Descriptions such as alien, hostile, and violent only hint at the conditions you would encounter if you could travel back in time 4.6 billion years to the newly formed Earth. At first, you would see a waterless surface pockmarked with craters and littered with volcanic rocks. Over the next billion years, you would watch as rocky debris left over from the formation of the solar system—some of it the size of a small planet—pummeled Earth. You would see volcanoes spewing clouds of noxious gases and monstrous rivers of lava. But sooner than you might imagine in such an unstable world, you would notice a thick atmosphere enveloping the planet and water pooling in low places. Then, about 3.5 billion years ago, you would discover that you were no longer the only life form on Earth.

Until the early 1970's, information about Early Earth was so sparse that geologists did not even have a name for the planet's first billion or so years. Many geologists now refer to this period as the Hadean Eon—after Hades, the ancient Greek underworld.

By 2006, scientists had developed a better understanding of the forces that had transformed Earth from an arid wasteland to a world whose oxygen-rich atmosphere nourishes an astonishing variety of life. They had been able to create a rough time line for the major events of the Hadean Eon, though many of the details remained unclear. They had discovered just how severe, violent, and long-lasting the bombardment period was. And they had found evidence that Earth's life forms and water had appeared much earlier than previously believed.

Most important, they had radically changed their theories about the origin of life. In the mid-1900's, scientists commonly assumed that life appeared in the warm, shallow seas of a primitive Earth whose temperatures were similar to those of today. By 2006, however, they had learned that some organisms, known as extremophiles, can thrive under conditions that would instantly kill human beings and most other life forms. At one time, scientists assumed that all life depended on the sun for energy, but they now know that some extremophiles get their energy from chemical reactions in hot springs bubbling up from the ocean floor. Other extremophiles live in oxygen-free environments deep underground. Under which of these—or other still unknown conditions—did the first life forms appear? Finally, scientists no longer wonder how Earth came to be so suitable for life. They know that from its first appearance, life has profoundly shaped the atmosphere, oceans, and other physical conditions on Earth.