155][156]
About 20 million years later (340 Ma[116]:293–296), the amniotic egg evolved, which could be laid on
land, giving a survival advantage to tetrapod embryos. This resulted in the divergence
of amniotes from amphibians. Another 30 million years (310 Ma[116]:254–256) saw the divergence of
the synapsids (including mammals) from the sauropsids (including birds and reptiles). Other
groups of organisms continued to evolve, and lines diverged—in fish, insects, bacteria, and so
on—but less is known of the details.[citation needed]

Dinosaurs were the dominant terrestrial vertebrates throughout most of the Mesozoic

After yet another, the most severe extinction of the period (251~250 Ma), around 230 Ma,
dinosaurs split off from their reptilian ancestors.[157]The Triassic–Jurassic extinction event at
200 Ma spared many of the dinosaurs,[22][158] and they soon became dominant among the
vertebrates. Though some mammalian lines began to separate during this period, existing
mammals were probably small animals resembling shrews.[116]:169
The boundary between avian and non-avian dinosaurs is not clear, but Archaeopteryx,
traditionally considered one of the first birds, lived around 150 Ma.[159]
The earliest evidence for the angiosperms evolving flowers is during the Cretaceous period,
some 20 million years later (132 Ma).[160]
Extinctions[edit]
The first of five great mass extinctions was the Ordovician-Silurian extinction. Its possible cause
was the intense glaciation of Gondwana, which eventually led to a snowball earth. 60% of marine
invertebrates became extinct and 25% of all families.[citation needed]
The second mass extinction was the Late Devonian extinction, probably caused by the evolution
of trees, which could have led to the depletion of greenhouse gases (like CO2) or
the eutrophication of water. 70% of all species became extinct.[citation needed]
The third mass extinction was the Permian-Triassic, or the Great Dying, event was possibly
caused by some combination of the Siberian Traps volcanic event, an asteroid impact, methane
hydrate gasification, sea level fluctuations, and a major anoxic event. Either the proposed Wilkes
Land crater[161] in Antarctica or Bedout structure off the northwest coast of Australia may indicate
an impact connection with the Permian-Triassic extinction. But it remains uncertain whether
either these or other proposed Permian-Triassic boundary craters are either real impact craters
or even contemporaneous with the Permian-Triassic extinction event. This was by far the
deadliest extinction ever, with about 57% of all families and 83% of all genera killed.[162][163]
The fourth mass extinction was the Triassic-Jurassic extinction event in which almost
all synapsids and archosaurs became extinct, probably due to new competition from
dinosaurs.[citation needed]
The fifth and most recent mass extinction was the K-T extinction. In 66 Ma, a 10-kilometer
(6.2 mi) asteroid struck Earth just off the Yucatán Peninsula – somewhere in the south western
tip of then Laurasia – where the Chicxulub crater is today. This ejected vast quantities of
particulate matter and vapor into the air that occluded sunlight, inhibiting photosynthesis. 75% of
all life, including the non-avian dinosaurs, became extinct,[164] marking the end of the Cretaceous
period and Mesozoic era.[citation needed]
Diversification of mammals[edit]
Further information: Evolution of mammals
The first true mammals evolved in the shadows of dinosaurs and other large archosaurs that
filled the world by the late Triassic. The first mammals were very small, and were probably
nocturnal to escape predation. Mammal diversification truly began only after the Cretaceous-
Paleogene extinction event.[165] By the early Paleocene the earth recovered from the extinction,
and mammalian diversity increased. Creatures like Ambulocetus took to the oceans to eventually
evolve into whales,[166] whereas some creatures, like primates, took to the trees.[167] This all
changed during the mid to late Eocene when the circum-Antarctic current formed between
Antarctica and Australia which disrupted weather patterns on a global scale.
Grassless savannas began to predominate much of the landscape, and mammals such
as Andrewsarchus rose up to become the largest known terrestrial predatory mammal
ever,[168] and early whales like Basilosaurus took control of the seas.[citation needed]
The evolution of grass brought a remarkable change to the Earth's landscape, and the new open
spaces created pushed mammals to get bigger and bigger. Grass started to expand in the
Miocene, and the Miocene is where many modern- day mammals first appeared.
Giant ungulates like Paraceratherium and Deinotherium evolved to rule the grasslands. The
evolution of grass also brought primates down from the trees, and started human evolution. The
first big cats evolved during this time as well.[169] The Tethys Sea was closed off by the collision of
Africa and Europe.[170]
The formation of Panama was perhaps the most important geological event to occur in the last
60 million years. Atlantic and Pacific currents were closed off from each other, which caused the
formation of the Gulf Stream, which made Europe warmer. The land bridge allowed the isolated
creatures of South America to migrate over to North America, and vice versa.[171] Various species
migrated south, leading to the presence in South America of llamas, the spectacled
bear, kinkajous and jaguars.[citation needed]
Three million years ago saw the start of the Pleistocene epoch, which featured dramatic climactic
changes due to the ice ages. The ice ages led to the evolution of modern man in Saharan Africa
and expansion. The mega-fauna that dominated fed on grasslands that, by now, had taken over
much of the subtropical world. The large amounts of water held in the ice allowed for various
bodies of water to shrink and sometimes disappear such as the North Sea and the Bering Strait.
It is believed by many that a huge migration took place along Beringia which is why, today, there
are camels (which evolved and became extinct in North America), horses (which evolved and
became extinct in North America), and Native Americans. The ending of the last ice age
coincided with the expansion of man, along with a massive die out of ice age mega-fauna. This
extinction, nicknamed "the Sixth Extinction", has been going ever since.[citation needed]
Human evolution[edit]
Hominin timeline
view • discuss • edit
-10 —
–
-9 —
–
-8 —
–
-7 —
–
-6 —
–
-5 —
–
-4 —
–
-3 —
–
-2 —
–
-1 —
–
0—
Hominini
 Nakalipithecus
Ouranopithecus
Sahelanthropus
Orrorin
Ardipithecus
Australopithecus
Homo habilis
Homo erectus
H. heidelbergensis
Homo sapiens
Neanderthals
←
Earlier apes
←
Gorilla split
←
Possibly bipedal
←
Chimpanzee split
←
Earliest bipedal
←
Stone tools
←
Exit from Africa
←
Earliest fire use
←
Earliest cooking
←
Earliest clothes
←
Modern speech
←
Modern humans

