Biogeochemical record of ancient humans

The geochemical record of humans extends back to their first appearance on the landscape some one million years ago. Biochemical remains from humans, however, can be found in the fossil record for only the last 100 000 years, and even then these remains are only fragmentary. Stable carbon and nitrogen isotopes in organic and inorganic fossils of humans are providing a great source of information on ancient human biogeochemistry. Two case studies demonstrating the utility of isotopic analyses for tracing the history of human behavior are presented. In the first, human skeletons from Easter Island were analyzed to determine whether the diet included foods from the marine environment. Isotopic records show that a sizeable influence from marine N can be detected in the ecosystem of this island. In a second study, the duration of nursing by prehistoric people was pinpointed with isotopic analysis. A study of modern humans was compared with analyses of two skeletal populations — Archaic Indians living 5000 years BP and Plains Indians from 1600 AD. Duration of nursing was the same in both populations, even though the Archaic Indians did not practice horticulture, whereas the Plains Indians grew crops. The geochemical record on the population growth of the human species has many implications. Recent developments include the analysis of stable isotopes in individual compounds from human remains. Carbon and nitrogen isotopic compositions of amino acids have been used to determine the source of dietary C with a C4 plant influence on human collagen. Also, the relative distribution of C and N isotopes in amino acids of these humans is compared with those in collagen from modern humans. Human diet and feeding behavior differ from that of almost all other animals (with the exception of primates and possibly cetaceans), because of the humanʹs ability to think and resulting societal pressures. Finally, when human remains are scarce, the search for human activity reverts to materials that serve as proxies for paleostudies. The future of this field lies in expanding tools of biomedicine to the study of ancient DNA. Expansion of isotopic techniques includes the study of lipids in skeletal remains and associated archaeological materials. The challenge will be to broaden the techniques and samples available for analysis to proxy materials not directly associated with skeletal remains so that an even older human record can be read.