Birth Date of Man

Geologists divide the earth's history into a sequence of periods: Permian, Jurassic, etc. But seldom do they agree on the age of each period, and a particularly annoying question mark is the Pleistocene, an epoch of intermittent ice ages during which man became true man. The geological dating system that uses the decay of uranium and other radioactive elements to tell the age of very ancient rocks is much too vague for the comparatively short Pleistocene. Dating by carbon 14, which is fine for recent times, reaches back only 60,000 years--not nearly enough.

A new system promises to pinpoint the Pleistocene. Developed at the University of Miami by Dr. John Rosholt of the U.S. Geological Survey and Italian-born Dr. Cesare Emiliani, it depends on the fact that a tiny amount of uranium is dissolved in all sea water. When it slowly decays radioactively, it yields protoactinium 231 and thorium 230, both of which attach themselves to sediment particles and sink slowly to the bottom. There they in turn decay, but protoactinium 231 decays faster than thorium 230. The age of sediment on the ocean floor can therefore be determined by measuring the relative abundance of the two isotopes.

By dating the ocean-bottom sediments, Rosholt and Emiliani estimate that the last warm interglacial Pleistocene period extended from 100,000 B.C. to 67,000 B.C., with its temperature peak coming about 93,000 B.C. Since the oldest skull fragments of Homo sapiens (true man) are believed to date from the warmest part of the last interglacial period, this date, 93,000 B.C., can be considered the provisional birth date of the human race.

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