   #copyright

History of Earth

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   The planet Earth, photographed in the year 1972.
   Enlarge
   The planet Earth, photographed in the year 1972.

   The history of Earth covers approximately 4 billion years
   (4,567,000,000 years), from Earth’s formation out of the solar nebula
   to the present. This article presents a broad overview, summarizing the
   leading scientific theories. Due to the difficulty of comprehending
   very large amounts of time, the analogy of a single 24-hour period will
   be used, beginning exactly 4.567 billion years ago, at the formation of
   Earth, and ending now. Each second of this period represents
   approximately 53,000 years (or 53 millennia). The Big Bang and origin
   of the universe, estimated at occurring 13.7 billion years ago, is
   equivalent to taking place almost three days ago—two whole days before
   our clock began to tick.

Origin

   Artist’s impression of a protoplanetary disc forming around a binary
   star system.
   Enlarge
   Artist’s impression of a protoplanetary disc forming around a binary
   star system.

   Earth formed as part of the birth of the solar system: what eventually
   became the solar system initially existed as a large, rotating cloud of
   dust and gas. It was composed of hydrogen and helium produced in the
   Big Bang, as well as heavier elements produced by stars long gone.
   Then, about 4.6 billion years ago (fifteen to thirty minutes before our
   imaginary clock started), a nearby star probably became a supernova.
   The explosion sent a shock wave toward the solar nebula and caused it
   to contract. As the cloud continued to rotate, gravity and inertia
   flattened the cloud into a proto- planetary disc, perpendicular to its
   axis of rotation. Most of the mass concentrated in the middle and began
   to heat up. The impossibility of kinetic heat, produced by the infall
   of matter escaping caused the centre to heat up sufficiently to enable
   the centre of the concentration to produce its own internal heat source
   through nuclear fusion of hydrogen into helium, starting as a T Tauri
   star, our early sun. Meanwhile, as gravity caused matter to condense
   around dust particles, the rest of the disc started to break up into
   rings. Small fragments collided and became larger fragments. These
   included one collection approximately 150 million kilometers from the
   centre: Earth. As the Sun condensed and heated, fusion began, and the
   resulting T Tauri solar wind cleared out most of the material in the
   disc that had not already condensed into larger bodies.

Moon

   Animation (not to scale) of Theia forming in Earth’s L5 point and then,
   perturbed by gravity, colliding to help form the moon. The animation
   progresses in one-year steps making Earth appear not to move. The view
   is of the south pole.
   Enlarge
   Animation (not to scale) of Theia forming in Earth’s L[5] point and
   then, perturbed by gravity, colliding to help form the moon. The
   animation progresses in one-year steps making Earth appear not to move.
   The view is of the south pole.

   The origin of the Moon is still uncertain, although much evidence
   exists for the giant impact hypothesis. Earth may not have been the
   only planet forming 150 million kilometers from the Sun. It is
   hypothesized that another collection occurred 150 million kilometers
   from both the Sun and the Earth, at the fourth or fifth Lagrangian
   point. This planet, named Theia, is thought to have been smaller than
   the current Earth, probably about the size and mass of Mars. Its orbit
   may at first have been stable but destabilized as Earth increased its
   mass by the accretion of more and more material. Theia swung back and
   forth relative to Earth until, finally, an estimated 4.533 billion
   years ago (perhaps 12:05 a.m. on our clock), it collided at a low,
   oblique angle. The low speed and angle were not enough to destroy
   Earth, but a large portion of its crust was ejected. Heavier elements
   from Theia sank to Earth’s core, while the remaining material and
   ejecta condensed into a single body within a couple of weeks. Under the
   influence of its own gravity, this became a more spherical body: the
   Moon. The impact is also thought to have changed Earth’s axis to
   produce the large 23.5° axial tilt that is responsible for Earth’s
   seasons. (A simple, ideal model of the planets’ origins would have
   axial tilts of 0° with no recognizable seasons.) It may also have sped
   up Earth’s rotation and initiated the planet’s plate tectonics.

