Martin Brasier of Oxford and his team have just described some very ancient fossils discovered in rocks dated to 3.4 billion years of age. The article is unfortunately behind a paywall, so I am unable to review it, but Jerry Coyne lays out the evidence neatly:
As the authors note, “determining the biogenicity [biological origin] of putative Archaean microfossils is notoriously difficult.” How do we know that these things are real remnants of bacteria and not just inclusions or artifacts? There are several independent lines of evidence, none conclusive but together building a very solid case:
- They look like cells, being cell-shaped, cell-sized, and forming chains of spheroids that look like chains of both well-established fossil bacteria and modern bacteria. Some can even be seen “dividing” or expelling their contents after cell damage (see figure above).
- The variation in size of the bodies is small—smaller than you’d expect if they were abiological inclusions. A uniformity of size, however, is expected if they’re all members of one living species.
- The cell “walls” of the microfossils, too, are of uniform thickness, unlike that of artifacts like silica grains coated with carbon.
- The geochemistry of the bacteria and surrounding rock supports the idea that these are true organisms. This involves not only the isotopic nature of the carbon, but the presence of nitrogen, a crucial biomarker, within the cell walls.
This reminds me very much of Margulis’ endosymbiotic theory and its multiple lines of evidence. Of course, that idea was a cornerstone achievement of 20th century biology whereas this recent discovery pushes the known origins of life back about 200 million years, but that does not mean it isn’t important. Braiser’s find goes to demonstrate the nature of early bacteria. Many of the features in these fossils show that ancient bacteria operated in ways similar to certain bacteria today – if you find something that works, keep it up. The find also demonstrates that life can come to be quite quickly. Earth is only 4.54 billion years old and probably was not hospitably cool enough for life for its first half billion years. Given this 3.4 billion year date plus the fact that these fossils are fairly complex, it is not a stretch to say life dates back even further, probably quite soon after it was possible for it to do so on Earth.
Finally, I have to reflect Coyne’s curiosity that this was not published in Nature or Science, but rather Nature Geoscience. It’s still in a prestigious enough journal, but the paper is important enough where it deserves publication that comes with a higher profile. I’m sure the authors attempted to reach the best level possible, so who knows what the deal is.