The infant Earth was a harsh place for life to begin. The oceans were covered with ice, the climate was as forbidding as modern Antarctica. And every few tens of millions of years, giant asteroids barreled into the planet, boiling away the oceans and veiling the globe with clouds of vaporized rock. Each impact would have sterilized the surface, yet evidence shows life survived the bombardment. Geophysicist Norman Sleep of Stanford University theorizes that early organisms endured in an unusual sanctuary—deep underground or out in space—and repopulated the surface when conditions improved.
Studies of lunar craters indicate Earth was struck by 15 to 20 huge asteroids, each about 150 miles wide, between 4.5 billion and 3.8 billion years ago, when the first solid evidence of life appears. But Sleep's research reveals that chemical conditions below Earth's surface were suitable for heat-loving microbes that derive energy from hydrogen and methane. Called thermophiles, they still thrive in hot springs and ocean vents. During the heavy bombardment, thermophiles could have holed up a half mile underground, where temperatures hover around 200 degrees Fahrenheit. Hardy microbes might also have been able to survive being blasted into space and returning millennia later.
A ring-shaped lake in Manicouagan, Canada, photographed from the space shuttle, marks the spot where a giant asteroid struck 210 million years ago. Earlier impacts may have repeatedly wiped out all surface life.Photograph courtesy of Lunar and Planetary Institute
Thermophiles are primitive organisms that lie at the roots of two of the three major branches of life, Bacteria and Archaea. That primacy suggests to Sleep that these hardy hangers-on gave rise to all the other living things that later overtook Earth. The diverse set of genes they carry implies that the thermophiles have evolved considerably from simpler life-forms that came before them. Sleep thinks life probably began almost as soon as Earth formed, enduring repeated crises while brutal conditions kept weeding out the weaklings. "The last common ancestor of present life is an incredibly complicated organism already; it looks like a survivor," he says.