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Some 4.5 billion years ago, the solar system took shape inside a chrysalis of gas and dust. Small objects formed first, then slammed into one another to create the planets. Early on, the energy unleashed by these violent collisions turned the embryonic earth into a molten ball. For a billion years thereafter, the young planet's gravitational field attracted all sorts of celestial garbage. Icy comets screamed in from the outermost reaches of the solar system, while asteroids and meteorites spiraled down like megaton bombs.
Some of these asteroids could have been the size of present-day continents, says planetary scientist Christopher Chyba, a White House fellow, and the asteroids' impact would have generated sufficient heat to vaporize rock, boil the oceans and fling into the atmosphere a scalding shroud of steam. Such a cataclysm would have obliterated all living things.
Yet after a billion years, when the solar system was swept nearly clean and the primordial bombardment ended, life was already flourishing. UCLA's Schopf has identified the imprints of 11 different types of microorganisms in the 3.5 billion-year-old rocks of Western Australia. Many of the fossils closely resemble species of blue-green algae found all over the world today. Still older rocks in Greenland hint of cellular life that may have come into existence a few hundred million years earlier -- perhaps 3.8 billion years ago.
At that time, scientists believe, life-threatening asteroids were still periodically pummeling the planet. Verne Oberbeck and colleagues at NASA Ames Research Center estimate that the interval between major impacts could have been as short as 3 million to 6 million years -- much too brief a time to give life a leisurely incubation. This means, says Oberbeck, that the chemistry needed to green the planet must have been fast, and it must have been simple. * That being the case, he asks, why wouldn't life have arisen more than once?
THE POINT OF ORIGIN
Where could life have sprouted and still been relatively safe from all but the largest asteroids? For the answer, many researchers are looking to strange, chimney-like structures found in the depths of oceans. These sit atop cracks in the ocean floor, known as hydrothermal vents, that lead to subterranean chambers of molten rock. The result is an underwater geyser: cold water plunges down through some of the cracks, and hot water gushes out through others. Fifteen years ago, when scientists began using submarines to explore these seemingly hostile environments, they were startled to discover extensive ecosystems filled with strange organisms, including giant tube worms and blind shrimp. Even more interesting, according to analysis of their RNA, the sulfur-eating microorganisms that anchor the food chain around the vents are the closest living link to the first creatures on earth. The only other life-forms that archaic are microbes living in surface steam baths like Yellowstone's Octopus Spring.