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dailynews.yahoo.com December 07 04:21 PM EST
Catastrophe, Mother of Evolution: Life Survived Early Bombardment
By Robert Roy Britt
Senior Science Writer, SPACE.com
Mounting evidence from seafloor critters to ancient soil and even Moon rocks suggests that life on early Earth survived heavy bombardment from space rocks, pointing to an earlier origin for terrestrial life and opening wider the window of possibilities for where life might exist in the cosmos.
Early Earth was a lousy place to live. The young solar system was teeming with comets and asteroids; many ended their travels by slamming into our fledgling planet, destroying entire continents and kicking up deadly clouds that circled the globe.
The chaos ended about 3.8 billion years ago. And some scientists have long held that only after things quieted down did life get going. But while others have argued for years of fossilized evidence that life existed as far back as 3.5 billion years ago, efforts to pin down an exact date for the origin of life on Earth have so far proved elusive.
In one recent study, scientists found signatures of biological activity in rock estimated to be 3.85 billion years old. While not a discovery of life, or even fossils, the finding is among the oldest evidence that life was around back when things were rough.
"We're learning all the time that life, in some form or other, is incredibly resilient, albeit fluid -- episodically morphing into new and better adapted forms rather than succumbing fragilely to the slightest little stress," says UC Berkeley geologist Paul Renne.
The 3.85 billion-year-old rock, from an island in Greenland, was dated by researchers from the Australian National University.
Steve Mojzis, of NASA's Astrobiology Institute, studied the rock and found higher-than-normal amounts of carbon-12, a form of carbon that is used by microorganisms to construct their own organic building blocks. Finding a bunch of it crammed between layers of rock is a strong hint that life may have been present.
Meanwhile, a paper in the December 1 issue of the journal Science reports strong evidence to support the belief that these microorganisms, if they existed 3.85 billion years ago, would in fact have found themselves on a planet under attack.
Attack from space
Barbara Cohen of the University of Tennessee analyzed four rock fragments that had been blasted off the Moon and found on Earth. Cohen and her colleagues found evidence of at least seven separate huge lunar impacts between 2.76 and 3.92 billion years ago, bolstering the already strong belief in the cataclysmic period known as the Late Heavy Bombardment.
Previous studies of rocks returned by Apollo astronauts had already established the time frame, but those rocks all came from the near side of the Moon, near the equator. Cohen analyzed rocks that were randomly ejected from the Moon, showing that the bombardment was a total lunar event.
Cohen and others agree that if the Moon was getting pummeled, so was Earth. Because Earth is larger, it would have been hit by at least 10 times as many space rocks.
"And we're talking large impactors 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's surface."
So maybe whatever critters that might have been around weren't on the surface. Three possibilities have been put forth: Life warmed its hypothetical hands on deep-sea hydrothermal vents; it burrowed deep inside the Earth; or -- in an exotic twist -- microbes took a hiatus in space.
Biologists have created a tree of life showing that all living things can be traced back to heat-loving organisms, called hyperthermophiles. Studying genetic material to create the tree, they say that life either began in hot water, or it spent some time there before evolving into slugs, oak trees, and rocket scientists.
These "original organisms," clustered around deep-sea hydrothermal vents, where Earth belches hot water and minerals into the ocean, might have endured for long periods 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 impact events would have boiled the oceans dry. Seafloor microbes would have been naked to the chaos.
Next page: Microbial space vacation
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." |
