Australian geologists say they have found ancient microbial fossils in 3.7 billion-year-old rocks in Greenland. The finding, reported in Nature last month, is some 200 million years older than previously accepted fossils.
Previously, the best-known evidence for life in these rocks has been a fractionation of stable carbon isotopes. All organisms fractionate carbon isotopes, and the carbon isotope record of sedimentary rocks is good evidence for the presence of life as we know it.
The fractionation of isotopes in Greenland is less than in younger rocks, where values are consistent with known microbial metabolisms.
But since time, heat and pressure generally drive values in this direction, the researchers have interpreted the Greenland data as evidence that life had evolved by 3.7 billion years ago.
The evidence so far
The newly reported fossils are known as stromatolites. Stromatolites are layered mounds that form when mats of microorganisms precipitate minerals and trap and bind sediment particles.
By doing this, they modify the local environment in ways we would not expect from non-biological processes.
But many non-biological processes can mimic the shape and texture of genuine stromatolites. Such processes include the formation of sand ripples on the seafloor, the precipitation of calcium carbonate minerals on the seafloor and deformation that occurs after the rocks are deposited.
That’s why geologists must be very careful when they interpret stromatolites.
In rare cases, the actual bodies of individual microbes have been found in ancient stromatolites. And we can even watch stromatolites forming before our eyes in some modern environments including Shark Bay in Western Australia.
But truly ancient stromatolites are typically poorly preserved and their interpretation is not straightforward.
Interpretations based on biology have resulted from years of detailed study and intense scrutiny supplied by prominent sceptics. The oldest generally accepted examples are 3.49 billion years old, in the Pilbara region of Western Australia.
In the latest study, the overall scene is set by sedimentary features that suggest the rocks were deposited in a shallow-water marine environment. This is where stromatolites are often found.
Here, the researchers discovered dome- and cone-like structures made of dolomite (calcium magnesium carbonate) and quartz. Dolomite is a mineral found in many ancient stromatolites.
More data needed
But the study is preliminary and there is little additional information available. Further work is needed and until then, geobiologists are likely to maintain a degree of scepticism.
Stromatolites are by definition laminated (layered). At best, the Greenland features show a lamination that is considerably less well-defined than accepted examples from younger rocks. Wrinkled mat surfaces, trapped organic matter, or other features associated with biological activity are not reported.
The overall shape of a stromatolite can also be influenced by biological processes. For example, the movement of microbes inhabiting the microbial mat may produce characteristic shapes. A limited number of slices through the rock leaves the 3D shape of the Greenland features unclear.
Careful investigation of the composition and texture of a suspected stromatolite and the surrounding rocks can help geologists determine how the structure formed.
During their growth on lake or ocean floors, material washed in from elsewhere can accumulate in and around stromatolites, and this material is usually made of different elements and minerals.
Elemental mapping in two-dimensions would show the distribution of elements in greater detail. Micro-x-ray fluorescence instruments are being developed at the NASA Jet Propulsion Laboratory to perform this and similar tasks on Mars, during future rover missions.
But we needn’t be so high-tech. Traditionally, very thin slices of rock are examined for mineral content using microscopes. More thin sections of the Greenland features are likely to reveal additional details.
So, if not stromatolites, what else might these Greenland features be?
Currently, there isn’t enough information to rule out several alternative hypotheses, including flame structures and non-biological mineral precipitates.
Life on Mars?
If they are shown to be genuine fossils, one of the potential implications is that life may have been less affected by early meteorite impacts than previously thought.
The 200 million year age difference between the Greenland features and well-studied biosignatures in younger rocks could change our understanding of early solar system habitability, and the timing of the evolution of photosynthesis.
This in turn may have implications for the search for life on Mars, since much of the surface of Mars is thought to date from this time.
The landing site selection process for NASA’s next Mars rover mission is currently underway. Ancient lake deposits that might preserve stromatolites are competing with rocks recording underground environments.
Discoveries like this one can influence the focus of multi-billion-dollar science investigations.
It is often said that extraordinary claims require extraordinary evidence, and I expect the debate surrounding these features will continue until more evidence emerges.