The Earth is 4.54 billion years old. How long did it take life to arise after that? While there are tantalizing hints of life from Greenland dating back about 3.75 billion years ago (“bya”; these hints come from carbon deposits that may have been formed by living organisms), there’s nothing like a fossil to establish the existence of life. And, of course, the earliest organisms were very simple, like bacteria. Fossils of bacteria or any single cell—”microfossils”—are hard to detect in the rocks, for small particles, like carbon-encrusted mineral grains, can easily look like bacterial “microfossils.”
This difficulty in distinguishing true bacterial fossils from simple inclusions led to a big scientific kerfuffle in 2002, when Bill Schopf claimed to have found the world’s oldest fossils, of cyanobacteria (once called “blue green algae”), in Australian rocks dated 3.465 billion years ago. That would have made them the world’s earliest fossils, but Martin Brasier, of Oxford, took exception, claiming that these were mere particles of heated graphite. There were even claims that Schopf had selectively published his data, leaving out photographs of the “bacteria” that didn’t look so bacterial. You can read about the 2002 controversy here. Time has come down on the side of Brasier, with most paleobiologists agreeing that the objects Schopf described were not living creatures.
Ironically, though, Brasier is the senior (last) author on a new paper by David Wacey et al. in Nature Geoscience (see report at the BBC here) that does show, pretty conclusively, the oldest known fossils on Earth, dated at 3.4 billion years old. This makes them only a tad younger than the fossils claimed by Schopf but 200 million years older than the oldest reliable microfossils known up to now. And the fossils described by Wacey et al. are much better documented than were Schopf’s, demonstrating that prokaryotic organisms had already evolved only a billion years after Earth had formed. These are also bacteria, of course, but their association with pyrites, and their morphology, also show that they were sulfur-metabolizing bacteria.
The new microfossils were found in a sandstone formation in Western Australia; the photograph from the BBC shows the study site, and notes that the new fossils were found at the base of these ridges:
Figure 1. Examples of spheroidal/ellipsoidal microfossils from the SPF (samples SP9D2, SPE1, SPV3a–c). a,b,e, Clusters of cells, some showing cell wall rupturing (arrows in a,b), folding or invagination (arrow in e). c,d,h, Chains of cells with cellular divisions (arrows). f,i–j, Cells attached to detrital quartz grains, exhibiting cell wall rupturing and putative escape of cell contents (arrow in f), preferred alignment of cells parallel to the surface of the quartz grain (arrows in i), and constriction or folding between two compartments (arrow in j). g, Large cellular compartment with folded walls (arrows).
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.
One of the more exciting features of these fossils is their co-occurrence with small grains of pyrite, a compound of sulfur and iron that is a byproduct of bacteria that metabolize sulfur. This supports a biological origin not only because the tiny pyrite grains are found in conjunction with the microfossils, but because such grains are also seen in association with modern sulfur-metabolizing bacteria.
I judge the paper a very good piece of work: the authors are careful in their conclusions and state explicitly that no single feature of these objects prove that they’re bacterial microfossils, but that the total weight of evidence strongly supports that conclusion. I agree. So we have the oldest fossils on earth, which means that simple life, not yet having the complexity of “true” cells, must have evolved well before 3.5 billion years ago. And perhaps the earliest cellular life metabolized sulfur rather than oxygen.
Does this mean that these bacteria were members of the Archaea, primitive single-celled organisms that often inhabit extreme environments, rather than members of true “bacteria”? Many biologists think that organisms with “true cells” (members of the Eukaryota, like us) are more closely related to the group Archaea than to true bacteria. As far as I can see, we can’t determine whether the cells of Wacey et al. are real bacteria or Archaea. (Note that although the authors call these “Archaean microfossils,” that does not mean they are members of the biological group Arachaea—only that they occurred during the eon that geologists call the “Archaean,” which lasted from 3.80-2.5 bya.) But never mind for the nonce—they’re still the oldest indisputable forms of life known on Earth.
I am very curious, though, why this paper was published in Nature Geoscience rather than a higher profile journal like Science or Nature itself. Both of those journals publish findings that are far less significant than this one, and this finding certainly deserves a berth in the very highest-profile journals. It is, after all, a report of the oldest known life on Earth. I suspect there are some editorial dynamics here that we don’t know about.
Wacey, D.,M. R. Kilburn, M. Saudners, J. Cliff, and M. D. Brasier. 2011. Microfossils of sulphur-metabolizing cells in 3.4-billion-year-old rocks of Western Australia. Nature Geoscience online: doi:10.1038/ngeo1238