Pleistocene Geochronology and Palaeothermometry from Protein Diagenesis in Ostrich Eggshells: Implications for the Evolution of Modern Humans

Proteinaceous residues incorporated within the crystal structure of ostrich eggshells (OES) are retained without loss over geological time exceeding 10 million years. Degradation of the polypeptides, including hydrolysis to smaller peptide fragments and eventual release of free amino acids, decomposition, and racemization and epimerization occur at regular, predictable rates dependent on ambient temperature. The extent of isoleucine epimerization (aIle/Ile ratio) in OES follows linear first-order reversible kinetics in controlled-temperature laboratory simulations of time up to an aIle/Ile ratio in excess of 1.0. The hydrolysis of leucine also follows a predictable pattern, but deviates from first-order kinetics. A non-linear mathematical model has been developed that adequately describes the pattern of leucine hydrolysis through a wide temperature range. Arrhenius parameters were derived from laboratory experiments combined with rate constant values found for $^{14}$C-dated OES from stratified caves in southern Africa. These parameters for isoleucine epimerization and leucine hydrolysis differ by ca. 10%, allowing the simultaneous solution of the two equations for temperature, independent of sample age. Although the uncertainty of the simultaneous temperature is relatively high ($\pm $ 10 degrees C), it provides an effective means of identifying burned samples. If sample age is known, palaeotemperatures (the integrated thermal history experienced by an eggshell as opposed to an `instantaneous' temperature) can be calculated with a precision of better than $\pm $ 1 degrees C. The ages of levels at Border Cave, South Africa, from which anatomically modern human skeletal remains have been recovered, are dated by the extent of isoleucine epimerization in associated OES. The reaction is calibrated in the upper levels by a series of concordant radiocarbon dates on charcoal at 38 ka before present (BP). The amino acid dates on deeper levels indicate that the Howiesons Poort stratum at Border Cave is more than 70 ka old, and that anatomically modern humans occupied the site as early as 100 ka ago.