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Intro | Precambrian Eon | Paleozoic Era | Mesozoic Era | Cenozoic Era

Precambrian Eon: (4,550-543 mya)

Hadean | Archaean | Proterozoic

The Precambrian eon is vast, almost unimaginably so. In fact, before some fossil discoveries were made late in the 20th century, it was considered unknowable -- evolution's dark ages. The Precambrian encompasses 86 percent of the history of Earth. As its name implies, this includes all of geological time prior to the Cambrian period.



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.

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.


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.


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.


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.


-> Go to the Archaean Era

Intro | Precambrian Eon | Paleozoic Era | Mesozoic Era | Cenozoic Era

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