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Hadean Era (4,550-3,850 mya)
The Precambrian's oldest era, the Hadean, predates most of the
geologic record. During the Hadean, the solar system forms out of gas and dust, the
sun begins to emit light and heat, and Earth takes shape. Meteors and other galactic
debris shower the planet over its first half-billion years, making it entirely
uninhabitable.
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4,550 mya: Birth of the solar system
More than 10 billion years since the universe exploded into
existence from nothing, the solar system -- including the Sun, the nine planets,
their moons, and various asteroids -- forms from interstellar gas and dust.
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Prelude to life (4,550-3,900 mya)
Planet Earth is very hot at its formation. As it cools and its
mass increases, its gravitational field strengthens. This attracts meteorites and
other debris, which will bombard the planet for at least 500 million years, producing
enough energy and heat to vaporize any water or melt any rock that may be present.
Gravity also helps separate heavy elements from lighter ones.
Iron sinks to form Earth's core, while silicon, magnesium, and aluminum gradually
rise toward the surface.
Gases released from magma (molten rock) inside Earth escape
through cracks in the surface and collect in the early atmosphere. The likely
presence of methane and ammonia among the gases makes for conditions that would
be highly toxic to life as we know it. Because there is little or no free oxygen
-- hence, no protective ozone layers -- damaging ultraviolet rays shower Earth
at full strength.
As the meteorite bombardment finally slows, Earth cools, and
its surface hardens as crust. Water condenses in the atmosphere, and torrential
rains fall. Over several millions of years of continuous rain, oceans form. By
about 3,900 million years ago (mya), Earth's environment has been transformed
from a highly unstable state into a more hospitable place.
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4,500 mya: Escaping gases build early atmosphere
Gases released from magma inside Earth collect to form an atmosphere
probably composed of nitrogen, carbon dioxide, water vapor, methane, and ammonia, with
little or no free oxygen.
4,500 mya: Meteorite bombardment
Meteorites and other debris from space begin a 500-million-year
bombardment of Earth.
4,400 mya: Earth's core forms
Iron sinks deep inside Earth and forms its core. Lighter elements
like silica, magnesium, and aluminum rise to the surface, and later harden to form the
outer crust.
4,200 mya: Great oceans form
Water condenses in the cooling atmosphere, and heavy rains pour down
on the planet. After several hundred million years of falling rain, great oceans form.
4,055 mya: Oldest known rocks
Rocks older than 3,500 million years old have been found on each of
Earth's continents. The oldest ones, from 4,055 mya, were found in what is now Canada's
Northwest Territories. Because Earth is continuously recycling old crust and replacing
it anew, discoveries of rocks as ancient as these are highly unusual.
3,850 mya: Evidence of life
In southwest Greenland, undisturbed volcanic rock 3,850 million years
old lies atop sedimentary rock. Within this deeper (and older) layer are minerals that
contain telltale signs of past biological activity: carbon isotope ratios that occur
only if life has been present. The act of piecing together plausible explanations of an
event or discovery from the evidence at hand is known as inference.
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Life's origins (3,850 mya)
Evidence preserved in rock layers in present-day Greenland tells
us that life existed at least 3,850 million years ago (mya). While this helps us
understand when life began, it doesn't explain how life began.
Scientists agree that certain conditions were needed for living
cells to evolve from the gases and water thought to be present on the early Earth.
These include a concentrated supply of organic chemicals, the most important of
which was cyanide; energy to fuel reactions between these chemicals; protection from
extreme heat; and some sort of biological catalysts to encourage the building of
proteins and assist in reproduction.
One of the more compelling hypotheses offered to
explain how living cells formed describes an "RNA world." It holds that chemical
reactions sparked by the introduction of energy produced RNA (ribonucleic acid)
sequences called ribozymes. Some of these ribozymes helped assemble proteins --
the workhorses of living cells. Other ribozymes helped RNA replicate itself. The
primitive cells of this "RNA world," like cells today, probably had water-repellent
outer membranes to hold together and protect their contents.
If life evolved near the planet's surface, ultraviolet radiation
or lightning could have provided the vital spark. But if, as many scientists now
think, it evolved underwater, heat generated by Earth itself -- like that found
today in hydrothermal vents along volcanic ridges in the deep sea -- probably
supplied the energy.
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