The Hadean eon

   Volcanic eruptions would have been common in Earth's early days.
   Enlarge
   Volcanic eruptions would have been common in Earth's early days.

   The early Earth, during the very early Hadean eon, was very different
   from the world known today. There were no oceans and no oxygen in the
   atmosphere. It was bombarded by planetoids and other material left over
   from the formation of the solar system. This bombardment, combined with
   heat from radioactive breakdown, residual heat, and heat from the
   pressure of contraction, caused the planet at this stage to be fully
   molten. Heavier elements sank to the centre while lighter ones rose to
   the surface, producing Earth's various layers (see " Structure of the
   Earth"). Earth's early atmosphere would have comprised surrounding
   material from the solar nebula, especially light gases such as hydrogen
   and helium, but the solar wind and Earth's own heat would have driven
   off this atmosphere.

   This changed when Earth was about 40% its present radius, and
   gravitational attraction allowed the retention of an atmosphere which
   included water. Temperatures plummeted and the crust of the planet was
   accumulated on a solid surface, with areas melted by large impacts on
   the scale of decades to hundreds of years between impact. Large impacts
   would have caused localized melting and partial differentiation, with
   some lighter elements on the surface or released to the moist
   atmosphere.

   The surface cooled quickly, forming the solid crust within 150 million
   years (around 12:45 a.m. on our clock). From 4 to 3.8 billion years ago
   (around 3 to 4 a.m.), Earth underwent a period of heavy asteroidal
   bombardment. Steam escaped from the crust while more gases were
   released by volcanoes, completing the second atmosphere. Additional
   water was imported by bolide collisions, probably from asteroids
   ejected from the outer asteroid belt under the influence of Jupiter's
   gravity. The planet cooled. Clouds formed. Rain gave rise to the oceans
   within 750 million years (3.8 billion years ago, around 4:00 a.m. on
   our clock), but probably earlier. (Recent evidence suggests the oceans
   may have begun forming by 4.2 billion years ago-1:50 a.m. on our
   clock.) The new atmosphere probably contained ammonia, methane, water
   vapor, carbon dioxide, and nitrogen, as well as smaller amounts of
   other gases. Any free oxygen would have been bound by hydrogen or
   minerals on the surface. Volcanic activity was intense and, without an
   ozone layer to hinder its entry, ultraviolet radiation flooded the
   surface.

Beginnings of life

   The replicator in virtually all known life is deoxyribonucleic acid.
   DNA is far more complex than the original replicator and its
   replication systems are highly elaborate.
   Enlarge
   The replicator in virtually all known life is deoxyribonucleic acid.
   DNA is far more complex than the original replicator and its
   replication systems are highly elaborate.

   The details of the origin of life are unknown, though the broad
   principles have been established. It has been proposed that life, or at
   least organic components, may have arrived on Earth from space (see “
   Panspermia”), while others argue that terrestrial origins are more
   likely. The mechanisms by which life would initially arise are
   nevertheless held to be similar. If life arose on Earth, the timing of
   this event is highly speculative—perhaps it arose around 4 billion
   years ago (around 3:00 a.m. on our clock). Somehow, in the energetic
   chemistry of early Earth, a molecule (or even something else) gained
   the ability to make copies of itself–the replicator. The nature of this
   molecule is unknown, its function having long since been superseded by
   life’s current replicator, DNA. In making copies of itself, the
   replicator did not always perform accurately: some copies contained an
   “error.” If the change destroyed the copying ability of the molecule,
   there could be no more copies, and the line would “die out.” On the
   other hand, a few rare changes might make the molecule replicate faster
   or better: those “strains” would become more numerous and “successful.”
   As choice raw materials (“food”) became depleted, strains which could
   exploit different materials, or perhaps halt the progress of other
   strains and steal their resources, became more numerous.

