History of Earth
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Based on the available evidence, scientists have been able to
reconstruct detailed information about the planet's past. Earth is
believed to have formed around 4.55×109 years ago out of the solar
nebula, along with the Sun and other planets. The moon formed soon
afterwards. Initially molten, the outer layer of the planet cooled,
resulting in the solid crust.
Outgassing and volcanic activity produced the primordial atmosphere;
condensing water vapor, augmented by ice delivered by comets,
produced the oceans. The highly energetic chemistry is believed to
have produced a self-replicating molecule around 4×109 years ago,
and half a billion years later, the last common ancestor of all life
lived. |
The development of photosynthesis allowed the sun's energy to be
harvested directly; the resultant oxygen accumulated in the
atmosphere and gave rise to the ozone layer. The incorporation of
smaller cells within larger ones resulted in the development of
complex cells called eukaryotes. Cells within colonies became
increasingly specialized, resulting in true multicellular organisms.
With the ozone layer absorbing harmful ultraviolet radiation, life
colonized the surface of Earth.
Over hundreds of millions of years, continents formed and broke up
as the surface of Earth continually reshaped itself. Since the
Cambrian explosion, about 535 million years ago, there were 5
distinct mass extinctions. The last one occurred 65 million years
ago, when a meteorite collision probably triggered the extinction of
the (non-avian) dinosaurs and other large reptiles, but spared small
animals such as mammals, which then resembled shrews.
Over the last 65 million years, mammalian life diversified, and
several million years ago, a small African ape gained the ability to
stand upright. As brain size increased, these hominids developed the
use of tools and language. The development of agriculture, and then
civilization allowed humans to affect the Earth in a short timespan
like no other life form had before, affecting both the nature and
quantity of other life forms as well as global climate.
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