The origin of life on earth appears more and more inevitable — or does it?

These have been exciting times for scientists striving to learn about the origin of life. The possible discovery of fossil microbes in a Martian meteorite has already grabbed the public's attention; another provocative recent study suggests that life may have arisen on the earth far earlier than previously thought. Such work has sharpened one of the most fascinating and least tractable debates in modern science: Is life a remarkable aberration, or is it a likely, perhaps even certain, outcome of the laws of nature?

On one side are scientists such as the British astronomer Fred Hoyle, who once declared that the emergence of a living cell from an inanimate chemical soup is about as likely as the assembly of a 747 by a whirlwind passing through a junkyard. Let's call them improbabilists. On the other side are inevitabilists, who believe the origin of life was a plausible consequence of basic chemical processes. "My guess is that life probably isn't all that rare and the origin of life probably wasn't all that improbable," remarks the inevitabilist Richard Dawkins of the University of Oxford in his latest book, Climbing Mount Improbable.

One favorite argument of inevitabilists concerns the rapidity with which life emerged on the earth after the planet's formation 4.5 billion years ago. The earlier that life began, the more inevitable it must have been — or so the argument goes. Inevitabilists have thus been greatly heartened by the recent report, published in the November 7 issue of Nature, which pushes the date for the existence of terrestrial life back to at least 3.85 billion years ago.

This finding comes from an international team of scientists that found what it believes to be chemical traces of life — carbon isotopes in a ratio characteristic of living organisms — in rocks from Akilia, an island off the coast of Greenland. The rocks, which have been dated at 3.85 billion years of age, are the most ancient known to science.

Previously, the oldest evidence for life was fossilized microbes from a rock formation in Australia that is 3.5 billion years old. The new, earlier date is particularly significant, because some evidence indicates that during this period the earth was still a hellish place. In the late 1980s geologists studying craters on the moon concluded that until roughly 3.8 billion years ago, the moon and, by inference, the earth were being constantly blasted by meteorites; some of the terrestrial impacts would supposedly have been large enough to incinerate any life.

If life did indeed exist during this era, it was not necessarily the precursor to modern organisms; it may have been wiped out by a meteor impact only to spring up again elsewhere, notes John M. Hayes of the Woods Hole Oceanographic Institution. But the existence of any kind of biological organism 3.85 billion years ago suggests that life developed with "breathtaking rapidity," Hayes declares in a commentary on the new finding in Nature.

Indeed, this report, together with the much heralded announcement by other scientists this past August that they had found what might be fossilized remofnants of life in a meteorite from Mars, implies that life is a robust and possibly ubiquitous phenomenon. At least, that is the contention of such leading inevitabilists as Stuart Kauffman, a biochemist and complexity theorist at the Santa Fe Institute.

Nevertheless, the improbabilist position still has a few rather substantial facts in its favor. First, the evidence that life existed on earth 3.85 billion years ago — like the evidence for ancient life on Mars — remains far from conclusive. The claims rest on subtle and possibly ambiguous analyses of chemical traces. As various researchers have pointed out, non-biological processes could have produced these putative fossil remnants on both worlds.

Moreover, the underlying uniformity of life on the earth (most notably, all modern organisms share the same DNA-based mechanism for genetic transmission) suggests that life emerged here only once during the planet's entire history. There is no evidence that any other type of life developed independently, even for a brief period.

And despite the immensely powerful tools associated with modern biotechnology, scientists still cannot transform inanimate matter into life in the laboratory. Pondering this problem, the Nobel laureate Francis Crick once lamented that the origin of life seems "a miracle, so many are the conditions which would have to be satisfied to get it going." As a partial solution, Crick proposed in his 1981 book, Life Itself, that the first organisms might have been brought to the earth by intelligent aliens. That answer, of course, only shifts the problem of life's origin to some as yet unknown but perhaps more conducive setting elsewhere in the universe.

Crick's argument echoed the so-called panspermia hypothesis developed more than a century earlier by the Swedish physicist Svante Arrhenius, who thought that the seeds of terrestrial life drifted here from elsewhere in the universe. Arrhenius's grandson, Gustaf Arrhenius of the Scripps Institution of Oceanography in La Jolla, Calif., continues to wrestle with the questions that so fascinated his ancestor: he headed the team that analyzed the Akilia rocks.

Gustaf Arrhenius neither rejects nor adheres to his grandfather's theory of life's origin. He is also an agnostic on the whole question of life's probability — or lack thereof — noting that there is simply too little evidence to settle the issue. He contends, for example, that the lunar data do not conclusively demonstrate that the earth was battered by meteorites until 3.8 billion years ago; no signs of severe impacts show up in the few terrestrial rocks remaining from that period. It is therefore quite possible that the earth was suited for life much sooner after its formation.

Arrhenius hopes that future missions to Mars may help to resolve questions over the "late bombardment" scenario. Such missions could also determine whether microbial life did indeed once take hold there — or whether it even still exists. The current Mars Pathfinder and Mars Global Surveyor missions are trailblazers that cannot search directly for life, however; the robotic probes needed to undertake the necessary experiments are still a decade or more away. Until at least then, Arrhenius says, the debate between the inevitabilists and the improbalists will remain "pure speculation."