History of Earth

The natural 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 Earth to
the present, and its
divisions chronicle some
definitive events of Earth Earth's history with time-spans of the eons to scale. Ma means "million years
history. Earth formed ago".
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 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, 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 538.8 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]

Earth's crust has constantly changed since its formation, as has life 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 Earth's continents
and oceans and the life they harbor.

Eons
In geochronology, time is generally measured in mya (million years ago), 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)

Earth is formed out of debris around the solar protoplanetary disk. There is no life.
Temperatures are extremely hot, with frequent volcanic activity and hellish-looking
4,540–
Hadean environments (hence the eon's name, which comes from Hades). The atmosphere is
4,000
nebular. Possible early oceans or 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, in a
4,000– process known as abiogenesis. The continents of Ur, Vaalbara and Kenorland may
Archean
2,500 have existed around this time. The atmosphere is composed of volcanic and
greenhouse gases.
The name of this eon means "early life". 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
2,500–
Proterozoic and possibly earlier forms of fungi form around this time. The early and late phases of
538.8
this eon may have undergone "Snowball Earth" periods, in which all of the planet
suffered below-zero temperatures. The early continents of Columbia, Rodinia and
Pannotia, in that order, may have existed in this eon.

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

known as the Cambrian explosion. Pangaea forms and later dissolves into Laurasia
and Gondwana, which in turn dissolve into the current continents. Gradually, life
538.8– expands to land and familiar forms of plants, animals and fungi begin appearing,
Phanerozoic
present including annelids, insects and reptiles, hence the eon's name, which means "visible
life". Several mass extinctions occur, among which birds, the descendants of non-
avian dinosaurs, and more recently mammals emerge. Modern animals—including
humans—evolve at the most recent phases of this eon.

Geologic time scale

The history of Earth can be organized chronologically according to the geologic time scale, which is split
into intervals based on stratigraphic analysis.[2][21]

The following five timelines show the geologic time scale to scale. The first shows the entire time from
the formation of Earth to the present, but this gives little space for the most recent eon. 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, the most recent period is expanded in the fourth timeline, and the most
recent epoch is expanded in the fifth timeline.
(Horizontal scale is millions of years for the above timelines; thousands of years for the timeline below)

Solar System formation

The standard model for the formation of the
Solar System (including Earth) is the solar
nebula hypothesis.[22] 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
elements ejected by supernovae. About
4.5 Ga, the nebula began a contraction that
An artist's rendering of a protoplanetary disk
may have been triggered by the shock wave
from a nearby supernova.[23] 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.[24]

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.[24] Earth formed in this manner about 4.54 billion years ago
(with an uncertainty of 1%)[25][26][4] and was largely completed within 10–20 million years.[27] In June
2023, scientists reported evidence that the planet Earth may have formed in just three million years, much
faster than the 10−100 million years thought earlier.[28][29] Nonetheless, 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 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 Earth's 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

The first eon in Earth's history, the Hadean, begins with 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 Earth's crust and 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 Artist's conception of Hadean Eon
Bombardment, began about 4.1 Ga, and concluded around 3.8 Ga, Earth, when it was much hotter and
at the end of the Hadean. [40] In addition, volcanism was severe inhospitable to all forms of life.
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 Earth already had oceans or seas
at that time.[34]

By the beginning of the Archean, 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 Earth's surface.[43]

Formation of 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 Earth[49]) and
a small metallic core. Second, 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] Artist's impression of the enormous
collision that probably formed the
The collision released about 100 million times more energy than Moon
the more recent Chicxulub impact that is believed to have caused
the extinction of the non-avian dinosaurs. It was enough to
vaporize some of Earth's outer layers and melt both bodies.[50][1]: 256 A portion of the mantle material was
ejected into orbit around 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 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
Mantle convection, the process that drives plate tectonics, is a
result of heat flow from Earth's interior to 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 Artist's impression of a Hadean
mantle was much hotter than today, probably around 1,600 °C landscape with the relatively newly
(2,910 °F),[56]: 82 so convection in the mantle was faster. Although formed Moon still looming closely
over Earth and both bodies
a process similar to present-day plate tectonics did occur, this
sustaining strong volcanism.
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, which formed when 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 Geologic map of North America,

