A frustrating start for life on earth.

A frustrating start for life on earth

Comets and meteors striking the earth frequently during its early history may have repeatedly exterminated the first living cells from the planet, forcing life to originate over and over again, according to two earth scientists who have examined the record of craters on both earth and the moon.

The records indicate that during the first billion years of the earth's history, the frequency of large impacts declined until leveling off around 3-1/2 billion years ago. Small meteors, the most numerous objects to hit the earth, would have caused local catastrophes, upsetting a small portion of the young planet, which scientists believe was largely covered by oceans. But larger impacts could have sterilized the whole earth by sending water and dust into the atmosphere, creating huge waves and raising the planet's temperature.

As primitive biological molecules were arranging themselves into living cells, frequent impacts may have disrupted the environment before these cells developed the ability to reproduce. Therefore, only when the developing cells had enough time between impacts to become self-replicating would they establish a foothold on the planet, suggest Kevin A. Mahler and David J. Stevenson in the Feb. 19 NATURE.

Basing their suggestion on the crater record, the researchers say life could have evolved on land or in shallow seas as far back as 4 billion to 3.7 billion years ago. On the other hand, if life began near hydrothermal vents in the deep sea, it would have escaped some of the smaller global catastophes. Life in this locale could have started as early as 4.2 billion years ago.

Mahler and Stevenson, from the California Institute of Technology in Pasadena, suggest that organisms that formed near hot springs would be better equipped for surviving the elevated temperatures after large impacts. "If life could have evolved in or near the midocean-ridge hydrothermal systems, then it probably began there," say the researchers.