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THE first bacteria on Earth had a much easier time of it than previously
thought, despite living through massive asteroid bombardments.

Between 4.15 and 3.8 billion years ago, storms of asteroids battered the Moon
and Earth. While this 鈥渓ate heavy bombardment鈥� was going on, the Earth鈥檚 surface
was uninhabitable, or so researchers used to think.

That assumption was challenged a few years ago when Steve Mojzsis of the
University of California at Los Angeles discovered hints of ancient
life鈥攊n the form of telltale ratios of carbon isotopes鈥攊n rocks that
appeared to be more than 3.85 billion years old.

Now Ariel Anbar and Gail Arnold of the University of Rochester in New York
State have found that these rocks, from Akilia Island off Greenland, contain
surprisingly small amounts of the metal iridium. When asteroids strike they
deposit iridium, so Anbar reckons that while these rocks were forming, asteroids
were not raining down as heavily, making the surface more hospitable to
life.

鈥淒uring most of this late heavy bombardment the Earth was a perfectly
reasonable place for microbes to live,鈥� Anbar says. The worst impacts were 10 to
100 million years apart, and a handful of hardy bacteria could have survived at
the bottom of the oceans to repopulate the planet.

It would have been 鈥渁 very bad time to try to develop any sort of advanced
life鈥�, agrees Kevin Zahnle of the NASA Ames Research Center in California. 鈥淏ut
not a bad time for bacteria.鈥� The work will be published in a forthcoming issue
of the Journal of Geophysical Research鈥擯lanets.