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But the amount of organic matter that can be carried by a meteorite is exceedingly small -- too small, many scientists believe, to have spawned life. For this reason, Chyba argues that a far more important source may have been interplanetary dust particles floating around in the era when earth was forming. Even today, he notes, countless tiny particles -- each potentially carrying a payload of organic compounds -- fall to earth like cosmic snowflakes, and their collective mass outweighs the rocky softball-size meteorites by a ratio of 100,000 to 1. Comets, black with carbon, could also have flown in some raw material. Whether it would have helped to spark life no one knows, since the chemical makeup of comets remains largely a mystery.
| And there's another possibility: big objects smashing into earth could have changed the composition of the atmosphere in significant -- albeit temporary -- ways. "Plow a big iron asteroid into earth," argues Kevin Zahnle of NASA Ames Research Center, "and you will certainly get interesting things happening, because all that iron is going to react with all the stuff that it hits." Such conditions, Zahnle speculates, might have briefly created the methane-filled atmosphere that Miller envisioned.
THE PRIMORDIAL CHEMISTRY LAB
Life's beginnings did not have the benefit of Miller's glass bottles, test tubes and vials. So how did nature bring the right ingredients for life together in an orderly fashion? One possibility recently suggested by Louis Lerman, a researcher at Lawrence Berkeley Laboratory, is that bubbles in the ocean served as miniature chemical reactors. Bubbles are ubiquitous, Lerman notes; at any given time, 5% of the ocean surface is covered with foam. In addition, bubbles tend to collect and concentrate many chemicals essential to life, including such trace metals as copper and zinc and salts like phosphate. Best of all, when bubbles burst, they forcibly eject their accumulated molecules into the atmosphere, where other scientists feel the most important chemistry takes place.
Biologist Harold Morowitz of George Mason University in Fairfax, Virginia, suspects that life arose in a less ephemeral chemistry lab than a bursting bubble. He focuses on Janus-faced molecules found in nature called amphiphiles. These molecules have one side with an affinity for water and another side that is repelled by water. Bobbing in the primitive oceans, the molecules would have hidden their water-hating sides away by curling into tiny spheres. These spheres, known as vesicles, would have provided an ideal setting for chemical reactions and could have been precursors to the first cells. "Once you have these little vesicles," says Morowitz, "you're on the way to life."