History of Earth

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,
Ma means "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 the 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 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.

Contents
Eons
Geologic time scale
Solar System formation
Hadean and Archean Eons
Formation of the Moon
First continents
Oceans and atmosphere
Origin of life
Proterozoic Eon
Oxygen revolution
Snowball Earth
Emergence of eukaryotes
Supercontinents in the Proterozoic
Late Proterozoic climate and life
Phanerozoic Eon
Tectonics, paleogeography and climate
Cambrian explosion
Colonization of land
Evolution of tetrapods
Extinctions
Diversification of mammals
Human evolution
See also
Notes
 References
Further reading
External links

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)
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 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 have
Archean
2,500 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 and
2,500–
Proterozoic possibly earlier forms of fungi form around this time. The early and late phases of this
541
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 the 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 expands to
541– land and familiar forms of plants, animals and fungi begin appearing, including annelids,
Phanerozoic
present 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 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 five 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, the most recent period is
expanded in the fourth timeline, and the most recent epoch is expanded in the fifth timeline.
 Millions of Years (1st, 2nd, 3rd, and 4th)
Thousands of years (5th)

Solar System formation

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
elements ejected by supernovae. About
4.5 Ga, the nebula began a contraction that
may have been triggered by the shock wave An artist's rendering of a protoplanetary disk
from 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 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 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, Artist's conception of Hadean Eon
at the end of the Hadean.[40] In addition, volcanism was severe Earth, when it was much hotter and
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 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

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 Artist's impression of the enormous
phenomena, one is widely accepted: The giant impact hypothesis collision that probably formed the
proposes that the Moon originated after a body the size of Mars Moon
(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 non-avian 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

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] Geologic map of North America,
color-coded by age. From most
The initial crust, formed when the Earth's surface first solidified, recent to oldest, age is indicated by
totally disappeared from a combination of this fast Hadean plate yellow, green, blue, and red. The
tectonics and the intense impacts of the Late Heavy Bombardment. reds and pinks indicate rock from the
However, it is thought that it was basaltic in composition, like Archean.
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

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
Graph showing range of estimated partial pressure
the impact which created the Moon, the molten Earth
of atmospheric oxygen through geologic time [61]
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.[63]: 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 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

One of the reasons for

Life timeline
interest in the early Quaternary ice age*
atmosphere and ocean is 0— Flowers Birds Primates ←
← Earliest apes / humans
P
that they form the –h Mammals
a Dinosaurs
conditions under which life —n
e P ← Karoo ice age*
first arose. There are many –r l Arthropods Molluscs ← Earliest tetrapods
models, but little o a ← Andean glaciation*
−500 — z n ← Cambrian explosion
consensus, on how life t
– oi s ← Ediacaran biota
emerged from non-living ← Cryogenian ice age*
chemicals; chemical —c ← Earliest animals
systems created in the – ← Earliest plants
laboratory fall well short of −1000 —
the minimum complexity Multicellular life
–
for a living —
organism.[70][71] –r
P

The first step in the −1500 — ot ← Earliest fungi

emergence of life may – er ← Earliest multicellular life
have been chemical —o
z
reactions that produced –o Eukaryotes
many of the simpler i
−2000 — c
organic compounds,
–
including nucleobases and
amino acids, that are the — ← Huronian glaciation*
← Atmospheric oxygen
building blocks of life. An –
experiment in 1953 by −2500 —
Stanley Miller and Harold –
Urey showed that such —
molecules could form in an Photosynthesis ← Pongola glaciation*
–
atmosphere of water,
methane, ammonia and −3000 —
A
hydrogen with the aid of –r
c
sparks to mimic the effect —h
of lightning.[72] Although –ea
atmospheric composition −3500 — n ← Earliest oxygen
was probably different
–
from that used by Miller
and Urey, later experiments — Single-celled life
with more realistic – ← LHB meteorites
compositions also managed −4000 — ← Earliest life
to synthesize organic –H
a
molecules.[73] Computer —d
Water
e
simulations show that –a
n ← Earliest water
extraterrestrial organic
← Earth
−4500 — formed
molecules could have (4540 mya)
formed in the (million years ago) *Ice Ages
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

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.[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

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

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

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]

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 The replicator in virtually all
absorbing additional lipids and dividing. The formation of the earliest cells known life is
may have been aided by similar processes.[96] deoxyribonucleic acid. DNA
is far more complex than the
A similar hypothesis presents self-replicating iron-rich clays as the original replicator and its
progenitors of nucleotides, lipids and amino acids.[97] replication systems are
highly elaborate.

