Prehistoric diets: down to the bone.

Several scientists have proposed, quite reasonably, that prehistoric hunter-gatherers moved back and forth between coastal and inland camps in order to exploit seasonally abundant food sources. New evidence indicates, however, that some humans living from 2,000 to over 8,000 years ago on the southwestern Cape of South Africa did no such thing.

Measurements of food intake, reflected in stable carbon isotope ratios of human bones from that time period, suggest that there were distinct coastal and inland populations that did not travel great distances during the year. This surprising finding casts doubt on assumptions about the diets of hunter-gatherers elsewhere, say archaeologists Judith C. Sealy and Nikolaas J. van der Merwe of the University of Cape Town in South Africa.

All previous archaeological evidence in the southwestern Cape pointed to summer occupation at inland sites and winter occupation in caves near the ocean. Prehistoric inaldn deposits are rich in the remains of plants that are best eaten in late summer; also present are the bones of tortoises, which are most active and easiest to collect in summer. Coastal sites contain remains of limpet and mussel shells, which are safer to collect in the winter, when they are less likely to be rendered toxic by outbreaks of a minute type of poisonous algae. Nearly all the jaws of Cape fur seals found in these camps are those of yearlings that died in summer months (the seals are born in November).

The South African researchers took a closer look at these and other foods known to be important to prehistoric humans in the southwestern Cape. By calculating the ratio of stable carbon isotopes in a wide array of coastal and inland foods, they found that a marine-based diet has an isotope "signature" substantially different from a land-based one.

They then measured isotope ratios in the bones of 18 prehistoric human skeletons, 14 of which were uncovered at coastal sites and 4 at inland mountain deposits. A seasonally shifting population would have had similar ratios falling between the marine and terrestrial "signatures," assert the researchers in the May 9 NATURE. Instead, the coastal skeletons reflect a predominantly marine diet and the mountain skeltons an almost entirely land-based diet. Thus, say the investigators, a nomigratory pattern of life appears to have characterized these people.

The sample is too small to identify sub-groups that may have seasonally migrated, they ad, but the further use of isotope ratios in South Africa and elsewhere will provide important food for thought about prehistoric diets.