Earth is often described as having had three atmospheres. The first color-coded by age. From most
recent to oldest, age is indicated by
atmosphere, captured from the solar nebula, was composed of
yellow, green, blue, and red. The
light (atmophile) elements from the solar nebula, mostly hydrogen
reds and pinks indicate rock from
and helium. A combination of the solar wind and Earth's heat the Archean.
would have driven off this atmosphere, as a result of which the
atmosphere is now depleted of these elements compared to cosmic
abundances.[61] 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]: 256 Finally, the third atmosphere, rich in oxygen, emerged when bacteria began to
produce oxygen about 2.8 Ga.[62]: 83–84, 116–117

In early models for the formation of the atmosphere and

ocean, the second atmosphere was formed by outgassing of
volatiles from 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 Earth formed.[66] The
new atmosphere probably contained water vapor, carbon The pale orange dot, an artist's
dioxide, nitrogen, and smaller amounts of other gases.[67] impression of the early Earth which might
have appeared orange through its hazy
Planetesimals at a distance of 1 astronomical unit (AU), the methane rich prebiotic second
distance of Earth from the Sun, probably did not contribute atmosphere.[63][64] Earth's atmosphere at
any water to Earth because the solar nebula was too hot for this stage was somewhat comparable to
ice to form and the hydration of rocks by water vapor would today's atmosphere of Titan.[65]
have taken too long.[66][68] The water must have been
supplied by meteorites from the outer asteroid belt and some
large planetary embryos from beyond 2.5 AU.[66][69] 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 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
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 the Sun has become 30% brighter since its
formation 4.5 billion years ago.[70] Many models indicate that the early Earth should have been covered
in ice.[71][66] 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. It is hypothesized that there also existed an organic haze created from the products of methane
photolysis that caused an anti-greenhouse effect as well.[72] Another greenhouse gas, ammonia, would
have been ejected by volcanos but quickly destroyed by ultraviolet radiation.[62]: 83

Origin of life
One of the reasons for
interest in the early Life timeline
atmosphere and ocean is 0— ← Quaternary ice age*
that they form the P Primates ← Earliest hominoid
Flowers Birds
h
conditions under which life a
–n Mammals
first arose. There are many
e Dinosaurs
models, but little —r
consensus, on how life o ← Karoo ice age*
z P
emerged from non-living –o l Arthropods Molluscs ← Earliest tetrapods
i a ← Hirnantian ice age*
chemicals; chemical −500 — c n
t ← Cambrian explosion
systems created in the s ← Ediacaran biota
–
laboratory fall well short of ← Cryogenian ice age*
the minimum complexity — ← Earliest animals
for a living – ← Earliest plants
organism. [73][74]
−1000 — Multicellular life
–
The first step in the
—
emergence of life may have P
–r
been chemical reactions
−1500 — o ← Earliest fungi
that produced many of the t
–e
simpler organic r
—o
compounds, including Eukaryotes
–z
nucleobases and amino o
−2000 — i ← Sexual reproduction
acids, that are the building ← Multicellular life
–c
blocks of life. An Huronian glaciation*
— ←
experiment in 1952 by ← Atmospheric oxygen
–
Stanley Miller and Harold −2500 —
Urey showed that such –
molecules could form in an — Photosynthesis
– ← Pongola glaciation*
atmosphere of water,
methane, ammonia and −3000 — A
r
–c
hydrogen with the aid of
—h
sparks to mimic the effect –ea
of lightning.[75] Although −3500 — n ← Earliest oxygen
atmospheric composition –
was probably different from — Single-celled life
– ← LHB meteorites
that used by Miller and −4000 — ← Earliest fossils
Urey, later experiments –H
with more realistic —a Water
d
e ← LUCA
li
 –e ← Earliest water
compositions also managed −4500 — ← Earth formed
to synthesize organic a
(million years
n ago) *Ice Ages
molecules. [76] Computer
simulations show that
extraterrestrial organic molecules could have formed in the protoplanetary disk before the formation of
Earth.[77]

