In his book, Davies suggests that huge impacts, known to fling dust and rocks into space, might have also carried microbes to safe refuge. Years later, after the planet became livable again, a few lucky microbes might have returned from the heavens to recolonize. Recent studies have shown that microorganisms could in fact survive the brunt of an impact and the rigors of space travel.

Still, Davies figures there was an easier way for life to endure catastrophe.

"The most plausible refuge is the deep subsurface, by which I mean more than 1 kilometer (0.6 miles) down in the crust, either in the basalt of the seabed, or on any land that may have existed during the bombardment," Davies told SPACE.com. "Merely being on the sea floor [near a hydrothermal vent] would not have provided adequate protection from the largest impactors, since these would have created a rock-vapor atmosphere that would have boiled the oceans dry."

If any of these scenarios is true, then catastrophe could be called the mother of evolution. It remains unknown, however, whether the events of the Late Heavy Bombardment forced life to evolve, or if it got wiped out several times and had to spring forth over and over.

"We have no idea whether the origin of life was a gigantic chemical fluke, unique in the universe, or an expected result of inherently bio-friendly laws," Davies says. "Given this uncertainty on the likelihood of life emerging, it is clearly more plausible in the current state of our knowledge to conjecture that life may have survived multiple impacts by a lifeboat mechanism, or other refuge, rather than re-emerging from scratch in each window of opportunity."

Maybe it wasn't so bad after all

More new research supports the idea that life could have hidden out long enough to hang on during the worst of times. Kevin Zahnle of NASA's Ames Research Center, working with Mojzis and Ariel Anbar at NASA's Astrobiology Institute, used computer models to study the effect of impacts during the Late Heavy Bombardment.

In taking measurements of the 3.85 billion-year-old terrestrial rock from Greenland, Ph.D. student Gail Arnold, working with Anbar, did not find any signs of iridium in the rock, which should have been present if there had been any large impact events around that time. (The element iridium is prevalent in comets and asteroids.)

Their work, to be published in an upcoming issue of the Journal of Geophysical Research, suggests that impacts large enough to cause worldwide disaster were few and far between. And even after the worst events, life would only have needed to hang on for about 10,000 years before Earth rolled out its bio-welcome mat again, according to the computer models.

"As long as life could hold out in niches during episodic catastrophic times (every 30 million years or so), it could have inhabited the surface during most of the 'heavy bombardment' era," says Anbar, a University of Rochester researcher.

The growing view of life's tenacity means life itself may have grown, and still be growing, in more places than scientists previously thought.

"This greatly extends the number of potential habitats in the solar system and beyond," says Davies, the author and physicist. "For example, life may still exist deep beneath the surface of Mars. Subsurface life on Europa is also possible. Even lunar subsurface life is not totally absurd, if liquid water can exist."

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