Last universal common 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]
Cross-section through a
liposome
Proterozoic Eon
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.

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.[100]: 377

Most of the life that covers the surface of the Earth depends
directly or indirectly on photosynthesis. The most common form, Lithified stromatolites on the shores
oxygenic photosynthesis, turns carbon dioxide, water, and sunlight of Lake Thetis, Western Australia.
into food. It captures the energy of sunlight in energy-rich Archean stromatolites are the first
molecules such as ATP, which then provide the energy to make direct fossil traces of life on Earth.
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 extremophile organisms
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 A banded iron formation from the
the appearance of life. The timing of oxygenic photosynthesis is 3.15 Ga Moodies Group, Barberton
more controversial; it had certainly appeared by about 2.4 Ga, but Greenstone Belt, South Africa. Red
some researchers put it back as far as 3.2 Ga.[101] The latter layers represent the times when
"probably increased global productivity by at least two or three oxygen was available; gray layers
orders of magnitude".[103][104] Among the oldest remnants of were formed in anoxic
oxygen-producing lifeforms are fossil stromatolites.[103][104][61] circumstances.

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
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 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.[110][111]

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,[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. The earliest fossils possessing features
typical of fungi date to the Paleoproterozoic era, some 2.4 ago;
Chloroplasts in the cells of a moss
these multicellular benthic organisms had filamentous structures
capable of anastomosis.[113]

Around this time, the first proto-mitochondrion was formed. A bacterial cell related to today's
Rickettsia,[114] 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.[115]

A similar event occurred with photosynthetic cyanobacteria[116] entering large heterotrophic cells and
becoming chloroplasts.[105]: 60–61 [117]: 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,[118][119] though none of them are widely
accepted.[120]

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.[121][122] 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[123], the first multicellular plants emerged, probably green algae.[124] Possibly by around 900
Ma[117]: 488 true multicellularity had also evolved in animals.

At first, it probably resembled today's sponges, which have totipotent cells that allow a disrupted organism
to reassemble itself.[117]: 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.

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.[125]: 370

Throughout the history of the 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.[125]: 370 [126]
Rodinia may have been preceded by Early-Middle Proterozoic
continents called Nuna and Columbia.[125]: 374 [127][128]

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.[129]: 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).[125]: 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.[130]

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.[131] The intensity and mechanism of both glaciations are
still under investigation and harder to explain than the early
Proterozoic Snowball Earth.[132] Most paleoclimatologists think
the cold episodes were linked to the formation of the
supercontinent Rodinia.[133] Because Rodinia was centered on the
equator, rates of chemical weathering increased and carbon dioxide
(CO2 ) was taken from the atmosphere. Because CO2 is an A 580 million year old fossil of
important greenhouse gas, climates cooled globally. In the same Spriggina floundensi, an animal from
way, during the Snowball Earths most of the continental surface the Ediacaran period. Such life forms
was covered with permafrost, which decreased chemical could have been ancestors to the
weathering again, leading to the end of the glaciations. An many new forms that originated in
the Cambrian Explosion.
alternative hypothesis is that enough carbon dioxide escaped
through volcanic outgassing that the resulting greenhouse effect
raised global temperatures.[133] Increased volcanic activity resulted
from the break-up of Rodinia at about the same time.

The Cryogenian period was followed by the Ediacaran period, which was characterized by a rapid
development of new multicellular lifeforms.[134] 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.[135] 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.

Phanerozoic Eon
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.[136]

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.

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.[137] It ended with the
Cretaceous–Paleogene extinction event that wiped out the dinosaurs..
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 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.[138] 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.

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.[139] 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.

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
extinctions took place, in which many brachiopods, trilobites,
existed from about 300 to 180 Ma.
Bryozoa and corals disappeared. These marine species could
The outlines of the modern
probably not contend with the decreasing temperature of the sea
continents and other landmasses are
water.[140] indicated on this map.