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.[78]

Replication first: RNA world

Even the simplest members of the three modern domains of life use DNA to record their "recipes" and a
complex array of RNA and 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.[79]
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.[80] 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.[81] Ribozymes remain as the main components of ribosomes, the "protein
factories" of modern cells.[82]

Although short, self-replicating RNA molecules have been artificially produced in laboratories,[83] doubts
have been raised about whether natural non-biological synthesis of RNA is possible.[84][85][86] 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.[87][88] Other pre-RNA replicators have been posited, including
crystals[89]: 150 and even quantum systems.[90]

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.[91]

Metabolism first: iron–sulfur world

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.[92]: 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.[93]
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.[94]

Membranes first: Lipid world

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.[95] 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.[96] The replicator in virtually all
known life is
deoxyribonucleic acid. DNA
The clay theory is far more complex than
Some clays, notably montmorillonite, have properties that make them the original replicator and
plausible accelerators for the emergence of an RNA world: they grow by its replication systems are
highly elaborate.
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.[97] Although
this idea has not become the scientific consensus, it still has active supporters.[98]: 150–158 [89]

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.[99]

A similar hypothesis presents self-replicating iron-rich clays as the

progenitors of nucleotides, lipids and amino acids.[100]

Last universal common ancestor

It is believed that of this multiplicity of protocells, only one line survived. Cross-section through a
Current phylogenetic evidence suggests that the last universal ancestor liposome
(LUA) lived during the early Archean eon, perhaps 3.5 Ga or
earlier.[101][102] 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.[103]
Proterozoic Eon
The Proterozoic eon lasted from 2.5 Ga to 538.8 Ma
(million years) ago.[105] In this time span, cratons grew
into continents with modern sizes. The change to an
oxygen-rich atmosphere was a crucial development.
Artist's impression of Earth during the later
Life developed from prokaryotes into eukaryotes and Archean, the largely cooled planetary crust and
multicellular forms. The Proterozoic saw a couple of water-rich barren surface, marked by volcanoes
severe ice ages called Snowball Earths. After the last and continents, features already round
Snowball Earth about 600 Ma, the evolution of life on microbialites. The Moon, still orbiting Earth
Earth accelerated. About 580 Ma, the Ediacaran biota much closer than today and still dominating
formed the prelude for the Cambrian Explosion. Earth's sky, produced strong tides.[104]

Oxygen revolution
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.[106]: 377

Most of the life that covers the surface of Earth depends directly Lithified stromatolites on the shores
or indirectly on photosynthesis. The most common form, oxygenic of Lake Thetis, Western Australia.
photosynthesis, turns carbon dioxide, water, and sunlight into Archean stromatolites are the first
food. It captures the energy of sunlight in energy-rich molecules direct fossil traces of life on Earth.
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.[107] 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 extremophile organisms
are restricted to otherwise inhospitable environments such as hot
springs and hydrothermal vents.[106]: 379–382 [108] A banded iron formation from the
3.15 Ga Moodies Group, Barberton
Greenstone Belt, South Africa. Red
The simpler anoxygenic form arose about 3.8 Ga, not long after
layers represent the times when
the appearance of life. The timing of oxygenic photosynthesis is
oxygen was available; gray layers
more controversial; it had certainly appeared by about 2.4 Ga, but were formed in anoxic
some researchers put it back as far as 3.2 Ga.[107] The latter circumstances.
"probably increased global productivity by at least two or three
orders of magnitude".[109][110] Among the oldest remnants of
oxygen-producing lifeforms are fossil stromatolites.[109][110][111]
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. [112]: 50–51 [62]: 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.[113][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.[113]
Graph showing range of estimated partial
Snowball Earth pressure of atmospheric oxygen through geologic
time [111]
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, Earth began to receive more
heat from the Sun in the Proterozoic eon. However, 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 Artist's rendition of an oxinated fully-
Huronian glaciation, may have been global. Some scientists frozen Snowball Earth with no remaining
liquid surface water.
suggest this was so severe that Earth was frozen over from the
poles to the equator, a hypothesis called Snowball Earth.[114]

