|Archaeopteryx from the Late Jurassic of Southern Germany.|
Fossils provide us with the evidence that narrates the story of decent with modification that is the evolution of life on Earth. Unravelling genomes and reconstructing molecular phylogenies can now precisely measure the evolutionary distance between organisms in the tapestry of extant species. The DNA that defines life is a fragile molecule, unable to resist even the gentlest ravages of geological time. The molecule of life is recovered from rare samples no older than 1 million years old, and then only in exceptional circumstances. The proteome might be the next logical focus, as proteins are more robust and might leave tantalizing evidence for the very building blocks of life. Here the frustration is also evident to those who study such ancient molecules, as anything older than 10 million years is rare. Is there another way that we can unpick the biological codec concealed within fossil remains?
However, the fossil remains that litter deep time are not so easy to characterize, but have the potential to constrain much of what we know record about the evolution of life on Earth.
|Synchrotron-Rapid Scanning X-Ray Fluorescence Map of Archaeopteryx scanned at beam line 6-2 at SSRL|
Fossils are indeed partially composed of chemistry that directly links them to the organisms from which the fossils remains came. They really cannot be considered minerals (a solid naturally occurring inorganic substance), but are truly ‘geobiological’ composites of both inorganic and organic molecules that were constructed through biological and post-burial processes that preserve the fossil through deep time. The alteration that occurs to the biological tissue through subsequent mineralization rarely overprints the organic composition of an organism completely. Our team at the College of Charleston, University of Manchester and also at the Stanford Synchrotron Radiation Lightsource (Stanford University, USA) have been chemically mapping fossils (above) using multiple imaging techniques to elucidate these geobiological composites we commonly know as fossils.
The carbon cycle is remarkably efficient at recycling organic material, but under certain preservational circumstances, some of the chemical building blocks of an organism make it through this taphonomic (literally "burial laws") filter. In exceptionally preserved fossils, it is possible that remnants of structural proteins and associated organic molecules survive and can be mapped to help resolve original biological structures. The potential for new techniques to compositionally or spatially resolve such ‘chemical fossils’ is being realized with the recognition of elemental and organic residues that once comprised living tissue. Until the advent of techniques sensitive enough to resolve trace amounts of organic compounds and organically bound elements, it was difficult to untangle potential material transfer from microbes, geochemical fluids and the contamination from sampling and/or conservation techniques applied to a sample. However, the advent of synchrotron-based imaging and infrared spectroscopy has revolutionized sample analysis, enabling high-resolution scans that spatially resolve reaction aureoles, precipitates, etc. The suite of de novo techniques available to paleontology is completely changing our understanding of what constitutes a ‘fossil’.