Mounting evidence fromseafloor critters to ancient soil and even Moon rocks suggests that life onearly Earth survived heavy bombardment from space rocks, pointing to an earlierorigin for terrestrial life and opening wider the window of possibilities forwhere life might exist in the cosmos.

Early Earth was a lousyplace to live. The young solar system was teeming with cometsand asteroids;many ended their travels by slamming into our fledgling planet, destroyingentire continents and kicking up deadly clouds that circled the globe.

The chaos ended about 3.8billion years ago. And some scientists have long held that only after thingsquieted down did life get going. But while others have argued for years offossilized evidence that life existed as far back as 3.5 billion years ago,efforts to pin down an exact date for the origin oflife on Earth have so far proved elusive.

In one recent study,scientists found signatures of biological activity in rock estimated to be 3.85billion years old. While not a discovery of life, or even fossils, the findingis among the oldest evidence that life was around back when things were rough.

"We're learning allthe time that life, in some form or other, is incredibly resilient, albeitfluid -- episodically morphing into new and better adapted forms rather thansuccumbing fragilely to the slightest little stress," says UC Berkeleygeologist Paul Renne.

The 3.85 billion-year-oldrock, from an island in Greenland, was dated by researchers from the AustralianNational University.

Steve Mojzis, of NASA'sAstrobiology Institute, studied the rock and found higher-than-normal amountsof carbon-12, a form of carbon that is used by microorganisms to constructtheir own organic building blocks. Finding a bunch of it crammed between layersof rock is a strong hint that life may have been present.

Meanwhile, a paper in theDecember 1 issue of the journal Science reports strong evidence tosupport the belief that these microorganisms, if they existed 3.85 billionyears ago, would in fact have found themselves on a planet under attack.

Attack from space

Barbara Cohen of theUniversity of Tennessee analyzed four rock fragments that had been blasted offthe Moonand found on Earth. Cohen and her colleagues found evidence of at least seven separatehuge lunar impacts between 2.76 and 3.92 billion years ago, bolstering thealready strong belief in the cataclysmic period known as the Late HeavyBombardment.

Previous studies of rocksreturned by Apollo astronauts had already established the time frame, but thoserocks all came from the near side of the Moon, near the equator. Cohen analyzedrocks that were randomly ejected from the Moon, showing that the bombardmentwas a total lunar event.

Cohen and others agree thatif the Moon was getting pummeled, so was Earth. Because Earth is larger, itwould have been hit by at least 10 times as many space rocks.

"And we're talking largeimpactors that would make craters the size of continents," Cohen told SPACE.com."During the cataclysm, 17,000 craters were created in 200 million years[on Earth]. It would not have been a great time to be living on the Earth'ssurface."

So maybe whatever crittersthat might have been around weren't on the surface. Three possibilitieshave been put forth: Life warmed its hypothetical hands on deep-seahydrothermal vents; it burrowed deep inside the Earth; or -- in an exotic twist-- microbes took a hiatus in space.

Biologists have created atree of life showing that all living things can be traced back to heat-lovingorganisms, called hyperthermophiles. Studying genetic material to create thetree, they say that life either began in hot water, or it spent some time therebefore evolving into slugs, oak trees, and rocket scientists.

These "originalorganisms," clustered around deep-sea hydrothermal vents, where Earthbelches hot water and minerals into the ocean, might have endured for longperiods without even noticing the dismal surface conditions.

But physicist Paul Davies,author of The Fifth Miracle: The Search for the Origin and Meaning of Life,dismisses hydrothermal vents as potential hideouts. He says the largest impactevents would have boiled the oceans dry. Seafloor microbes would have beennaked to the chaos.

Next page: Microbialspace vacation