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.[115]

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 Earth is thought to have been covered with ice apart from the highest mountains,
and average temperatures were about −50 °C (−58 °F).[116] The snowball may have been partly due to the
location of the supercontinent 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 Cryogenian 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 Earth theory, even at the height of the ice ages there was still open water at the
Equator.[117][118]

Emergence of eukaryotes
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,[119] the
Neomura split into the Archaea and the Eukaryota. Eukaryotic
cells (Eukaryota) are larger and more complex than prokaryotic
cells (Bacteria and Archaea), and the origin of that complexity is
only now becoming known.[120] The earliest fossils possessing
features typical of fungi date to the Paleoproterozoic era, some 2.4 Chloroplasts in the cells of a moss
Ga ago; these multicellular benthic organisms had filamentous
structures capable of anastomosis.[121]

Around this time, the first proto-mitochondrion was formed. A bacterial cell related to today's
Rickettsia,[122] 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.[123]

A similar event occurred with photosynthetic cyanobacteria[124] entering large heterotrophic cells and
becoming chloroplasts.[112]: 60–61 [125]: 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,[126][127] though none of them are widely
accepted.[128]
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. Around 1.1 Ga, 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[129], the
first multicellular plants emerged, probably green algae.[130] Possibly by around 900 Ma[125]: 488 true
multicellularity had also evolved in animals.[131]

At first, it probably resembled today's sponges, which have totipotent cells that allow a disrupted
organism to reassemble itself.[125]: 483–487 As the division of labor was completed in the different lineages
of multicellular organisms, cells became more specialized and more dependent on each other.[132]

Supercontinents in the Proterozoic

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.[133]: 370

Throughout the history of Earth, there have been times when A reconstruction of Pannotia
continents collided and formed a supercontinent, which later broke (550 Ma).
up into new continents. About 1000 to 830 Ma, most continental
mass was united in the supercontinent Rodinia.[133]: 370 [134]
Rodinia may have been preceded by Early-Middle Proterozoic continents called Nuna and
Columbia.[133]: 374 [135][136]

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.[137]: 321–322 The evidence for it is a phase of continental collision known 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).[133]: 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.[138]
Late Proterozoic climate and life
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.[139] The intensity and mechanism of both glaciations are
still under investigation and harder to explain than the early
Proterozoic Snowball Earth.[140] Most paleoclimatologists think
the cold episodes were linked to the formation of the
supercontinent Rodinia.[141] Because Rodinia was centered on the
equator, rates of chemical weathering increased and carbon
A 580 million year old fossil of
dioxide (CO2) was taken from the atmosphere. Because CO2 is an
Spriggina floundensi, an animal
important greenhouse gas, climates cooled globally.[142] from the Ediacaran period. Such life
forms could have been ancestors to
In the same way, during the Snowball Earths most of the the many new forms that originated
continental surface was covered with permafrost, which decreased in the Cambrian Explosion.
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.[141] Increased
volcanic activity resulted from the break-up of Rodinia at about the same time.[143]

The Cryogenian period was followed by the Ediacaran period, which was characterized by a rapid
development of new multicellular lifeforms.[144] 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.[145] 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.[146]

Phanerozoic Eon
The Phanerozoic is the current eon on Earth, which started approximately 538.8 million years ago. It
consists of three eras: The Paleozoic, Mesozoic, and Cenozoic,[105] and is the time when multi-cellular
life greatly diversified into almost all the organisms known today.[147]

The Paleozoic ("old life") era was the first and longest era of the Phanerozoic eon, lasting from 538.8 to
251.9 Ma.[105] During the Paleozoic, many modern groups of life came into existence. Life colonized the
land, first plants, then animals. Two significant 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.[148]