   Several different models have been proposed explaining how a replicator
   might have developed. Different replicators have been posited,
   including organic chemicals such as modern proteins of nucleic acids,
   phospholipids, crystals, or even quantum systems. There is currently no
   method of determining which of these models, if any, closely fits the
   origin of life on Earth. One of the older theories, and one which has
   been worked out in some detail, will serve as an example of how this
   might occur. The high energy from volcanoes, lightning, and ultraviolet
   radiation could help drive chemical reactions producing more complex
   molecules from simple compounds such as methane and ammonia. Among
   these were many of the relatively simple organic compounds that are the
   building blocks of life. As the amount of this “ organic soup”
   increased, different molecules reacted with one another. Sometimes more
   complex molecules would result—perhaps clay provided a framework to
   collect and concentrate organic material. The presence of certain
   molecules could speed up a chemical reaction. All this continued for a
   very long time, with reactions occurring more or less at random, until
   by chance there arose a new molecule: the replicator. This had the
   bizarre property of promoting the chemical reactions which produced a
   copy of itself, and evolution proper began. Other theories posit a
   different replicator. In any case, DNA took over the function of the
   replicator at some point; all known life (with the exception of some
   viruses and prions) use DNA as their replicator, in an almost identical
   manner (see genetic code).

The first cell

   A small section of a cell membrane. This modern cell membrane is far
   more sophisticated than the original simple phospholipid bilayer (the
   small blue spheres with two tails). Proteins and carbohydrates serve
   various functions in regulating the passage of material through the
   membrane and in reacting to the environment.
   Enlarge
   A small section of a cell membrane. This modern cell membrane is far
   more sophisticated than the original simple phospholipid bilayer (the
   small blue spheres with two tails). Proteins and carbohydrates serve
   various functions in regulating the passage of material through the
   membrane and in reacting to the environment.

   Modern life has its replicating material packaged neatly inside a
   cellular membrane. It is easier to understand the origin of the cell
   membrane than the origin of the replicator, since the phospholipid
   molecules that make up a cell membrane will often form a bilayer
   spontaneously when placed in water. Under certain conditions, many such
   spheres can be formed (see “ The bubble theory”). It is not known
   whether this process preceded or succeeded the origin of the replicator
   (or perhaps it was the replicator). The prevailing theory is that the
   replicator, perhaps RNA by this point (the RNA world hypothesis), along
   with its replicating apparatus and maybe other biomolecules, had
   already evolved. Initial protocells may have simply burst when they
   grew too large; the scattered contents may then have recolonized other
   “bubbles.” Proteins that stabilized the membrane, or that later
   assisted in an orderly division, would have promoted the proliferation
   of those cell lines. RNA is a likely candidate for an early replicator
   since it can both store genetic information and catalyze reactions. At
   some point DNA took over the genetic storage role from RNA, and
   proteins known as enzymes took over the catalysis role, leaving RNA to
   transfer information and modulate the process. There is increasing
   belief that these early cells may have evolved in association with
   underwater volcanic vents known as “ black smokers”. or even hot, deep
   rocks. However, it is believed that out of this multiplicity of cells,
   or protocells, only one survived. Current evidence suggests that the
   last universal common ancestor lived during the early Archean eon,
   perhaps roughly 3.5 billion years ago (5:30 a.m. on our imaginary
   clock) or earlier.^, This “LUCA” cell is the ancestor of all cells and
   hence all life on Earth. 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
   all 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 in lateral gene transfer.

Photosynthesis and oxygen

   The harnessing of the sun’s energy led to several major changes in life
   on Earth.
   Enlarge
   The harnessing of the sun’s energy led to several major changes in life
   on Earth.

   It is likely that the initial cells were all heterotrophs, using
   surrounding organic molecules (including those from other cells) as raw
   material and an energy source. As the food supply diminished, a new
   strategy evolved in some cells. Instead of relying on the diminishing
   amounts of free-existing organic molecules, these cells adopted
   sunlight as an energy source. Estimates vary, but by about 3 billion
   years ago (around 8:00 a.m. on our clock), something similar to modern
   photosynthesis had probably developed. This made the sun’s energy
   available not only to autotrophs but also to the heterotrophs that
   consumed them. Photosynthesis used the plentiful carbon dioxide and
   water as raw materials and, with the energy of sunlight, produced
   energy-rich organic molecules ( carbohydrates).

