There’s a new paper in Nature that has everyone excited, for it reports what is said to be the earliest evidence for microbial life—”microbial structures” dated 3.7 billion years ago. The paper, by Allen P. Nutman et al. (reference and free link at bottom), describes what are said to be ancient traces of stromatolites—layered colonies of cyanobacteria that trap sediments and are thus fossilized—from a part of southwest Greenland that harbors old rocks.
The earliest previous evidence for microbial life are microfossils dated at 3.4-3.5 billion years old, coming from the Strelley Pool formation of West Australia. (Wacey et al., Nature Geoscience 4:698-702). The Nutman et al. finding, if true, pushes back the known existence of cells by 200-300 million years, no small chunk of time. (There is some evidence, though not very convincing, for carbon of biological origin dating back 4.1 billion years.)
What is the new evidence for 3.7 billion-year-old life? It’s largely structures in dated rocks that Nutman et al. interpret as stromatolites, structures like those shown below (“strom” means “stromatolite”). The pointy structures are identified as the remains of ancient stromatolites, though I wonder why the middle one isn’t labeled “strom”:
Nutman et al. give other evidence too, including isotopic data, the presence of minerals that said to be biogenic, and the presence of layers (“lamellae”) in the stromatolite-looking bits. But the most touted (and convincing to others) evidence are pictures like those above.
I checked with some well known paleontologists and sedimentologists, however, and they don’t find even the “fossil” data very convincing. (I’ll withhold their names for the time being.) The pointy bits above, they say, could be “flame structures“: simple deformation of clay or mud that occurs when it’s pushed up by heavier overlying layers of sand. This could produce (and has produced) the kind of structures seen in the photo above, but without any presence of life. Further, the layers in the structures might not represent layers of ancient microbes, but simply layers in the underlying mud that, after all, could be produced by successive sedimentation events.
The rest of the evidence, I’m told, may be suggestive of life but hardly convincing. The paper is tough going, which you’ll see if you read it, so all I want to do is note that the evidence for life given in this paper is questioned by some experts.
Nevertheless, we still have pretty good evidence for bacterial cells existing 3.4-3.5 billion years ago, and such cells are pretty complex. That means that life got started pretty soon after the Earth cooled down, roughly 4.3 billion years ago. These cells, after all, had to have undergone a very long period of evolution from the initial replicating molecule (or whatever it was) that constituted the first “life”.
So take this 3.7 billion year date with a grain of NaCl. That doesn’t mean it’s wrong, just that there are formidable problems with finding solid evidence for life in ancient rocks. Not many of those rocks exist on Earth any more, and those that do could have been changed or deformed in a way that would make life hard to detect. Further, the best evidence for life are microfossils like those shown below, but even these are somewhat controversial. Proving that such structures are fossil bacteria rather than inclusions or artifacts is often hard to do.
Nevertheless, the photos below, and other data from the Wacey et al. paper, have convinced most paleontologists that there were microbial cells around 3.4-3.5 billion years ago.
Why do paleontologists fight bitterly about the “first” cells if it’s only a mere matter of 300 million years (!)? Well, there’s cachet to be gained by finding the earliest good evidence for life, but, beyond that, finding complex cells soon after Earth cooled down gives us a good time scale for how long it takes to go from simple chemicals to “life” (I see the origin of “life” as a somewhat subjective point, as it varies depending on your definition). Pushing dates of cells further back tells us that that transition could be even faster than we once envisioned.
h/t: Latha Menon
Nutman, A. P., V. C. Bennett, C. R. L. Friend, M. J. Van Kranendonk, and A. R. Chiva. 2016. Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures. Nature, published August 2016, doi:10.1038/nature19355