P
l
e
i
s
t
o
c
e
n
e

P
l
i
o
c
e
n
e

M
i
o
c
e
n
e

H
 o

s
Axis scale: million years

Also see: Life timeline and Nature timeline

Main article: Human evolution

A small African ape living around 6 Ma was the last animal whose descendants would include
both modern humans and their closest relatives, the chimpanzees.[116]:100–101 Only two branches of
its family tree have surviving descendants. Very soon after the split, for reasons that are still
unclear, apes in one branch developed the ability to walk upright.[116]:95–99 Brain size increased
rapidly, and by 2 Ma, the first animals classified in the genus Homo had appeared.[149]:300 Of
course, the line between different species or even genera is somewhat arbitrary as organisms
continuously change over generations. Around the same time, the other branch split into the
ancestors of the common chimpanzee and the ancestors of the bonobo as evolution continued
simultaneously in all life forms.[116]:100–101
The ability to control fire probably began in Homo erectus (or Homo ergaster), probably at least
790,000 years ago[172] but perhaps as early as 1.5 Ma.[116]:67 The use and discovery of controlled
fire may even predate Homo erectus. Fire was possibly used by the early Lower
Paleolithic (Oldowan) hominid Homo habilis or strong australopithecines such
as Paranthropus.[173]

A reconstruction of human history based on fossil data.[174]

It is more difficult to establish the origin of language; it is unclear whether Homo erectuscould
speak or if that capability had not begun until Homo sapiens.[116]:67 As brain size increased, babies
were born earlier, before their heads grew too large to pass through the pelvis. As a result, they
exhibited more plasticity, and thus possessed an increased capacity to learn and required a
longer period of dependence. Social skills became more complex, language became more
sophisticated, and tools became more elaborate. This contributed to further cooperation and
intellectual development.[175]:7 Modern humans (Homo sapiens) are believed to have originated
around 200,000 years ago or earlier in Africa; the oldest fossils date back to around
160,000 years ago.[176]
The first humans to show signs of spirituality are the Neanderthals (usually classified as a
separate species with no surviving descendants); they buried their dead, often with no sign of
food or tools.[177]:17 However, evidence of more sophisticated beliefs, such as the early Cro-
Magnon cave paintings (probably with magical or religious significance)[177]:17–19 did not appear
until 32,000 years ago.[178] Cro-Magnons also left behind stone figurines such as Venus of
Willendorf, probably also signifying religious belief.[177]:17–19 By 11,000 years ago, Homo
sapiens had reached the southern tip of South America, the last of the uninhabited continents
(except for Antarctica, which remained undiscovered until 1820 AD).[179] Tool use and
communication continued to improve, and interpersonal relationships became more intricate.[citation
needed]

Civilization[edit]
Main articles: History of the world and Cradle of civilization
Further information: History of Africa, History of the Americas, History of Antarctica, and History
of Eurasia

Vitruvian Man by Leonardo da Vinciepitomizes the advances in art and science seen during the
Renaissance.