   Moreover, oxygen was produced as a waste product of photosynthesis. At
   first it became bound up with limestone, iron, and other minerals.
   There is substantial proof of this in iron-oxide rich layers in
   geological strata that correspond with this time period. The oceans
   would have turned to a green colour while oxygen was reacting with
   minerals. When the reactions stopped, oxygen could finally enter the
   atmosphere. Though each cell only produced a minute amount of oxygen,
   the combined metabolism of many cells over a vast period of time
   transformed Earth’s atmosphere to its current state.

   This, then, is Earth’s third atmosphere. Some of the oxygen was
   stimulated by incoming 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 ultimately the
   land: without the ozone layer, ultraviolet radiation bombarding the
   surface would have caused unsustainable levels of mutation in exposed
   cells. Besides making large amounts of energy available to life-forms
   and blocking ultraviolet radiation, the effects of photosynthesis had a
   third, major, and world-changing impact. Oxygen was toxic; probably
   much life on Earth died out as its levels rose (the “ Oxygen
   Catastrophe”). Resistant forms survived and thrived, and some developed
   the ability to use oxygen to enhance their metabolism and derive more
   energy from the same food.

Endosymbiosis and the three domains of life

   Some of the pathways by which the various endosymbionts might have
   arisen.
   Enlarge
   Some of the pathways by which the various endosymbionts might have
   arisen.

   Modern taxonomy classifies life into three domains. The time of the
   origin of these domains are speculative. 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
   billion years ago (around 2:00 p.m. on our clock), 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 coming to light. Around this time
   period a bacterial cell related to today’s Rickettsia entered a larger
   prokaryotic cell. Perhaps the large cell attempted to ingest the
   smaller one but failed (maybe due to the evolution of prey defenses).
   Perhaps the smaller cell attempted to parasitize the larger one. In any
   case, the smaller cell survived inside the larger cell. Using oxygen,
   it was able to metabolize the larger cell’s waste products and derive
   more energy. Some of this surplus energy was returned to the host. The
   smaller cell replicated inside the larger one, and soon a stable
   symbiotic relationship developed. Over time the host cell acquired some
   of the genes of 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. Symbiosis
   developed between the larger cell and the population of smaller cells
   inside it to the extent that they are considered to have become a
   single organism, the smaller cells being classified as organelles
   called mitochondria. A similar event took place with photosynthetic
   cyanobacteria entering larger heterotrophic cells and becoming
   chloroplasts.^, Probably as a result of these changes, a line of cells
   capable of photosynthesis split off from the other eukaryotes some time
   before one billion years ago (around 6:00 p.m. on our clock). There
   were probably several such inclusion events, as the figure at right
   suggests. Besides the well-established endosymbiotic theory of the
   cellular origin of mitochondria and chloroplasts, it has been suggested
   that cells gave rise to peroxisomes, spirochetes gave rise to cilia and
   flagella, and that perhaps a DNA virus gave rise to the cell nucleus,^,
   though none of these theories are generally accepted. During this
   period, the supercontinent Columbia is believed to have existed,
   probably from around 1.8 to 1.5 billion years ago (2:30–4:00 p.m.); it
   is the oldest hypothesized supercontinent.

Multicellularity

   Volvox aureus is believed to be similar to the first multicellular
   plants.
   Enlarge
   Volvox aureus is believed to be similar to the first multicellular
   plants.