The continents Laurentia and Baltica collided between 450 and

400 Ma, during the Caledonian Orogeny, to form Laurussia (also
known as Euramerica).[141] 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.

Cambrian explosion

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 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 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.

During the Cambrian, the first vertebrate animals, among them the
first fishes, had appeared.[117]: 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 Trilobites first appeared during the
Ordovician. The colonisation of new niches resulted in massive Cambrian period and were among the
body sizes. In this way, fishes with increasing sizes evolved during most widespread and diverse groups
the early Paleozoic, such as the titanic placoderm Dunkleosteus, of Paleozoic organisms.
which could grow 7 meters (23 ft) long.

The diversity of life forms did not increase greatly because of a series of mass extinctions that define
widespread biostratigraphic units called biomeres.[142] After each extinction pulse, the continental shelf
regions were repopulated by similar life forms that may have been evolving slowly elsewhere.[143] By the
late Cambrian, the trilobites had reached their greatest diversity and dominated nearly all fossil
assemblages.[144]: 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[145] had probably colonized the land as early as 2.6
Ga[146] 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 Artist's conception of Devonian flora
apart a short 50 million years later. [147] Fish, the earliest
vertebrates, evolved in the oceans around 530 Ma.[117]: 354 A
major extinction event occurred near the end of the Cambrian period,[148] which ended 488 Ma.[149]

Several hundred million years ago, plants (probably resembling algae) and fungi started growing at the
edges of the water, and then out of it.[150]: 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.[151] 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,[152] 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.[153]

Evolution of tetrapods
At the end of the Ordovician period, 443 Ma,[22] additional
extinction events occurred, perhaps due to a concurrent ice
age.[140] Around 380 to 375 Ma, the first tetrapods evolved from
fish.[154] Fins evolved to become limbs that the first tetrapods used
to lift their heads out of the water to breathe air. This would let Tiktaalik, a fish with limb-like fins
them live in oxygen-poor water, or pursue small prey in shallow and a predecessor of tetrapods.
water.[154] They may have later ventured on land for brief periods. Reconstruction from fossils about
Eventually, some of them became so well adapted to terrestrial life 375 million years old.
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.[155] Plants evolved seeds, which dramatically accelerated their spread on land, around this time
(by approximately 360 Ma).[156][157]

About 20 million years later (340 Ma[117]: 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[117]: 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.[158] The Triassic–Jurassic extinction event at
200 Ma spared many of the dinosaurs,[22][159] 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. [117]: 169

The boundary between avian and non-avian dinosaurs is not clear,

Dinosaurs were the dominant
but Archaeopteryx, traditionally considered one of the first birds, terrestrial vertebrates throughout
lived around 150 Ma.[160] most of the Mesozoic

The earliest evidence for the angiosperms evolving flowers is

during the Cretaceous period, some 20 million years later (132 Ma).[161]

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.

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[162] 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.[163][164]

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.

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 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,[165] 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.[166] 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,[167] whereas some creatures, like
primates, took to the trees.[168] 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,[169] and early whales like Basilosaurus
took control of the seas.

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.[170] The Tethys Sea
was closed off by the collision of Africa and Europe.[171]

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.[172] 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 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 is nicknamed "the Sixth Extinction".

