A New Clock

It is one of life's great unexplained chemical quirks: the 20 amino acids found in all living things almost always have the same geometric configuration. The structure can best be demonstrated by crystallizing the amino acids and passing polarized light through them; the light waves are always rotated to the left. Yet after an organism dies, its amino acids undergo a radical change. The lefthanded molecules gradually become righthanded. Although it has long been known that the mirror-like reversal progresses steadily as time passes, a practical use for the phenomenon has now been suggested: it can be used as a geological "clock" to fill important gaps in the earth's fossil records.

The new dating technique was conceived by Chemist Jeffrey Bada of the Scripps Institution of Oceanography while he was trying to date some fossil-laden sediment from the ocean floor. The standard method for determining the age of fossils is the so-called carbon 14 clock, which is based on the ratio of ordinary carbon atoms to atoms of the radioactive isotope carbon 14 found in the specimen. The carbon 14 atoms decay at a known rate and are not replenished after the creature dies; thus the proportion of ordinary carbon to carbon 14 slowly increases. But the carbon clock only works up to about 50,000 years after the death of the organism. After that time, almost all the carbon 14 has vanished, and other radioactive clocks must be used. Trouble is, the alternative clocks are usually accurate only with objects at least a million years old. That leaves many pages of the past still partially closed to scientists, including some of the most critical years in man's own evolution.

Bada decided amino acids might help open those pages. Using standard lab equipment, he found that it was an easy matter to measure the ratio of left-handed to righthanded molecules in a common amino acid called isoleucine, and he was able to estimate the age of fossils from that ratio. What is more, his tests required only a tiny sampling of material and could be completed in a few hours. There is one serious hitch, he reports in Earth and Planetary Science Letters. Because the rate at which amino acids change their configuration varies significantly with heat, the temperature history of the specimen must be taken into account. Still, if an object has been sheltered in a cave or buried in the ocean floor, that should not be a great problem: the fossil is not likely to have undergone any great temperature variations that would upset the age calculations.

To check his theory, Bada dated a number of objects, including an ancient hominid bone dug up from East Africa's Olduvai Gorge by Anthropologist Louis S.B. Leakey. Its age, based on amino-acid dating, turned out to be 135,000 years--almost exactly the same as that deduced by Leakey from indirect geological evidence. Bada is still incredulous over the seemingly accurate results obtained by using his new clock. "It was so obvious and simple," he says, "I was just amazed that it hadn't been discovered before."

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