While life remained stuck at the single-celled level for a billion years on an oxygen-poor Earth, evolution on Mars might have led much more quickly to microscopic animals. But then, about 3.5 billion years ago, the planet turned chilly, and the Martians - if they ever existed - would have been snuffed out in any of several ways.

The likely suspect, in Wood's eyes, is a fragment from Biela's Comet, which had been circling the sun every six years and nine months before a close encounter with Jupiter caused it to break into two large fragments in 1845. During its next passage, astronomers noted a 1.5-million mile, 15-day gap between the two pieces.