It's a long way from Primaevifilum The press has recently carried some announcements of th

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It's a long way from Primaevifilum ---------------------------------- The press has recently carried some announcements of the discovery of signs of early life in the north-west of Western Australia. This is a short summary of the discovery based on material from two articles in Science[1,2]. The interview with William Schopf on the ABC radio's (the Australian one) "Science Show"[3] which I mentioned in an earlier post was disappointing, and added little to what I'd heard from the edited highlights of the same interview the week before. The site is at Chinaman Creek near Marble Bar, about 250km SE of Port Hedland, in the Pilbara region of West Australia, a region "underlain by a 30-km-thick sequence of relatively well-preserved sedimentary and volcanic rocks that are ~3000 to ~3500 million years old"[1]. The samples in which the fossils were found were taken in 1982 and 1986, and an initial description of microfossils in them was published by Schopf in 1987[2]. The recent article, which has prompted the press activity, lists a further 8 species, and constrains the dating better. But the discovery doesn't in fact alter the date of the earliest known microfossils significantly; they are from the same formations as described in the 1987 paper. The fossils are in chert (flint), bedded in the Apex basalt. The "maximum age for the Apex chert of ~3470Ma is constrained bu U-Pb zircon ages (3465+-3Ma and 3471+-5Ma) from the stratigraphically underlying Duffer formation. A minimum age for the fossiliferous rocks of ~3460Ma is provided by a U-Pb zircon date of 3458+-1.9Ma for the immediately overlying Panorama Formation. Thus the fossiliferous Apex chert is evidently about 3465Ma. [refs to dating sources and table omitted]" [1]. There is evidence that the fossils predate the deposition of the chert, and were initially preserved in older rocks, which were eroded and redeposited to form the chert. The microfossils described in [1] are filamentary colonies, and Schopf classifies them tentatively as about 2/3 cyanobacteria (microfossils described in [2] were also so classified), the rest being classified as "probable bacteria" or undifferentiated prokaryotes; the distinction being made on filament width. [2] also described some spheroidal colonies. [1] Schopf, J. W, "Microfossils of the early Archaen Apex chert: New evidence of the antiquity of life", Science, 260:640-646, 30 Apr 1993. [2] Schopf, J. W, "Early Archaen (3.3-billion to 3.5-billion-year-old) microfossils from the Warrawoona group, Australia", Science, 237:70-72, 3(?) Jul 1987. [3] Williams, R, Interview with Schopf, "Science Show", Radio National, 8 May 1993, Australian Broadcasting Commission. -- Peter Lamb (prl@csis.dit.csiro.au)

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