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.