Conclusion
Thermal setting and geologic age have been commonly used as proxies for predicting molecular sequence preservation potential3, 5, 6, 13. Late Pleistocene and Holocene specimens from cooler regions, especially permafrost deposits, have been shown to generally possess the highest preservation potential for molecular sequence information5, 6, 8, 14, 15. However, depositional environments are influenced by other variables including moisture17-21 and oxygen content17-20, 22, 23, ion species present, and sediment composition16-20. These confounding variables limit the usefulness of thermal setting and geologic age as proxies outside of a broad scale.
Direct analysis of fossil and sub-fossil molecular histology is a potential answer to this limitation. The molecular histology of a specimen’s preserved cells and tissues reflects the cumulative effects of environmental variables upon its constituent biomolecules, including DNA and protein sequences17, 20, 27. Observed degradation of cell and tissue molecular histology is hypothesized to correlate with constituent biomolecules having undergone degradation. This agrees with the limited data in the primary literature on the correlation of molecular histology with sequence preservation potential2, 33, 57, 62. Thus, the preserved state of fossil/sub-fossil molecular histology is predicted to be an accurate proxy for molecular sequence preservation. A potential limitation to this approach is that some aspects of molecular histology may be beyond resolution or limit of detection for current molecular methods. However, modern molecular instrumentation regularly functions on the micro- and nanoscale in terms of resolution and limit of detection63, 64, 68, 69, 78-80, 88, thus minimizing this limitation as a potential obstacle. The use of fossil/sub-fossil molecular histology as a proxy for sequence preservation has potential to elucidate why ancient specimens of some formations and timepoints preserve sequences while others do not; such understanding would facilitate the selection of ancient specimens for use in future ancient DNA and paleoproteomic studies.