   Archaeans, bacteria, and eukaryotes continued to diversify and to
   become more sophisticated 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 billion
   years ago (6:15 p.m. on our clock), the supercontinent Rodinia was
   assembling. The plant, animal, and fungi lines had all split, though
   they still existed as solitary cells. Some of these lived in colonies,
   and gradually some 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 (around 7:00 p.m. on our clock), the first
   multicellular plants emerged, probably green algae. Possibly by around
   900 million years ago (7:15 p.m. on our clock), true multicellularity
   had also evolved in animals. At first it probably somewhat resembled
   that of today’s sponges, where all cells were totipotent and a
   disrupted organism could reassemble itself. As the division of labor
   became more complete in all lines of multicellular organisms, cells
   became more specialized and more dependent on each other; isolated
   cells would die. Many scientists believe that a very severe ice age
   began around 770 million years ago (7:56 p.m.), so severe that the
   surface of all the oceans completely froze (Snowball Earth).
   Eventually, after 20 million years (8:02 p.m.), enough carbon dioxide
   escaped through volcanic outgassing; the resulting greenhouse effect
   raised global temperatures. By around the same time, 750 million years
   ago, Rodinia began to break up.

Colonization of land

   For most of Earth’s history, there were no multicellular organisms on
   land. Parts of the surface may have vaguely resembled this view of
   Mars, one of Earth’s neighboring planets.
   Enlarge
   For most of Earth’s history, there were no multicellular organisms on
   land. Parts of the surface may have vaguely resembled this view of
   Mars, one of Earth’s neighboring planets.

   As we have already seen, the accumulation of oxygen in Earth’s
   atmosphere resulted in the formation of ozone, forming a layer that
   absorbed much of the sun’s ultraviolet radiation. As a result,
   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. Prokaryotes had likely colonized the land as early as 2.6
   billion years ago (10:17 a.m.), prior even to the origin of the
   eukaryotes. For a long time, the land remained barren of multicellular
   organisms. The supercontinent Pannotia formed around 600 million years
   ago and then broke apart a short 50 million years later (from about
   8:50 p.m. to 9:05 p.m. on our imaginary clock). Fish, the earliest
   vertebrates, evolved in the oceans around 530 million years ago (9:10
   p.m). A major extinction event occurred near the end of the Cambrian
   period, which ended 488 million years ago (9:25 p.m.).

   Several hundred million years ago, plants (probably resembling algae)
   and fungi started growing at the edges of the water, and then out of
   it. The oldest fossils of land fungi and plants date to 480–460 million
   years ago (9:28–9:34 p.m.), though molecular evidence suggests the
   fungi may have colonized the land as early as 1000 million years ago
   (6:40 p.m.) and the plants 700 million years ago (8:20 p.m.). 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 million years ago
   (9:40 p.m.), perhaps thriving and becoming better adapted due to the
   vast food source provided by the terrestrial plants. There is also some
   unconfirmed evidence that arthropods may have appeared on land as early
   as 530 million years ago (9:12 p.m.). At the end of the Ordovician
   period, 440 million years ago (9:40 p.m.), additional extinction events
   occurred, perhaps as a result of a concurrent ice age. Around 380 to
   375 million years ago (10:00 p.m.) the first tetrapods evolved from the
   fish. It is thought that perhaps fins evolved to become limbs which
   allowed the first tetrapods to lift their heads out of the water to
   breathe air. This would let them survive in oxygen-poor water or pursue
   small prey in shallow water. They may have later ventured on land for
   brief periods. Eventually, some of them became so well adapted to
   terrestrial life that they spent their adult lives on land, although
   they hatched in the water and returned to lay their eggs. This was the
   origin of the amphibians. About 365 million years ago (10:04 p.m.),
   another period of extinction occurred, perhaps as a result of global
   cooling. Plants evolved seeds, which dramatically accelerated their
   spread on land, around this time (by approximately 360 million years
   ago or 10 o’clock).^,
   Pangaea, the most recent supercontinent, existed from 300 to 180
   million years ago. The outlines of the modern continents and other land
   masses are indicated on this map.
   Enlarge
   Pangaea, the most recent supercontinent, existed from 300 to 180
   million years ago. The outlines of the modern continents and other land
   masses are indicated on this map.

