Amino acid dating.

Investigation of amino acids in fossils over the past thirty years has revealed that residual amino acids may exist in fossils from throughout the Phanerozoic portion of the geologic column, that the amino acid pattern in a given fossil changes with age due to differences in stability among the twenty amino acids of which proteins are constructed, and that the ratio of right-handed to left-handed forms (D/L ratio) of amino acids increases with age from zero in the proteins of living organisms to the ratios which are characteristic of amino acids produced synthetically (the racemic ratios). The possibilities for using these characteristics as a means for determining fossil age are frustrated by variations of the amino acid pattern among individual living organisms of the same species, and by the critical dependency of the racemization probability for an amino acid molecule on temperature, water concentration in the environment, alkalinity of the environment, association with other molecules (free state or a component of a macromolecule), size of the macromolecule of which it may be a component, specific location in the structure of a macromolecule, catalytic effect of clay surfaces with which it may be associated, presence of aldehydes and metal ions, concentration of buffer compounds in the environment, and ionic strength of the environment. In spite of these complications, fossils of similar characteristics, and which have experienced similar conditions of preservation, can be placed in a relative age sequence on the basis of D/L ratios. Due to the strong dependency of racemization rates on temperature, water concentration, and alkalinity, uncertainties regarding conditions of preservation can leave amino-acid-based age relationships among even similar fossils open to question. At the present time there is insufficient knowledge concerning the effective average racemization rate in a fossil as a function of time to justify dependence on D/L ratios for a quantitative determination of age. The survival of amino acids in fossils from the Paleozoic era and the trend for the apparent racemization rate constant to decrease with conventional fossil age assignment raise a serious question concerning the accuracy with which radioisotope age data have been used to represent the real-time history of fossils.