The evolution of life on Earth has given rise to the endless forms most beautiful that weave a complex web of origin, diversification and extinction. Unraveling genomes and reconstructing molecular phylogenies can now measure the evolutionary distance between living species. However, the fossil remains that litter deep time and record the evolution of all life on Earth are not so easy to characterize. The DNA that so defines life is a fragile molecule, unable to resist even the gentlest ravages of geological time.
The
evolution of life on Earth has given rise to the endless forms most beautiful
that weave a complex web of origin, diversification and extinction. Unraveling
genomes and reconstructing molecular phylogenies can now measure the
evolutionary distance between living species. However, the fossil remains that
litter deep time and record the evolution of all life on Earth are not so easy
to characterize.
The DNA that so defines
life is a fragile molecule, unable to resist even the gentlest ravages of geological
time. The molecule of life is only recovered from rare samples no older than 1
million years, but only in rare and 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 hard to identify. However, the breakdown of organic
material through time can yield hydrocarbons that provide unequivocal evidence
that such molecules can survive, albeit in an altered state, through deep time.
Hydrocarbons in crude oil are composed mostly of alkanes, cycloalkanes and
several aromatic hydrocarbons with additional organic compounds containing
nitrogen, oxygen and sulfur, and dilute amounts of trace-metals (iron, nickel,
copper, zinc and vanadium). The composition of crude oil is a chemical ghost of
past life that might well be echoed in fossils.
The
very atoms that construct biological materials can and do survive the sands of
time, else we would not find fossils, but can these atoms be imaged to relay information
about the original organism? Recent work has shown that there are elemental biomarkers
that we can identify and map in both living and extinct organisms (plants and animals).
Such biomarkers are powerful tools when unlocking the puzzle of organismal
biology, physiology and the very biosynthetic pathways that built, regulated
and drove the evolution of life. For the very first time, synchrotron-based
imaging techniques are allowing us to piece together the complex relationships
between trace-metals, rare earth elements and the discrete tissue types that
comprise life, both past and present.
The
fragile paradigm that fossils merely represent shadows of past life clearly has
to shift, not with the promise of DNA or intact proteins, but the fundamental building blocks
of life, including organometallic complexes and the breakdown products of original
proteins. Through the analysis of exceptionally preserved fossils from
different ages and contrasting preservational environments, we have begun to
chemically resolve and map such discrete elemental biomarkers. These results are
permitting us to develop a model for degradation of biological tissue, improve
our ability to use biomarkers to resolve discrete biosynthetic pathways, and
possibly begin to ask new questions on the evolution of life on Earth. This elemental
“Rosetta stone” can help us translate the chemical residues that survive in
fossils to an intelligible record that provides detailed information on tissue
types, composition, trace metal inventories and their distribution through deep
time. While the genome and proteome leave scant evidence to resolve the biology
of life, the metallome has the potential to push back our understanding of life
by billions of years!