Immunological methods in molecular palaeontology

Variable amounts of proteins or protein fragments may survive in fossils for thousands and occasionally millions of years. The quantities are generally too small to determine accurate amino acid sequences, but even these small amounts may be immunologically detectable and may retain useful genetic information. A very sensitive solid phase radioimmunoassay has been used to analyse fossil proteins, particularly albumin and collagen, the most abundant animal proteins and those most likely to be found in fossil skin, muscle, bones and teeth. Species-specific proteins have been identified in the skin and muscle of mammoth, quagga and Tasmanian wolf, and in the bones of the extinct mastodon and Steller’s sea cow. Specific albumin has also been detected in bloodstains on ancient stone weapons and in fossilized urine from the middens of cave-dwelling pack rats, porcupines and hyraxes.

Accuracy of North American Human Skeleton Ages

Accelerator mass spectrometry (AMS) radiocarbon dates fail to provide conclusive evidence that all New World human fossils are younger than approximately 11,000 yr. Because fossil bones vary widely in preservation, their radiocarbon dates are not equally accurate. Molecular-level radiocarbon dating, which used individual amino acids to assess fossil diagenesis, revealed that dates on known-age, noncollagenous bone were underestimated by at least 2000 to 9000 yr. The significance is that >11,000-yr-old fossil bones with poor preservation would yield Holocene and not Pleistocene radiocarbon ages, regardless of what chemical pretreatment or 14C counting method was used. Irreplaceable evidence for Pleistocene-age fossils in the New World could be lost if the diagenesis of fossil bones is not evaluated before the bones are radiocarbon dated. In contrast, radiocarbon ages for collagenous fossils can be determined more accurately if 14C is measured in several individual amino acids that are isolated from collagenous bone protein. Molecular-level radiocarbon dating will greatly improve not only the accuracy of chronologies for human migrations and animal extinctions, but of all late Quaternary chronologies that are based upon the 14C dating of fossil proteins.

Fossil proteins in vertebrate calcified tissues

Selected fossil vertebrates and the enclosing sediments dating from 1300 years B.C. to approximately 400 million years ago were subjected to amino acid assay. The amino acid analyses revealed little evidence of intact collagen in fossils of Tertiary, Mesozoic or Palaeozoic age. There was, however, evidence of contemporary proteinaceous material which may have been derived from bacteria. In Palaeozoic material the analyses detected a general background of amino acids common to both fossils and sediments. The degree of racemization was routinely determined as a means of measuring modern contamination of geologically older samples. An electron microscope study of Quaternary (Pleistocene) collagen revealed a significant reduction of the 64 nm banding to about 50 nm. The same Pleistocene material gave amino acid compositional profiles typical of collagen. However, when this material was subjected to digestion by the proteolytic enzymes collagenase, pronase and subtilisin, the resulting peptide fingerprints showed small but significant differences from those obtained from modern collagen digests, indicating the possibility of changes having occurred during fossilization affecting susceptible cleavage sites in the molecule.