The amino acids present in living organisms consist mainly of the L-enantiomers. Over long periods of geological time, amino acids undergo a slow racemization, producing the D-enantiomers. The amino acids contained in fossil materials show an increasing extent of racemization with age. This increase can be measured, and the result can be used to date materials that contain amino acid. The authors demonstrate the application of their finding by dating fossil bones, mollusk, shells, and fossil food. -- AATA