Evidence of Archean life: Stromatolites and microfossils
Introduction
It has recently been suggested that “true consensus for life's existence” dates only from “the bacterial fossils of 1.9-billion-year-old Gunflint Formation of Ontario” (Moorbath, 2005). Evidently, all supposed evidences of earlier life, “the many claims of life in the first 2.0–2.5 billion years of Earth's history,” have been cast in doubt (Moorbath, 2005). Yet it is precisely during this period of Earth history, prior to 2000 Ma, that most workers have assumed that prokaryotic microbes originated and diversified to comprise Earth's earliest biosphere. If the fossil record is to make any contribution to defining life's early history, doubts such as those raised by Moorbath (2005) must be laid to rest. This prompts the fundamental first-order question addressed here: What fossil evidence exists for life's presence during the Archean Eon of Earth history, prior to 2500 Ma?
This discussion need not be exhaustive. Elsewhere in this issue of Precambrian Research, Sugitani and his colleagues (p. 228) report new finds of Archean microfossils and Allwood et al. summarize their recent in-depth studies of the stratigraphic setting and morphology, paleoecology, and biogenicity of ∼3400 Ma stromatolites (p. 198). Moreover, carbon isotopic evidence of Archean biologic activity and the known fossil records, both of Archean stromatolites and of microbial microscopic fossils, have recently been reviewed (Schopf, 2006a, Schopf, 2006b). Thus, the aims of this contribution need only be two-fold: (1) to summarize in broad-brush outline and to illustrate selected examples of the 48 occurrences of Archean stromatolites and 40 morphotypes of putative microfossils described from Archean deposits and (2) to provide new Raman-based evidence that demonstrates the cellularity of microbe-like filaments reported from brecciated chert of the ∼3465 Ma Apex Basalt (hereafter referred to informally as the “Apex chert”), one of the oldest putatively fossiliferous deposits yet reported and the subject of recent controversy (Brasier et al., 2002, Brasier et al., 2005, Schopf, 2004, Altermann, 2005, Altermann et al., 2006). Taken together, the data presented support the view that the “true consensus for life's existence” dates from ≥3500 Ma, not from some 1500 Ma later.
Section snippets
Preservation of the Archean rock record
As shown by Lowe (p. 177) in this issue of Precambrian Research, vanishingly few rock units have survived from the Archean to the present. Similarly, as Garrels and Mackenzie suggested some years ago (1971, p. 275), “about 90% of the Precambrian once deposited is gone,” surviving rocks petering out rapidly with increasing geologic age to produce a severely depleted Archean rock record. As currently known, only two relatively thick especially ancient Archean sedimentary sequences have survived
Archean stromatolites
As used here, the term “stromatolite” refers to accretionary sedimentary structures, commonly thinly layered, megascopic and calcareous, produced by the activities of mat-building communities of mucilage-secreting microorganisms, mainly photoautotrophic prokaryotes. Other definitions have been proposed, some similarly emphasizing the biogenic, organosedimentary nature of such structures (e.g., Awramik and Margulis, in Walter, 1976; Awramik, in Semikhatov et al., 1979, Buick et al., 1981),
Archean microfossils
Over recent decades, the rules for accepting Precambrian microfossil-like objects as bona fide have come to be well established; namely, that such objects be demonstrably biogenic, and indigenous to and syngenetic with the formation of rocks of known provenance and well-defined Precambrian age (Schopf and Walter, 1983, Schopf, 2004). Of these criteria, the most difficult to satisfy has been that of biogenicity (Hofmann and Schopf, 1983, Schopf and Walter, 1983, Mendelson and Schopf, 1992). A
Fossil-like filaments of the Apex chert
In the discussion below, we apply this multi-trait strategy to the putative fossils of the ∼3465 Ma Apex chert of the Pilbara Block of northwestern Western Australia (Schopf, 1992, Schopf, 1993). Questions have been raised about the paleoenvironment of the 11 taxa of microbe-like structures described from this deposit (Schopf, 1993), as well as about their chemical composition, mode of preservation, and putative biological morphology (Brasier et al., 2002, Brasier et al., 2005). These questions
Conclusions
Evidence for the existence of life during the Archean is firm. Consistent with the findings presented in other papers of this special issue of Precambrian Research, the data presented here – from diverse Archean stromatolite-bearing (Fig. 1, Fig. 2) and microfossiliferous deposits (Fig. 3, Fig. 5) – show that life was not only extant but was flourishing in the Archean. Further, new findings presented here support the biological interpretation of the microbe-like microstructures of the Apex
Acknowledgements
This discussion of Archean stromatolites and microfossils is in part an abridged version of Schopf (2006a), presented here in order to assure that this special issue of Precambrian Research includes fossil data from the entire Archean in addition to those from the more focused studies of Allwood et al. (p. 198) and Sugitani et al. (p. 228). We thank J. Shen-Miller and an anonymous reviewer for helpful comments on the manuscript, and we are particularly grateful to K. Grey for providing data
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