Light Stable Isotopes and the Reconstruction of Prehistoric Diets

Several stable isotope ratios have been measured in human skeletons to determine prehistoric diet. These include the isotopes of nitrogen, sulphur and strontium, which have been used to assess the importance of marine foods in human diets. The systematics of these isotopes in food chains are imperfectly understood, whereas that of carbon is quite well-known. Stable carbon isotope ratios (i.e., '3C/'2C ratios, reported as 6I3C values) are of particular relevance to terrestrial foodwebs because of the characteristic isotope signatures of plants with different photosynthetic chemistry. The two dominant photosynthetic pathways produce C, plants (all trees and woody shrubs; temperate and shaded grasses) and C, plants (subtropical and tropical grasses, except those in shaded forests). The carbon isotope ratios of the two plant groups are distinctive and are passed along the food chain to herbivores and carnivores. This fact has been used to trace the spread of, for example, maize agriculture in the woodlands of North America and in the tropical forests of South America, as well as rice agriculture in China. Such applications have invariably made use of bone collagen as sample material, which has restricted their use to the last 10,000 years or so. The alternative is to use substituted carbonates in biological apatite, particularly in tooth enamel, after appropriate chemical cleaning to remove diagenetic carbonates. Although still controversial, this method is being used with success to study the diets of early hominids of more than two million years ago.