Eggshells help date ancient human sites.
The use of a controversial technique to date an underappreciated type of ancient refuse -- ostrich eggshells -- may help resolve questions about modern human origins. A new study indicates that the technique, known as amino acid racemization (AAR), can provide reliable dates for eggshells found at early human sites from Africa to China.
AAR yields accurate ages of up to 200,000 years at tropical sites and up to 1 million years at cold-weather sites, asserts a research team led by anthropologist Alison S. Brooks of George Washington University in Washington, D.C., in the April 6 SCIENCE. Radiocarbon dates extend back only about 40,000 years.
After an organism dies, its amino acids realign themselves at a slow, relatively uniform rate to form a mirror image of their original molecular structure. AAR uses the proportion of the two structures as a biological clock. Because the realignment occurs more quickly and unpredictably in warm or moist climates, AAR dates are more accurate and extend back farther in cold regions. The method thus requires good estimates of a site's temperature history.
In laboratory tests, Brooks' team found ostrich eggshells prime candidates for AAR dating. Early humans apparently ate ostrich eggs and used the shells as water containers. The hardy shells are virtually impervious to chemical contamination through leaching, the scientists maintain. In the past, researchers have questioned AAR dates for bones and mollusks, citing possible contamination from groundwater and other sources.
Age estimates for eggshells at two African sites, one in the Kalahari desert and the other in the Sahara, closely match radiocarbon dates extending back as far as 35,000 years at the same sites, the scientists report. Eggshells found in older sediment layers at the Kalahari site date to between 65,000 and 85,000 years ago, according to AAR analyses based on temperature estimates going back about 200,000 years. Such "pre-radiocarbon" dates are harder to establish at the Sahara site, where the temperature history remains unclear, says study coauthor Gifford H. Miller of the University of Colorado in Boulder.
Ostrich eggshells at a South African cave containing anatomically modern human bones yield AAR ages of between 78,000 and 110,000 years, according to a recent analysis directed by Miller. However, another technique called electron spin resonance (SN: 4/29/89, p.263) yields an age estimate of only about 60,000 years at the same cave. Scientists have no consensus as to which date is more accurate.
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|Date:||Apr 7, 1990|
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