EARLIEST RECORDS OF LIFE

In recent years there has been an upsurge of interest in the oldest evidence of life, that preserved in Archean (>2,500-Ma-old) units. Such evidence is widely regarded to extend to at least ~3,500 Ma ago: molecular biomarkers and isotopic data have been interpreted as providing insight into the physiology of Archean microbes, and stromatolites and microbial fossils have been reported from more than 50 Archean geologic units, including 13 deposits in the 3,200- to 3,500-Ma-old age range.

Despite this body of evidence, some workers have claimed that “true consensus for life's existence” dates only from 1,900 Ma ago (Moorbath, 2005, Nature 434, 155), a timing for the beginning of the known fossil record more than a billion-and-a-half years younger than (and, thus, very much at odds with) that presented in modern textbooks.

It is certainly true that because of the scarcity of Archean rocks and their pervasive metamorphism -- results of geologic recycling and tectonism over their protracted history -- evidence of Archean life is markedly less abundant and well preserved than that known from the younger, Proterozoic, Precambrian. But the prime source of such uncertainty comes from doubts raised about the biological origin of certain of the oldest putative fossils known, microbe-like microscopic filaments of the ~3,465-Ma-old Apex chert of Western Australia. Such doubts have now been laid to rest. The paleoenvironment, carbonaceous composition, mode of preservation, and morphology of these permineralized organic-walled filaments, coupled with new evidence of their cellularity and geochemical state of preservation provided by two- and three-dimensional Raman imagery, establish their biogenicity (Schopf et al., 2007, Precambrian Research, in press). The Apex filaments, together with the presence in similarly aged deposits of stromatolites, microfossils, and carbon isotopic evidence of biological activity, document that the existence of biologic systems on Earth extends at least to ~3,500 Ma ago. And three-dimensional Raman imagery and SIMS analyses of quartz-enclosed apatite-hosted isotopically light graphitic carbon in ancient rocks from Akilia Island, SW Greenland, provide strong hints of life's existence at ~3,830 Ma ago, arguably the oldest indications of life now known (McKeegan et al., 2007, Geology 35, 591-594).