A few days ago I posted on the discovery of the world’s earliest known fossils—sulfur-metabolizing bacteria from a site in western Australia, with the bacterial “microfossils” dated at 3.4 billion years old. (Earth is 4.54 billion years old, which means that life originated no later than 1 billion years after Earth’s formation). The paper, whose reference and link is below, was by Dave Wacey et al., with Martin Brasier as the last (i.e., “senior”) author.
I was a bit concerned, then, when I saw P. Z. Myers had a few concerns about this paper at Pharyngula, concerns that “gave him pause.”
P.Z.’s first worry was that the fossil cells seemed too large to be prokaryotes. The paper of Wacey et al. shows, as P. Z. said:
. . . lots of cells with 10-30µm diameters. And the authors come right out and report that:
‘The size range is also typical of such assemblages, with small spheres and ellipsoids 5-25 µm in diameter, rare examples (<10) of larger cellular envelopes up to 80 µm in diameter, and tubes 7-20 µm across (see ref. 24).”
How odd. When I poke into the nervous system of an embryonic insect or fish, those are the sizes of cells I often see (well, except there aren’t many tubes of that size!). When I poke into a culture or embryo contaminated with bacteria, they’re much, much smaller. So maybe paleoarchaean bacteria tended to be larger?”
P. Z.’s second concern was that “reference 24” cited by Wacey et al. in support of the size ranges of fossil bacterial assemblages was in fact a paper by J. William Schopf. This disturbed Myers because Schopf’s earlier claim to have found the oldest microfossils on earth (3.465 billion years old) has since been discredited; as P.Z. said, Schopf’s microfossils form “a data set that’s widely considered artifactual now.” And Schopf’s largest cells were from his oldest samples, with cells getting smaller as the samples came from successively younger strata.
These considerations gave the estimable Dr. Myers some reservations about Wacey et al.’s conclusions, and at the end of his report he asks “…isn’t this just a little bit strange? Maybe there are some micro people out there who can reassure me that this isn’t a surprising result.”
I was a bit distressed about this since I hadn’t noticed those “problems.” I then checked them out since I don’t want to report stuff that’s wrong or dicey. So I contacted Dave Wacey and Martin Brasier about P. Z.’s reservations. And I’m happy to report that there’s no problem with the Wacey et al. paper. What appears below is a bit technical, and I’m posting it to put the record straight, but if you’re following the early-fossil literature, you should read it.
Here’s what Martin Brasier wrote me, which I post with his permission:
1) Re the large size of the bacterial fossils shown:
The first point is that the larger microfossils proved to be the easiest to image and showed more convincing cellular features, so that there is a bias towards the illustration of large forms in our Figures 1 and 2. In fact, the majority of the microfossils from the Strelley Pool arenite actually fit into the same sort of size distribution patterns as seen in the younger Gunflint and Bitter Springs cherts. This is clearly shown within our Supplementary Figure 6.
Regarding the suggestion that bacterial cells tend to be small, this approach no longer looks safe. Sulphur bacteria have members that are particularly large today (12 to 160 μm). They reach great size because of cell vacuoles within them (e.g., AHMAD, A., BARRY, J.P. and NELSON, D.C. 1999. Phylogenetic Affinity of a Wide, Vacuolate, Nitrate-Accumulating Beggiatoa sp. from Monterey Canyon, California, with Thioploca spp. Appl. Environ Microbiol. 65, 270-277). Hence unsheathed Beggiatoa filaments can often reach a width of 65 to 85 μm (range about 20-140 μm). Individual cells can reach 20 by 70 μm. Multiple Thioploca filaments (2-43 μm) usually occupy a single sheath (the part that preserves). Coccoid Thiomargarita namibiensis can be 100–300 µm wide, but can sometimes reach 750 µm.
It would be unwise to speculate about the possibilities of a larger cell size in early bacteria. But we note that microfossils reported from the 3.2 Ga Moodies Group of South Africa can be almost ten times the diameter of those found in the Strelley Pool rocks. See JAVAUX, E., MARSHALL, C.P. and BEKKER, A. (2010). Organic-walled microfossils in 3.2-billion-year-old shallow-marine siliciclastic deposits. Nature 463, 934-939.
2) Re the concerns of PZ Myers about use of a Bill Schopf reference
Reference 24 in our paper is NOT the Schopf (1993) article about the Apex microfossils – it is Schopf’s 2006 Phil Trans R Soc paper that was a summary of ALL microfossil claims from units older than 2.5 Ga known at that time. There were actually 40 occurrences of Archaean microfossils described by Schopf in this paper. From younger rocks, he describes 10-28 µm tubular sheaths from the Ghaap Dolomite and 16-23 micron coccoids from the Sheba Formation. (See his Table 2.) Those records are what we were referencing.
It is true that our views on some of these records has been, and remains, cautious. See, for example, BRASIER, M.D., McLOUGHLIN, N., GREEN, O., WACEY, D. 2006. A fresh look at the fossil evidence for early Archaean cellular life. In Cavalier-Smith, T., Brasier, M.D. & Embley, T..M. (Eds) Major Steps in Cell Evolution: Palaeontological, Molecular and Cellular evidence of their timing and global effects.Philosophical Transactions of the Royal Society, Series B, volume 361, 887-902. See also the book by David Wacey (200) “Early Life on Earth. A Practical Guide”. Springer, Dordrecht.
For those who are interested, our re-interpretation of the 3.46 Ga Apex microfossils has been set out afresh in a new monograph: BRASIER, M.D., GREEN, O.R., LINDSAY, J. McLOUGHLIN, N., STOAKES, C., BRASIER, A. & WACEY, D. 2011. Earth’s Oldest Putative Fossil Assemblage from the 3.5 Ga Apex Chert, Chinaman Creek, Western Australia: A Field and Petrographic Guide. Records of the Geological Survey of Western Australia, Perth. This is now available on the GSWA website and on the Academia website. Out views on those follows has not changed from that we set out in 2002. We saw no need to cite the latter paper as its arguments are widely known.