Throughout more than 90% of its history, Homo sapiens lived in small bands as nomadic hunter-
gatherers.[175]:8 As language became more complex, the ability to remember and communicate
information resulted, according to a theory proposed by Richard Dawkins, in a new replicator:
the meme.[180] Ideas could be exchanged quickly and passed down the generations. Cultural
evolution quickly outpaced biological evolution, and history proper began. Between 8500 and
7000 BC, humans in the Fertile Crescent in the Middle East began the systematic husbandry of
plants and animals: agriculture.[181] This spread to neighboring regions, and developed
independently elsewhere, until most Homo sapiens lived sedentary lives in permanent
settlements as farmers. Not all societies abandoned nomadism, especially those in isolated
areas of the globe poor in domesticable plant species, such as Australia.[182] However, among
those civilizations that did adopt agriculture, the relative stability and increased productivity
provided by farming allowed the population to expand.[citation needed]
Agriculture had a major impact; humans began to affect the environment as never before.
Surplus food allowed a priestly or governing class to arise, followed by increasing division of
labor. This led to Earth's first civilization at Sumer in the Middle East, between 4000 and 3000
BC.[175]:15 Additional civilizations quickly arose in ancient Egypt, at the Indus River valley and in
China. The invention of writing enabled complex societies to arise: record-keeping
and libraries served as a storehouse of knowledge and increased the cultural transmission of
information. Humans no longer had to spend all their time working for survival, enabling the first
specialized occupations (e.g. craftsmen, merchants, priests, etc...). Curiosity and education
drove the pursuit of knowledge and wisdom, and various disciplines, including science (in a
primitive form), arose. This in turn led to the emergence of increasingly larger and more complex
civilizations, such as the first empires, which at times traded with one another, or fought for
territory and resources.
By around 500 BC, there were advanced civilizations in the Middle East, Iran, India, China, and
Greece, at times expanding, at times entering into decline.[175]:3 In 221 BC, China became a single
polity that would grow to spread its culture throughout East Asia, and it has remained the most
populous nation in the world. The fundamentals of Western civilization were largely shaped
in Ancient Greece, with the world's first democratic government and major advances in
philosophy, science, and mathematics, and in Ancient Rome in law, government, and
engineering.[183] The Roman Empire was Christianized by Emperor Constantine in the early 4th
century and declined by the end of the 5th. Beginning with the 7th century, Christianization of
Europe began. In 610, Islam was founded and quickly became the dominant religion in Western
Asia. The House of Wisdom was established in Abbasid-era Baghdad, Iraq.[184] It is considered to
have been a major intellectual center during the Islamic Golden Age, where Muslim
scholars in Baghdad and Cairo flourished from the ninth to the thirteenth centuries until
the Mongol sack of Baghdad in 1258 AD. In 1054 AD the Great Schism between the Roman
Catholic Church and the Eastern Orthodox Church led to the prominent cultural differences
between Western and Eastern Europe.[citation needed]
In the 14th century, the Renaissance began in Italy with advances in religion, art, and
science.[175]:317–319 At that time the Christian Church as a political entity lost much of its power. In
1492, Christopher Columbus reached the Americas, initiating great changes to the new world.
European civilization began to change beginning in 1500, leading to
the scientificand industrial revolutions. That continent began to exert political and
cultural dominance over human societies around the world, a time known as the Colonial
era (also see Age of Discovery).[175]:295–299 In the 18th century a cultural movement known as
the Age of Enlightenment further shaped the mentality of Europe and contributed to
its secularization. From 1914 to 1918 and 1939 to 1945, nations around the world were
embroiled in world wars. Established following World War I, the League of Nations was a first
step in establishing international institutions to settle disputes peacefully. After failing to
prevent World War II, mankind's bloodiest conflict, it was replaced by the United Nations. After
the war, many new states were formed, declaring or being granted independence in a period
of decolonization. The United States and Soviet Union became the world's
dominant superpowers for a time, and they held an often-violent rivalry known as the Cold
War until the dissolution of the latter. In 1992, several European nations joined in the European
Union. As transportation and communication improved, the economies and political affairs of
nations around the world have become increasingly intertwined. This globalization has often
produced both conflict and cooperation.[citation needed]
Recent events[edit]
Astronaut Bruce McCandless IIoutside of the space shuttle Challengerin 1984

Main article: Modern history

See also: Modernity and Future
Change has continued at a rapid pace from the mid-1940s to today. Technological developments
include nuclear weapons, computers, genetic engineering, and nanotechnology. Economic
globalization, spurred by advances in communication and transportation technology, has
influenced everyday life in many parts of the world. Cultural and institutional forms such
as democracy, capitalism, and environmentalismhave increased influence. Major concerns and
problems such as disease, war, poverty, violent radicalism, and recently, human-caused climate
change have risen as the world population increases.[citation needed]
In 1957, the Soviet Union launched the first artificial satellite into orbit and, soon afterward, Yuri
Gagarin became the first human in space.Neil Armstrong, an American, was the first to set foot
on another astronomical object, the Moon. Unmanned probes have been sent to all the known
planets in the solar system, with some (such as Voyager) having left the solar system. Five
space agencies, representing over fifteen countries,[185] have worked together to build
the International Space Station. Aboard it, there has been a continuous human presence in
space since 2000.[186] The World Wide Web became a part of everyday life in the 1990s, and
since then has become an indispensable source of information in the developed world.[citation needed]