The Mesozoic ("middle life") era lasted from 251.9 Ma to 66 Ma.[105] 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.[149] It ended with the
Cretaceous–Paleogene extinction event that wiped out the dinosaurs.[150]
The Cenozoic ("new life") era began at 66 Ma, and is subdivided into the Paleogene, Neogene, and
Quaternary periods. These three periods are further split into seven subdivisions, with the Paleogene
composed of The Paleocene, Eocene, and Oligocene, the Neogene divided into the Miocene, Pliocene,
and the Quaternary composed of the Pleistocene, and Holocene.[151] 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.[152]

Tectonics, paleogeography and climate

At the end of the Proterozoic, the supercontinent Pannotia had
broken apart into the smaller continents Laurentia, Baltica, Siberia
and Gondwana.[153] During periods when continents move apart,
more oceanic crust is formed by volcanic activity. Because the
young volcanic crust is relatively hotter and less dense than the
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.

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 Pangaea was a supercontinent that
existed from about 300 to 180 Ma.
extinctions took place, in which many brachiopods, trilobites,
The outlines of the modern
Bryozoa and corals disappeared. These marine species could continents and other landmasses
probably not contend with the decreasing temperature of the sea are indicated on this map.
water.[154]

The continents Laurentia and Baltica collided between 450 and 400 Ma, during the Caledonian Orogeny,
to form Laurussia (also known as Euramerica).[155] 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)[21] 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)[21] and resulted in the formation of the last
supercontinent, Pangaea.[60]

By 180 Ma, Pangaea broke up into Laurasia and Gondwana.

Cambrian explosion
The rate of the evolution of life as recorded by fossils accelerated in the Cambrian period (542–
488 Ma).[21] The sudden emergence of many new species, phyla, and forms in this period is called the
Cambrian Explosion. It was a form of adaptive radiation, where vacant niches left by the extinct
Ediacaran biota were filled up by the emergence of new phyla.[156] The biological fomenting in the
Cambrian Explosion was unprecedented 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, skeletons or 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 period than about life
in older periods. Some of these Cambrian groups appear complex Trilobites first appeared during the
but are seemingly quite different from modern life; examples are Cambrian period and were among
the most widespread and diverse
Anomalocaris and Haikouichthys. More recently, however, these
groups of Paleozoic organisms.
seem to have found a place in modern classification.[157]

During the Cambrian, the first vertebrate animals, among them the
first fishes, had appeared.[125]: 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.[158]

The diversity of life forms did not increase significantly because of a series of mass extinctions that
define widespread biostratigraphic units called biomeres.[159] After each extinction pulse, the continental
shelf regions were repopulated by similar life forms that may have been evolving slowly elsewhere.[160]
By the late Cambrian, the trilobites had reached their greatest diversity and dominated nearly all fossil
assemblages.[161]: 34

Colonization of land
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 had probably colonized the land as early as 3 Ga[162][163]
even before the origin of the eukaryotes. For a long time, the land
remained barren of multicellular organisms. The supercontinent
Artist's conception of Devonian flora
Pannotia formed around 600 Ma and then broke apart a short
50 million years later.[164] Fish, the earliest vertebrates, evolved in
the oceans around 530 Ma.[125]: 354 A major extinction event occurred near the end of the Cambrian
period,[165] which ended 488 Ma.[166]

Several hundred million years ago, plants (probably resembling algae) and fungi started growing at the
edges of the water and then out of it.[167]: 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.[168] 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,[169] 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.[170]

Evolution of tetrapods
At the end of the Ordovician period, 443 Ma,[21] additional
extinction events occurred, perhaps due to a concurrent ice
age.[154] Around 380 to 375 Ma, the first tetrapods evolved from
fish.[171] Fins evolved to become limbs that the first tetrapods
Tiktaalik, a fish with limb-like fins
used to lift their heads out of the water to breathe air. This would and a predecessor of tetrapods.
let them live in oxygen-poor water, or pursue small prey in Reconstruction from fossils about
shallow water.[171] They may have later ventured on land for brief 375 million years old.
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 extinction occurred, perhaps as a result of global cooling.[172] Plants evolved
seeds, which dramatically accelerated their spread on land, around this time (by approximately
360 Ma).[173][174]

About 20 million years later (340 Ma[125]: 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[125]: 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.