Human evolution
A small African ape living Hominin timeline
around 6 Ma was the last 0— ←
← Modern humans
animal whose descendants –P
Homo sapiens Earliest clothes
would include both −0.5 — l
Neanderthals,Denisovans
modern humans and their e Homo heidelbergensis
– i
closest relatives, the
[117]: 100–101 −1 — s
chimpanzees.
– t
Only two branches of its o Homo erectus
−1.5 — c
family tree have surviving ← Earliest fire / cooking
– e
descendants. Very soon n
after the split, for reasons −2 — e
Homo habilis ← Dispersal beyond Africa
that are still unclear, apes in –
one branch developed the −2.5 —
ability to walk –
upright. [117]: 95–99 Brain −3 — Australopithecus
size increased rapidly, and –P
l ← Stone tools
by 2 Ma, the first animals −3.5 — i
classified in the genus – o
Homo had −4 — c ← Earliest bipedal
[150]: 300 e
appeared. Of – n H
Ardipithecus
course, the line between −4.5 — e
different species or even – o
genera is somewhat −5 — m
arbitrary as organisms – Hominini
continuously change over −5.5 — i
generations. Around the –
same time, the other branch n
−6 — Orrorin
split into the ancestors of i
–
the common chimpanzee
−6.5 — d
and the ancestors of the
–
bonobo as evolution M Sahelanthropus s
−7 — i
continued simultaneously
in all life – o
forms. [117]: 100–101 −7.5 — c
e
– n
The ability to control fire −8 — e Oreopithecus
probably began in Homo –
erectus (or Homo −8.5 — ← Chimpanzee split
ergaster), probably at least –
790,000 years ago [173] but −9 — Ouranopithecus ← Gorilla split
perhaps as early as –
1.5 Ma.[117]: 67 The use −9.5 —
and discovery of controlled – Nakalipithecus
fire may even predate −10 — ← Earlier apes
Homo erectus. Fire was (million years ago)
possibly used by the early
Lower Paleolithic
(Oldowan) hominid Homo habilis or strong australopithecines such as Paranthropus.[174]

It is more difficult to establish the origin of language; it is unclear whether Homo erectus could speak or if
that capability had not begun until Homo sapiens.[117]: 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.[176]: 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.[177]

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.[178]: 17 However, evidence of more sophisticated beliefs,
such as the early Cro-Magnon cave paintings (probably with
magical or religious significance)[178]: 17–19 did not appear until
A reconstruction of human history
32,000 years ago.[179] Cro-Magnons also left behind stone
based on fossil data.[175]
figurines such as Venus of Willendorf, probably also signifying
religious belief.[178]: 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).[180] 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.[176]: 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.[181] 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.[182] 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.[183] However, among those civilizations that did adopt
agriculture, the relative stability and increased productivity
Vitruvian Man by Leonardo da Vinci
provided by farming allowed the population to expand.
epitomizes the advances in art and
Agriculture had a major impact; humans began to affect the science seen during the
Renaissance.
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.[176]: 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.[176]: 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 in architect. The fundamentals of Western
civilization were largely shaped in Ancient Greece, with the world's first democratic government and major
advances in philosophy, science. Ancient Rome in law, government, and engineering.[184] 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.[185] 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.

In the 14th century, the Renaissance began in Italy with advances in religion, art, and science.[176]: 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).[176]: 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 democratic capitalist United States and the socialist Soviet Union became
the world's dominant superpowers for a time, and they held an ideological, 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.

Recent events

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 environmentalism have
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.

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 the two Voyager spacecraft) having left the Solar System. Five space agencies,
representing over fifteen countries,[186] have worked together to build the International Space Station.
Aboard it, there has been a continuous human presence in space
since 2000.[187] 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.

Astronaut Bruce McCandless II

outside of the Space Shuttle
Challenger in 1984

See also
2021 in the environment and Geological history of Earth – The
environmental sciences sequence of major geological events in
Chronology of the universe – History and Earth's past
future of the universe Global catastrophic risk – Hypothetical
Detailed logarithmic timeline future events that could damage human
Earth phase – Phases of the Earth as seen well-being globally
from the Moon Timeline of the evolutionary history of life –
Evolutionary history of life Current scientific theory outlining the major
events during the development of life
Future of Earth – Long term extrapolated
geological and biological changes Timeline of natural history – Wikipedia list
article

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,
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 (http
s://doi.org/10.1144%2FGSL.SP.2001.190.01.14). S2CID 130092094 (https://api.semanticsch
olar.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.gov/nysg
s/resources/images/geologicaltimescale.pdf) (PDF) (Report). International Commission on
Stratigraphy. Fig. 2. Archived from the original (http://www.nysm.nysed.gov/nysgs/resources/i
mages/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/1991
Sci...253..535W). doi:10.1126/science.253.5019.535 (https://doi.org/10.1126%2Fscience.25
3.5019.535). 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 Shockwave). Animated
story of life shows everything from the big bang to the formation of the Earth and the
 development of bacteria and other organisms to the ascent of man.
25 biggest turning points in Earth History (http://www.bbc.com/earth/bespoke/story/2015012
3-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 the 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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