BIOGENICITY OF WORLD'S OLDEST FOSSILS: HOW THE PROBLEM WAS SOLVED

Diverse, abundant, carbonaceous microscopic filaments described in 1993 from the ~3,465-Ma-old Apex chert of Western Australia (Schopf 1993) are recorded in many textbooks as the oldest fossils known. In 2002, however, the biogenicity of these sinuous microbe-like filaments was questioned, their presence ascribed to graphite thought to be derived from organics produced by abiotic Fischer-Tropsch-type syntheses in a submarine hydrothermal setting (Brasier et al. 2002). This nonbiologic interpretation is not correct -- recent studies establish that the filaments are bona fide fossil microbes and that the Apex organic matter is assuredly biogenic.

Consistent with paleobiologic evidence from other geologic units 3,200 to 3,500 Ma in age -- carbon isotopes, stromatolites, and microfossils (including those in other hydrothermal deposits) -- Raman imagery and confocal laser scanning microscopy (CLSM) show that the Apex fossils, like a great many other chert-permineralized filamentous Precambrian microbes, are three-dimensional, cylindrical, and composed of organic-walled cells that exhibit well defined cell lumina (Schopf et al. 2007). Raman establishes that they are composed of biogenic kerogen, not abiotic graphite. CLSM data rule out their emplacement by permeating organic fluids. And the CHONSP-composition and functional-group chemistry of the Apex organic matter indicate "that the Apex microbe-like features represent authentic biogenic organic matter" (De Gregorio et al. 2009, p. 631).

Other non-biological interpretations of the Apex fossils also fail. Reported clay mineral pseudofossils (Pinti et al. 2009) and hematite-infilled veinlets (Marshall et al. 2010) are not relevant to interpretation of the demonstrably carbonaceous Apex filaments, and the solid mineral crystallites synthesized by García-Ruiz et al. (2003) lack the transverse cell walls and biological cellularity of the Apex microbes.

This problem is solved. The Apex fossils are demonstrably biogenic.

References

Braiser et al. (2002) Nature 416:76-81

De Gregorio et al. (2009) Geology 37:631-634

Marshall et al. (2011) Nature Geosci. 4: 240-243

García-Ruiz et al. (2003) Science 302:1194-1197

Pinti et al. (2009) Nature Geosci. 2:640-643

Schopf (1993) Science 260:640-646

Schopf et al. (2007) Precambrian Res. 158:141-155