Amino Acid Stereochemistry and the First Americans

Abstract

The timing of the peopling of the New World remains one of the biggest questions in archaeology, one that is complicated by the issues of exactly dating the materials from the sites in question. The rate at which the amino acid l-aspartic acid racemizes into its enantiomeric d-form was suggested to be one method for directly determining the age of the fibrous protein collagen preserved within bone. The rate of conversion of l- to d-amino acids is dependent upon several structural and environmental factors, and the calibration of amino acid racemization (AAR) dating relies on reliable radiocarbon dates. AAR dating in the 1970s of Paleoindian bones from California suggested great antiquity, as much as 70 000 years before present, for the arrival of humans in North America. With the advent of accelerator mass spectrometry in the 1980s making possible only the dating of the collagen fraction of the same bones, this arrival was shifted back to the Holocene, more in line with the ‘Clovis first’ theory. However, poor preservation of collagen and issues that arise from radiocarbon calibration complicate the matter further. In the intervening years, several unquestionably pre-Clovis sites have been identified throughout both North and South America, though much remains to be learned. Amino acid racemization dating is not a lost cause, however. Recent developments have shown its utility in dating shell over geological time periods, and for determining age at death in forensic cases.