Life may have begun on Earth many times, only to be destroyed by repeated global catastrophes before it finally took hold and evolved to its present state, two geologists have inferred from a novel approach to the subject.

Their study indicates life could have originated as long as 4.2 billion years ago but that it might not have been able to escape catastrophe until 3.6 billion years ago.

The oldest known fossils are of microscopic, one-celled forms that date from about 3.5 billion years ago.

The new study, reported in last week's Nature, is based on hypothetical calculations about how often the young Earth experienced two rival phenomena. One is how long it might take for nonliving chemicals to assemble into a primitive form of life. The other is how often impacts of large meteorites would have blasted out a sun-blocking dust cloud analogous to the one that may have wiped out the dinosaurs. Such events, the geologists say, would have been "terminally uncomfortable for life."

The numbers, which the researchers concede are highly uncertain, are as follows:

For life to begin in the kind of chemical-rich hot water vents recently discovered on the ocean floor (a new and promising theory) -- between 100,000 and 1 million years. For life to arise in the classical "warm little pond" -- between 300,000 and 3 million years. For life to form in moist soil (a minority view) -- between 1 million and 10 million years.

The rate of meteorite bombardment of the early Earth, judging from the comparable bombardment of the Moon, was too high immediately after Earth formed, perhaps 4.6 billion years ago, to allow time for any scientifically plausible life-creating scenario. As the interval between major impacts lengthened, however, life could have had time to begin, but it would have been exterminated by the next big impact.

If life began in the deep ocean hot water, or hydrothermal, vents, its first appearance could have been between 4.2 billion and 4 billion years ago. If it arose at the surface, either in soil or a shallow pond, the first instance could have been as early as 4 billion to 3.7 billion years ago.

"If life could have evolved in or near the mid-ocean hydrothermal systems, then it probably began there," Kevin A. Maher and David J. Stevenson, both of the California Institute of Technology in Pasadena, wrote in their paper. " . . . It might have been more likely to propagate as well.

"Wherever life did first appear, it would seem possible that it was eradicated from at least the surface of the planet (perhaps several times), re-evolving in some new location or radiating from a preserved, more heat-tolerant population each time before it took possession of the Earth undisputed by impact events," the study said.