Sunday 4 September 2016

A New Contender For The Earliest Known Fossils

The identity of the oldest fossil is hotly contested. As palaeontology is the science which concerns itself with the remains of past life on the planet, the oldest definite traces of life are perhaps its most important pursuit. For decades the focus has been the rocks of the Pilbara craton in Western Australia. Ranging from 3 to 3.5 billion years old they are among the more ancient records of the early planet, and as they are largely sedimentary, have excellent fossil preservation.

New discoveries are always contentious, however. Recently the Apex Chert fossils, dated at 3.43 billion years old and hailed for years as the oldest fossils on earth, were shown to be no more than hydrothermal artefacts which happened to look like bacterial filaments.

The site in Isua where the fossils were identified
Since then, fossils from Pilbara have buffeted this title back and forth by a few tens of millions of years, most recently settling on 3.48-billion-year-old stromatolites from the Dresser Formation.

Yet a new discovery led by Professor Allen Nutman from the University of Wollongong, Australia, may push back the definitive record of life on earth by an incredible 220 million years.

Rocks older than those of the Pilbara craton are rare, but a significant amount exists in the Isua Supracrustal Belt in Greenland. The older sediments in the Belt have been largely metamorphosed and so have offered little opportunity for palaeontological study. The likely presence of stromatolites, however, has been identified in sediments dated to 3.7 billion years old.

A and B: the 3.7 billion year old Isua stromatolites. C and D:
younger, indisputable Australian stromatolites for comparison
The rocks were recently exposed by snowmelt and consist of dolomites and storm-wave generated breccias. Rare earth element signatures in the rocks suggest a marine setting. This image of a shallow sea fits with younger, uncontested stromatolite assemblages. The macroscopic nature of the stromatolites is a good start, but there are many processes which can create similarly shaped, layered structures.

Their biogenicity, however, is supported by four lines of evidence. The specimens are internally laminated in a way which could not have been generated by post-depositional processes; an isotopic and rare earth element signature which precludes a hydrothermal origin; the presence of an originally low temperature dolomite which requires a microbial origin; and on lapping of the dolomite matrix with the stromatolites, indication growth above the boundary of the sediment and the water, a non-sedimentary phenomenon.

The age of the stromatolites is the same as the most parsimonious estimates for the origin of life. Their complexity therefore indicates that the origin of life is older still. How much older is difficult to say. Preserved carbon isotope ratios indicate biological processes at 3.8 billion years old. Isotopic evidence by itself is not the strongest case, but these newly discovered stromatolites suggest that the signature may be genuine. Geological evidence shows that the early planet was much more hospitable than previously suspected and so the origin of life may well be over four billion years old. Such an age also has intriguing implications for how life may have affected early geochemical cycles on the planet to make it more hospitable, thus paving the way for later, more complex developments in evolutionary history.