Between the cataclysmic impact that created the Moon around 4.5 billion years ago and the first evidence of life 3.8 billion years ago, there may have been long periods during which life repeatedly spread across the globe, only to be nearly annihilated by the impact of large asteroids.

The early Earth, in other words, may have been an interrupted Eden -- a planet where life repeatedly evolved and diversified, only to be sent back to square one by asteroids 10 or 20 times wider than the one that hastened the dinosaurs' demise. When the surface of the Earth finally became inhabitable again, thousands of years after each asteroid impact, the survivors would have emerged from their hiding places and spread across the planet -- until another asteroid struck and the whole cycle was repeated.

"We know that large asteroid impacts can sterilize or partially sterilize planets," says Norman Sleep, a professor of geophysics at Stanford who will present the theory at the fall meeting of the American Geophysical Union in San Francisco on Friday, Dec. 14.

"An asteroid a few hundred kilometers in diameter will boil off much of the ocean and leave the rest of the ocean very hot, so all that will survive will be high-temperature organisms living deep in the subsurface," he says. Rock vapor and water would fill the atmosphere, killing off any life on the surface with temperatures upwards of 1,000 C (1,800 F).

The only organisms that could survive such an impact are thermophiles -- heat-loving microbes -- buried half a mile or more below the Earth's surface, where the effects of the burning atmosphere would have been muted to a survivable 100 C (212 F). Those organisms may have given rise to much of the life on today's Earth.

Sleep calls the region where those organisms would have lived the "Goldilocks Zone" -- deep enough for microbes to avoid the heat of the burning atmosphere, but not so deep that they ran afoul of the Earth's internal heat.

Since there are no records of life before 3.8 billion years ago, there is no direct proof that Sleep's theory is correct. But several strands of evidence are highly suggestive.

The first is that two of the three major branches of life that exist on Earth today -- Archaea, Bacteria and Eukarya -- began with organisms that were designed to live in extremely hot environments, the kinds that would have existed for millions of years after the impact of a large asteroid.

A glance at the names of modern members of the Archaea and Bacteria branches turns up an overwhelming number of "thermos" -- Thermococcus, Thermotoga, Thermoproteus and others. All of them thrive at temperatures above 80 C (176 F), with some managing to eke out an existence in conditions that would literally boil most organisms alive. (The current record-holder can survive in environments above 115 C [239 F], says Sleep.)

"The roots of these two branches of the tree are clearly thermophile, which is exactly what's going to survive in a large impact," says Sleep.

Where Eukarya -- the branch that includes yeast, worms, corn and humans -- fits into the story is less certain. "It's unclear whether Eukarya, which we are, has a thermophile root or not," says Sleep. "We may never have had a high-temperature-organism ancestor. But clearly two of the three branches look like asteroid survivors: very complex, highly-evolved organisms that are thermophile."

The second strand of evidence is geophysical. Although it has long been thought that early Earth would have been rendered lifeless by continual asteroid bombardment, there are now good reasons to believe that our planet was struck by fewer than 20 large asteroids between the time of the Moon-forming impact and the first fossil signs of life. That would leave hundreds of millions of years between each asteroid strike, during which complex organisms -- and life itself -- would be free to evolve.

When asteroids did strike, only those organisms that could find some kind of shelter would have survived. The most obvious refuge is deep within the Earth itself, but Sleep believes there may have been another, more exotic way for early organisms to survive such Earth-shattering catastrophes. Martian invaders Perhaps, says Sleep, some of the asteroids that struck the early Earth were large enough to destroy all life on the planet, even those organisms hidden deep within the crust. There was still at least one other place where life could have survived, even flourished, before returning to Earth: Mars. Although Mars is now a frigid desert, four billion years ago it may have been a warm, water-filled oasis as friendly to life as early Earth.

But could a microorganism really have survived the trip from Earth to Mars? To successfully complete the interplanetary journey, a microbe first would have to survive an asteroid impact powerful enough to free a chunk of rock from the grip of gravity. Once in space, the traveler would be faced with conditions harsher than anything found on Mars or Earth: total vacuum, subzero temperatures, harmful radiation and the passage of perhaps thousands of years before the interplanetary dart hit its target. Even then, the colonizing microbe would have to hope that some of its descendants were buried deep enough in the rock to avoid burning up in Earth's atmosphere.

Sleep says these factors make the trip difficult, but not impossible. Models have shown that the initial shock of ejection from a planet isn't necessarily deadly, especially for the hardiest microbes, and especially from a small planet like Mars where the atmosphere is thin and gravity is relatively weak. "You don't sterilize a milk bottle by throwing it off your roof," he explains.

And laboratory experiments have shown that earthly microbes, especially if hidden in cracks deep within a meteorite, can survive the harsh conditions of space at least for a few years. Of course, no one has tested whether they can survive for thousands of years, but there's no reason to think they can't, notes Sleep. "Conditions are not good for microorganisms, but they're not bad," he adds.

So it is possible that life came from another planet -- but did it really happen? So far there is no direct evidence of life on other planets or asteroids, although it is becoming clear that conditions exist, at least on Mars and Europa -- one of Jupiter's inner moons -- where microbes that live comfortably in Earth's harsher climates would have felt at home. As Sleep put its, Mars "is no more uninhabitable than Antarctica" -- uncomfortable for humans, but perfect for some microbes.

Conclusive evidence for or against the theory only will come when scientists can examine samples from other planets and asteroids, something that is still a long way off. But Sleep says he's not frustrated by the sometimes slow pace of studying early life.

"The origin of life is one of the fundamental problems of science, and it always has been. Living at a time when you can do that, it's not something I'm going to pass up," he says.