   Some twenty million years later (340 million years ago, 10:12 p.m. on
   our clock), the evolution of the amniotic egg allowed eggs to be laid
   on land, certainly a survival advantage for the tetrapod embryos. This
   resulted in the divergence of amniotes from amphibians. Another thirty
   million years (310 million years ago, 10:22 p.m.) saw the divergence of
   the synapsids (including mammals) from the sauropsids (including birds
   and non-avian, non-mammalian reptiles). Of course, other groups of
   organisms continued to evolve and lines diverged—in fish, insects,
   bacteria, and so on—but not as much is known of the details. 300
   million years ago (10:25 p.m.) the most recent supercontinent formed,
   called Pangaea. The most severe extinction event to date took place 250
   million years ago (10:40 p.m. on our clock), at the boundary of the
   Permian and Triassic periods; 95% of life on Earth died out, possibly
   as a consequence of the Siberian Traps volcanic event. The discovery of
   a crater hidden under the East Antarctic Ice Sheet has risen up a new
   theory that a meteor caused the mass extinction and possibly began the
   breakup of the Gondwana supercontinent by creating the tectonic rift
   that pushed Australia northward. But life persevered, and around 230
   million years ago (10:47 p.m. on our clock), dinosaurs split off from
   their reptilian ancestors. An extinction event between the Triassic and
   Jurassic periods 200 million years ago (10:56 p.m.) spared many of the
   dinosaurs, and they soon became dominant among the vertebrates. Though
   some of the mammalian lines began to separate during this period,
   existing mammals were probably all small animals resembling shrews. By
   180 million years ago (11:03 p.m.), Pangaea broke up into Laurasia and
   Gondwana. The boundary between avian and non-avian dinosaurs is not
   clear, but Archaeopteryx, traditionally considered one of the first
   birds, lived around 150 million years ago (11:12 p.m.). The earliest
   evidence for the angiosperms evolving flowers is during the Cretaceous
   period, some twenty million years later (132 million years ago, 11:18
   p.m.) Competition with birds drove many pterosaurs to extinction, and
   the dinosaurs were probably already in decline for various reasons
   when, 65 million years ago (11:39 p.m.), a 10-kilometer meteorite
   likely struck Earth just off the Yucatán Peninsula, ejecting vast
   quantities of particulate matter and vapor into the air that occluded
   sunlight, inhibiting photosynthesis. Most large animals, including the
   non-avian dinosaurs, became extinct., marking the end of the Cretaceous
   period and Mesozoic era. Thereafter, in the Paleocene epoch, mammals
   rapidly diversified, grew larger, and became the dominant vertebrates.
   Perhaps a couple million years later (around 63 million years ago,
   11:40 p.m.), the last common ancestor of the all primates lived. By the
   late Eocene epoch, 34 million years ago (11:49 p.m.), some terrestrial
   mammals had returned to the oceans to become animals such as
   Basilosaurus which would later give rise to the dolphins and whales.

Humanity

   Australopithecus africanus, an early hominid.
   Enlarge
   Australopithecus africanus, an early hominid.

   A small African ape living around six million years ago (11:58 p.m. on
   our clock) was the last animal whose descendants would include both
   modern humans and their closest relatives, the chimpanzees. Only two
   branches of his family tree have surviving descendants. Very soon after
   the split, for reasons that are still debated, apes in one branch
   developed the ability to walk upright. Brain size increased rapidly,
   and by 2 million years ago (11:59:22 p.m., or 38 seconds before
   midnight) the very first animals classified in the genus Homo had
   appeared. Of course, the line between different species or even genera
   is rather arbitrary as organisms continuously change over generations.
   Around the same time, the other branch split into the ancestors of the
   common chimpanzee and the ancestors of the bonobo as evolution
   continued simultaneously in all life forms. The ability to control fire
   likely began in Homo erectus (or Homo ergaster), probably at least
   790,000 years ago but perhaps as early as 1.5 million years ago
   (fifteen to twenty-eight seconds ago). 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. As
   brain size increased, babies were born sooner, 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 advanced, and tools became more
   elaborate. This contributed to further cooperation and brain
   development. Anatomically modern humans—Homo sapiens—are believed to
   have originated somewhere around 200,000 years (two seconds) ago or
   earlier in Africa; the oldest fossils date back to around 160,000 years
   ago. The first humans to show evidence of spirituality are the
   Neanderthals (usually classified as a separate species with no
   surviving descendants); they buried their dead, often apparently with
   food or tools. However, evidence of more sophisticated beliefs, such as
   the early Cro-Magnon cave paintings (probably with magical or religious
   significance) did not appear until some 32,000 years (0.6 seconds) ago.
   Cro-Magnons also left behind stone figurines such as Venus of
   Willendorf, probably also signifying religious belief. By 11,000 years
   (0.2 seconds) ago, Homo sapiens had reached the southern tip of South
   America, the last of the inhabited continents. Tool use and language
   continued to improve; interpersonal relationships became more complex.