After yet another, the most severe extinction of the period

(251~250 Ma), around 230 Ma, dinosaurs split off from their
reptilian ancestors.[175] The Triassic–Jurassic extinction event at
200 Ma spared many of the dinosaurs,[21][176] 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
[125]: 169
shrews.
Dinosaurs were the dominant
The boundary between avian and non-avian dinosaurs is unclear,
terrestrial vertebrates throughout
but Archaeopteryx, traditionally considered one of the first birds, most of the Mesozoic
lived around 150 Ma.[177]

The earliest evidence for the angiosperms evolving flowers is during the Cretaceous period, some
20 million years later (132 Ma).[178]
Extinctions
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.

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.[179]

The third mass extinction was the Permian-Triassic, or the Great Dying, event. The 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[180]
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 these or other proposed Permian-
Triassic boundary craters are real impact craters or even contemporary 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.[181][182]

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.[183]

The fifth and most recent mass extinction was the Cretaceous-Paleogene extinction event. In 66 Ma, a 10-
kilometer (6.2 mi) asteroid struck Earth just off the Yucatán Peninsula—somewhere in the southwestern
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,[184] marking the end of the Cretaceous period and Mesozoic era.

Diversification 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.[185]
By the early Paleocene Earth recovered from the extinction, and mammalian diversity increased.
Creatures like Ambulocetus took to the oceans to eventually evolve into whales,[186] whereas some
creatures, like primates, took to the trees.[187] 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 savanna began to predominate much of the landscape, and mammals such as
Andrewsarchus rose up to become the largest known terrestrial predatory mammal ever,[188] and early
whales like Basilosaurus took control of the seas.

The evolution of grasses brought a remarkable change to 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.[189] The Tethys Sea
was closed off by the collision of Africa and Europe.[190]
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.[191] Various species migrated south, leading to
the presence in South America of llamas, the spectacled bear, kinkajous and jaguars.

Three million years ago saw the start of the Pleistocene epoch, which featured dramatic climatic changes
due to the ice ages. The ice ages led to the evolution and expansion of modern man in Saharan Africa.
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 various water bodies 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 end
of the last ice age coincided with the expansion of man and a massive die out of ice age mega-fauna. This
extinction is nicknamed "the Sixth Extinction".

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.[101][125]: 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.[125]: 95–99 Brain size increased rapidly, and by
2 Ma, the first animals classified in the genus Homo
had appeared.[167]: 300 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
An artist's impression of ice age Earth at glacial
forms.[125]: 100–101 maximum.

The ability to control fire

probably began in Homo
erectus (or Homo ergaster), Hominin timeline
probably at least 0— ← Modern humans
[192] Homo sapiens ←
← Earliest clothes
790,000 years ago but Earliest rock art
P Denisovans
perhaps as early as – l Neanderthals
[125]: 67 H. heidelbergensis
1.5 Ma. The use and e
discovery of controlled fire −1 — i H. erectus
s
may even predate Homo t (H. antecessor)
erectus. Fire was possibly –o (H. ergaster)
c (Au. sediba) ← Earliest fire / cooking
used by the early Lower e ← Earliest language
−2 — n
Paleolithic (Oldowan) e H. habilis ← Dispersal beyond Africa
(H. rudolfensis)
–
(Au. garhi)
← Earliest sign of
 a est s g o
hominid Homo habilis or −3 — Homo
Australopithecus
strong australopithecines (Au. africanus) ← Earliest stone tools
–
such as Paranthropus.[193] P (Au. afarensis) P
l (Au. anamensis) a
It is more difficult to −4 — i
o r
establish the origin of c Ardipithecus H
← Earliest sign of
a
–e
language; it is unclear n
(Ar. ramidus) n Australopithecus
o
whether Homo erectus e t
−5 —
could speak or if that Hominini m
h
r
capability had not begun – i
until Homo sapiens.[125]: 67 o
(Ar. kadabba) np ← Earliest sign of
Ardipithecus
As brain size increased, −6 — Orrorin u
babies were born earlier, i
(O. praegens) s
before their heads grew too – (O. tugenensis) d
large to pass through the
M
pelvis. As a result, they −7 — i Sahelanthropus s ← Earliest bipedal
exhibited more plasticity, o
Graecopithecus
thus possessing an –c
e Oreopithecus
increased capacity to learn n
and requiring a longer −8 — e Chororapithecus

