WAS THE COSMOS SEEDED WITH LIFE?

Did

life emerge spontaneously on earth, or did it come from outer space? The scientific community is sharply split on the question, and the evidence from Mars not only heats up the debate but also adds a tantalizing third possibility: life-forms may have arisen on Mars first and then hitched a ride on a meteorite to Earth--or vice versa. As Stanford University chemist Richard Zare puts it, "Who is to say that we are not all Martians?"

Sound implausible? Consider the alternatives. Sir Fred Hoyle, the distinguished British astronomer, favors an even more radical theory. The idea, known as panspermia, is that billions of years ago, the solar system was peppered by biological "seeds," which took root wherever conditions were right. That would explain how life may have arisen at roughly the same time on Earth and on Mars. But it also raises awkward questions about where those seeds came from and what, or who, sent them flying through space.

Most scientists lean heavily toward the less disturbing theory that life arises spontaneously through commonplace chemical reactions. New findings over the past decade tend to support that idea. "Today life occurs on Earth everywhere you look," says Washington University geochemist Everett Shock. "It's in the Antarctic ice sheet. It's in hot springs. It's buried deep in the sea floor. Why not just assume it started here?"

There is something to the panspermia theory, however. Even scientists who reject it acknowledge that some of life's building blocks probably had extraterrestrial origins. Indeed, they now believe that everything from organic chemicals to amino acids, the constituents of proteins, was carried in by the comets, asteroids and meteorites. And if life happened to form elsewhere in the solar system first, muses biochemist Gerald Joyce of the Scripps Research Institute in La Jolla, California, then it's at least possible that something more complex could have been included in the cargo--not necessarily a living organism but a molecular precursor that could have given life on Earth "a kind of kick start."

What about the idea that Mars seeded Earth? Recent findings suggest there could have been substantial biological exchange between the planets. Every year, researchers calculate, two tons of Martian material rain down on Earth, and two tons of terrestrial rock smash into Mars. The chances that a primitive creature secreted in this rock may survive such a journey are beginning to look surprisingly good. It takes 10 million years or so for a piece of Earth to reach Mars, and some scientists argue, on the basis of organisms trapped in ancient amber, that bacteria can survive even longer.

But have organisms actually made such a journey? If scientists find living microbes on Mars, they may be able to answer this question. Should Martian life-forms base their genetic code on dna, for example, researchers could decipher the script and determine whether they were related to the microbes that populate Earth. And if those life-forms didn't rely on dna at all, "that would be even more fantastic," says molecular biologist Jack Szostak of Massachusetts General Hospital. For such a discovery would not only prove that life arose on Mars and Earth independently and, therefore, spontaneously, but it would also strongly suggest that life--unicellular life, at least--is not something rare and special in the universe but an ordinary event that occurs wherever there is enough water and light from a sunlike star.