The Precambrian Eon
The name means: "before the Cambrian period." This old, but still common term was originally used to refer to the whole period of Earth's history before the formation of the oldest rocks with recognizable fossils in them. In the last few decades, however, geologists have found that there are some hard-to-discern fossils in some Precambrian rocks, so this period also is now known as the Cryptozoic or "obscure life" Eon (from the words "crypt" = "hidden," and "zoon" = "life").

This eon covers almost 90% of the entire history of Earth. It has been divided into three eras: the Hadean, the Archean and the Proterozoic. Each era is very different.

Hadean ("Hades-like") Era (pictured above)
This era begins about 4.6 billion years ago with the formation of Earth from dust and gas orbiting the Sun. During this era the surface of Earth is like popular visions of Hades: oceans of liquid rock, boiling sulfur, and impact craters everywhere! Volcanoes blast off all over the place, and the rain of rocks and asteroids from space never ends. It is hard to take a step without falling into a pool of lava or getting hit by a meteor! The air is hot, thick, steamy, and full of dust and crud. But you can't breathe it anyway: its made of nothing but carbon dioxide and water vapor, with traces of nitrogen and smelly sulfur compounds! Any rocks that do form from cooling lavas are quickly buried under new lava flows or blasted to bits by yet another impact. Some people think that an asteroid as large as the planet Mars hit Earth near the beginning of the Hadean era, completely smashing and melting Earth and forming the Moon as part of the "splash!" Wow!

No one has found any rocks on Earth from this era. Only meteorites from space and moon rocks are this old. If any life formed on Earth during this era, it was probably destroyed.

The lava also is mostly cooled to form the ocean floor. The interior of Earth is still quite hot and active, as shown by the many erupting volcanoes. The volcanoes form lots of small islands in long chains. The islands are the only land surface. The continents have not formed yet. The islands are carried over the surface of Earth by the movement of rock deep in Earth's interior. (This movement results from the loss of heat from the deep interior and is called Plate Tectonics.) Occasionally the small islands collide with each other to form larger islands. Eventually these larger islands will collide to form the cores of the continents we know today.

Thank goodness those pesky asteroids and meteorites are mostly gone, so now impact craters only form occasionally.

What about life? If you look closely, you will see evidence of blue-green algae (actually simple bacteria) floating in the ocean. That's all there is! Just single-celled bacteria in the ocean. There is as yet no life on land. Life began in the ocean near the beginning of this era. The oldest known fossils--the remains of different types of bacteria--are in archean rocks about 3.5 billion years old.

The surface of Earth is still very active, but a few of the rocks that are forming now will actually survive to the present in spite of wind, rain, and remelting. In fact, geologists define the beginning of the Archean era as the age of the oldest rocks on Earth we can still find today.

Proterozoic ("Early Life") Era
Well, here we are about 700 million years ago, near the end of the longest time period in geologic history. It began about two billion years after the formation of Earth and lasted about another two billion years! So what has happened in all that time?

Hmmmmm. There is a lot more land to be seen. In fact, there are two supercontinents, one visible across the equator on this side of Earth and another one on the other side. These huge masses of land formed by collisions of the many, many islands made by volcanoes during the Archaean and most of the Proterozoic eras. Earth's interior has cooled some more, and there are fewer volcanoes than in the Archean. Even though the movements of Earth's surface we call Plate Tectonics are still very fast and continental collisions are frequent (every few hundred million years or so!), the centers or cores of the continents are now quite large and stable. In fact, geologists date the beginning of the Proterozoic Era by the age of the oldest continental rocks that have not been reheated or chemically altered.

Life has not changed much during the last two billion years, but the few changes are significant. Life is still found only in the ocean, but sometime around 1.7 billion years ago, single-celled creatures appeared that had a real nucleus. Another important change is about to happen: true multi-celled life is about to appear, some 30 million years before the end of the Proterozoic. These multi-celled creatures will have no hard parts like shells or teeth in their bodies, so their fossils will be hard to find.

The atmosphere is about the same, mostly nitrogen, with a little water vapor and carbon dioxide. But what's this? Free oxygen released by the algae floating in the oceans is beginning to collect in the air. These single-celled plants have been producing oxygen for about two billion years, but up until now the oxygen has been combining chemically with iron and other elements to form great mineral deposits around the world. Paradoxically, this oxygen, which we must have to live, is poisonous to most of the life forms living on Earth during the Proterozoic, so another great change in the types of life is about to occur.

Earth at this time is also very cold, with huge, bluish glacial ice sheets visible across the supercontinent, even in the normally warm equatorial regions!

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Last updated on April 28, 2005