period of dependence. Sivapithecus

– ← Chimpanzee split
Social skills became more
Ouranopithecus
complex, language became (Ou. turkae)
−9 — ← Gorilla split
more sophisticated, and (Ou. macedoniensis)
tools became more – Samburupithecus
elaborate. This contributed Nakalipithecus
to further cooperation and −10 — ← Earlier apes
intellectual (million years ago)

development.[195]: 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.[196]

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.[197]: 17 However, evidence of more sophisticated beliefs,
such as the early Cro-Magnon cave paintings (probably with
magical or religious significance)[197]: 17–19 did not appear until
32,000 years ago.[198] Cro-Magnons also left behind stone
figurines such as Venus of Willendorf, probably also signifying
A reconstruction of human history religious belief.[197]: 17–19 By 11,000 years ago, Homo sapiens had
based on fossil data.[194] reached the southern tip of South America, the last of the
uninhabited continents (except for Antarctica, which remained
undiscovered until 1820 AD).[199] Tool use and communication
continued to improve, and interpersonal relationships became more intricate.
Human history
Throughout more than 90% of its history, Homo sapiens lived in
small bands as nomadic hunter-gatherers.[195]: 8 As language
became more complex, the ability to remember and communicate
information resulted in a new replicator: the meme.[200] 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.[201] 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.[202] However, among those civilizations
that did adopt agriculture, the relative stability and increased
Vitruvian Man by Leonardo da Vinci
productivity provided by farming allowed the population to epitomizes the advances in art and
expand. science seen during the
Renaissance.
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.[195]: 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.[195]: 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. During this period, famous Hindu texts known as vedas came in existence in Indus valley
civilization. This civilization developed in warfare, arts, science, mathematics and architecture. The
fundamentals of Western civilization were largely shaped in Ancient Greece, with the world's first
democratic government and major advances in philosophy and science, and in Ancient Rome with
advances in law, government, and engineering.[203]

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, and since at least the
4th century Christianity has played a prominent role in the shaping of Western
civilization.[204][205][206][207][208][209][210][211] 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.[212] 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.[213]

In the 14th century, the Renaissance began in Italy with advances in religion, art, and science.[195]: 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 scientific and 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).[195]: 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.

See also
Chronology of the universe – History and Geological history of Earth – The
future of the universe sequence of major geological events in
Earth phase – Phases of Earth as seen Earth's past
from the Moon Global catastrophic risk – Hypothetical
Evolutionary history of life global-scale disaster risk
Future of Earth – Long-term extrapolated Timeline of the evolutionary history of life
geological and biological changes of planet Timeline of natural history
Earth

Notes
1. Pluto's satellite Charon is relatively larger,[44] but Pluto is defined as a dwarf planet.[45]