Civilization

   Throughout more than ninety percent of its history, Homo sapiens lived
   in small bands as nomadic hunter-gatherers. As language became more
   complex, the ability to remember and transmit information resulted in a
   new sort of replicator: the meme. Ideas could be rapidly exchanged and
   passed down the generations. Cultural evolution quickly outpaced
   biological evolution, and history proper began. Somewhere between 8500
   and 7000 BCE (0.20 to 0.17 seconds ago), humans in the Fertile Crescent
   in Mesopotamia began the systematic husbandry of plants and animals:
   agriculture. This spread to neighboring regions, and also developed
   independently elsewhere, until most homo sapeins 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. However, among those
   civilizations that did adopt agriculture, the relative security and
   increased productivity provided by farming allowed the population to
   expand. 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 BCE (around 0.10 seconds ago). Additional
   civilizations quickly arose in ancient Egypt and the Indus River
   valley.
   Vitruvian Man by Leonardo da Vinci epitomizes the advances in art and
   science seen during the Renaissance.
   Enlarge
   Vitruvian Man by Leonardo da Vinci epitomizes the advances in art and
   science seen during the Renaissance.

   Starting around 3000 BCE (0.09 seconds ago on our clock), Hinduism, one
   of the oldest religions still practiced today, began to take form.
   Others soon followed. 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—curiosity and education drove the pursuit of knowledge and
   wisdom. Various disciplines, including science (in a primitive form),
   arose. New civilizations sprang up, traded with one another, and
   engaged in war for territory and resources: empires began to form. By
   around 500 BCE (0.048 seconds ago), there were empires in the Middle
   East, Iran, India, China, and Greece, approximately on equal footing;
   at times one empire expanded, only to decline or be driven back later.

   In the 1300s (about 0.012 seconds ago), the Renaissance began in Italy
   with advances in religion, art, and science. Starting around 1500
   (0.0096 seconds ago), European civilization began to undergo changes
   leading to the scientific and industrial revolutions: that continent
   began to exert political and cultural dominance over human societies
   around the planet. From 1914 to 1918 (about 0.0017 seconds ago) and
   1939 to 1945 (about 0.0012 seconds ago), nations around the world were
   embroiled in world wars. Established following World War I, the League
   of Nations was a first step toward a world government; after World War
   II it was replaced by the United Nations. In 1992, several European
   nations joined together 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 discord, although increased
   collaboration has resulted as well.

Recent events

   After four and a half billion years, one of Earth’s life forms broke
   free of the biosphere. For the first time in history, Earth was viewed
   from the vantage of space.
   Enlarge
   After four and a half billion years, one of Earth’s life forms broke
   free of the biosphere. For the first time in history, Earth was viewed
   from the vantage of space.

   Change has continued at a rapid pace in the last millisecond of our
   notional 24-hour period, 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. Problems such as disease, war, global warming and poverty
   are still present.

   In 1957, the Soviet Union launched the first artificial satellite into
   orbit and, soon afterward, Yuri Gagarin became the first human in space
   and Neil Armstrong the first to set foot on another astronomical
   object, the Earth's Moon. Five space agencies, representing over
   fifteen countries, have worked together to build the International
   Space Station. Aboard it, there has been a continuous human presence in
   space since 2000.

   Retrieved from " http://en.wikipedia.org/wiki/History_of_Earth"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
