Fossils more than a couple hundred million years old are usually destroyed as the sediments are drawn under the Earth's crust and melted. However, investigators have been able to study the traces of the isotopes carbon-12 and carbon-13 and propose a time for the origin of life. In biological processes, carbon-13 is reduced, such that there is a reduced ratio of carbon-13 to carbon-12 that is indicative of life. This ratio, undisturbed by geological melting, was measured in rocks in Greenland known to be the oldest in the world. The rocks had been dated at 3.85 billion years ago, and interestingly, the reduced ratio of carbon-13 to carbon-12 in the rocks matches that characteristic of biological life. Scientists then concluded that life may have began 3.85 billion years ago, promptly after the Earth was inhabitable.
"Scientists Push Back Origin of Life by Millions of Years," by Malcolm W. Browne, The New York Times, November 7, 1996