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Further reading
Dalrymple, G.B. (1991). The Age of the Earth. California: Stanford University Press.
ISBN 978-0-8047-1569-0.
Dalrymple, G. Brent (2001). "The age of the Earth in the twentieth century: a problem
(mostly) solved" (http://sp.lyellcollection.org/content/190/1/205.abstract). Geological Society
of London, Special Publications. 190 (1): 205–221. Bibcode:2001GSLSP.190..205D (https://
ui.adsabs.harvard.edu/abs/2001GSLSP.190..205D). doi:10.1144/GSL.SP.2001.190.01.14 (h
ttps://doi.org/10.1144%2FGSL.SP.2001.190.01.14). S2CID 130092094 (https://api.semantic
scholar.org/CorpusID:130092094). Retrieved 2012-04-13.
Dawkins, Richard (2004). The Ancestor's Tale: A Pilgrimage to the Dawn of Life. Boston:
Houghton Mifflin Company. ISBN 978-0-618-00583-3.
Gradstein, F.M.; Ogg, James George; Smith, Alan Gilbert, eds. (2004). A Geological Time
Scale 2004. Reprinted with corrections 2006. Cambridge University Press. ISBN 978-0-521-
78673-7.
Gradstein, Felix M.; Ogg, James G.; van Kranendonk, Martin (2008). On the Geological
Time Scale 2008 (https://web.archive.org/web/20121028022719/http://www.nysm.nysed.go
v/nysgs/resources/images/geologicaltimescale.pdf) (PDF) (Report). International
Commission on Stratigraphy. Fig. 2. Archived from the original (http://www.nysm.nysed.gov/
nysgs/resources/images/geologicaltimescale.pdf) (PDF) on 28 October 2012. Retrieved
20 April 2012.
Levin, H.L. (2009). The Earth through time (9th ed.). Saunders College Publishing.
ISBN 978-0-470-38774-0.
Lunine, Jonathan I. (1999). Earth: evolution of a habitable world. United Kingdom:
Cambridge University Press. ISBN 978-0-521-64423-5.
McNeill, Willam H. (1999) [1967]. A World History (4th ed.). New York: Oxford University
Press. ISBN 978-0-19-511615-1.
Melosh, H. J.; Vickery, A. M. & Tonks, W. B. (1993). Impacts and the early environment and
evolution of the terrestrial planets, in Levy, H. J. & Lunine, Jonathan I. (eds.): Protostars and
Planets III, University of Arizona Press, Tucson, pp. 1339–1370.
Stanley, Steven M. (2005). Earth system history (2nd ed.). New York: Freeman. ISBN 978-0-
7167-3907-4.
Stern, T.W.; Bleeker, W. (1998). "Age of the world's oldest rocks refined using Canada's
SHRIMP: The Acasta Gneiss Complex, Northwest Territories, Canada". Geoscience
Canada. 25: 27–31.
Wetherill, G.W. (1991). "Occurrence of Earth-Like Bodies in Planetary Systems". Science.
253 (5019): 535–538. Bibcode:1991Sci...253..535W (https://ui.adsabs.harvard.edu/abs/199
1Sci...253..535W). doi:10.1126/science.253.5019.535 (https://doi.org/10.1126%2Fscience.2
53.5019.535). PMID 17745185 (https://pubmed.ncbi.nlm.nih.gov/17745185).
S2CID 10023022 (https://api.semanticscholar.org/CorpusID:10023022).
External links
Davies, Paul. "Quantum leap of life (https://www.theguardian.com/technology/2005/dec/20/c
omment.science)". The Guardian. 2005 December 20. – discusses speculation on the role
of quantum systems in the origin of life
Evolution timeline (http://www.johnkyrk.com/evolution.html) (uses Flash Player). Animated
story of life shows everything from the big bang to the formation of Earth and the
development of bacteria and other organisms to the ascent of man.
25 biggest turning points in Earth History (https://www.bbc.com/earth/bespoke/story/201501
23-earths-25-biggest-turning-points/) BBC
Evolution of the Earth (http://historystack.com/30_Major_Events_in_History_of_the_Earth).
Timeline of the most important events in the evolution of Earth.
The Earth's Origins (https://www.bbc.co.uk/programmes/p00547hl) on In Our Time at the
BBC
Ageing the Earth (https://www.bbc.co.uk/programmes/p005493g), BBC Radio 4 discussion
with Richard Corfield, Hazel Rymer & Henry Gee (In Our Time, Nov. 20, 2003)

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