A similar sulfur isotope story to that in the Pilbara emerges in the Barberton area from the work of Ohmoto et al. (1993). These authors sampled a shale and three black cherts from the uppermost Onverwacht Group (-3,300 Ma Mendon Formation) in the Barberton and used high resolution laser ablation mass spectroscopy to analyse individual pyrite grains. The range of 534S values (up to 12%o variation) from these pyrites was argued to be greater than that expected if they had formed from purely magmatic or hydrothermal H2S, and an origin from bacterial sulfate reduction was again invoked. This evidence of sulfate reduction from the Pilbara and Barberton predates previous evidence (Goodwin et al., 1976) by some 600-750 Ma and potentially allows calibration of an important deep branching node on the Tree of Life.
We remind the reader that the use of only one line of evidence, in this case isotopic evidence to constrain a key event in the evolution of life needs always to be regarded with care. Additional supporting lines of evidence from geochemistry, morphological remains and geological context are needed to strengthen the conclusions. Towards this end, multiple isotope systems are being developed and integrated with palaeontological observations from the Archean. For example, in younger c.2.7 Ga stromatolitic and non-stromatolitic sediments from the Belingwe Greenstone Belt of Zimbabwe, sulfur and carbon isotopes in conjunction with rare earth element patterns have been used to argue for the presence of bacterial and archaeal consortia which are water-depth dependant and capable of anoxy-genic photosynthesis, methanogenesis and methanotrophy (e.g. Grassineau et al., 2001). The sulfur and carbon isotopic systems discussed during this review have provided most of our isotopic data thus far for early Archean biological studies. Further isotopic systems are currently being developed which will hopefully further constrain Archean environmental and biological processes. A description of these is beyond the scope of this review and we refer the reader to the following: Nitrogen isotopes (Beaumont and Robert, 1999; Van Zuilen et al., 2005); Iron isotopes (Anbar, 2004; Dauphas et al., 2004, 2007; Johnson and Beard, 2006); Oxygen isotopes (Knauth and Lowe, 2003).
The potential existence of life on other planets is an exciting prospect. On Mars in particular which had a "warm wet" climate and similar environmental condition to the early Earth 4.5-3.5 million years ago, the possibility of the emergence of life is very real (e.g. McKay and Stoker, 1989). Given, however, the numerous problems and controversies we have highlighted when looking for earliest life here on Earth, what hope do we have for confidently identifying life elsewhere in the universe? We attempt to address this question here by looking in turn at three lines of evidence for life on other planets and the challenges that these face: first putative microfossils found in Martian meteorites; second microbial trace fossils created by endolithic bacteria on other planetary surfaces; and thirdly the insights stable isotope ratios measured on extra-terrestrial samples may yield. In each case we emphasize how critical analysis of this evidence goes hand-in-hand with re-examination of the early terrestrial rock record to provide the best strategy for seeking life beyond Earth.
To date, claims for extraterrestrial life have, unsurprisingly, been debated as vigorously, if not more so, than claims for the earliest evidence of life on Earth. The most famous of these is the claim for life in the ~4,500 Ma ALH84001 Martian meteorite found in Antarctica in 1984 (McKay et al., 1996). Here we summarize the four lines of evidence that were originally presented for fossil life in this meteorite and why each has been subsequently dismissed or re-interpreted. (1) Carbonate globules: the density and composition of the carbonate being comparable to phases associated with terrestrial bacteria. Carbonate by itself, however, is abundant in non-living materials. (2) Organic compounds known as polycyclic aromatic hydrocarbons that are created by bacteria were present in the meteorite. It is now thought that these were contaminants from the Antarctic environment (e.g. Jull et al., 1998). (3) Magnetite globules with morphologies only known to be produced by magnetotactic bacteria (Thomas-Keprta et al., 2001) were reported. Six criteria were subsequently outlined to test the biogenicity of magnetosomes; only a small proportion of this population satisfy these criteria and very few of the magnetosomes are aligned in chains (Weiss et al., 2004). Nevertheless, this remains probably the strongest line of evidence for biogenicity with Thomas-Keprta et al. (2000) reporting that 27% of the magnetite grains do indeed satisfy the criteria for biogenicity. (4) "Nanofossil"-like structures were described. These, however, were smaller than the accepted minimum size range for even the smallest cells capable of independent growth and are likely just mineral artefacts. ALH84001 continues to be studied but the consensus at present is that unambiguous biosignatures have yet to be found. There are also a number of other reports of candidate microfossils found in other carbonaceous meteorites (e.g. Hoover et al., 2004) although these have been rather less celebrated and debated by the Astrobiology community in comparison to ALH84001.
This debate has been re-awakened by the recent discovery in the Martian Nakhla meteorite of carbonaceous, tubular and bleb shaped microstructures that show some similarities to endolithic microtubes in volcanic glass (McKay et al., 2006; Gibson et al., 2006). New technology has allowed identification of distinct CN- signatures and an in situ carbon isotopic composition of -18%o to -20%o associated with these structures (Gibson et al., 2006). This carbonaceous phase appears to be distinct from a contaminating terrestrial phase which decomposes at much lower temperatures. It has therefore been suggested that the carbonaceous phase is either: (i) abiogenic, derived from an impact on Mars that also produced the fractures and veins in the Nakhla meteorite; or (ii) biogenic, the products of Martian euendoliths that may be similar to terrestrial, volcanic microborings. The latter is a plausible scenario as it is envisaged that the intense UV flux, absence of liquid water and freezing temperatures on the outer surface of Mars may have encouraged an endolithic mode of life (e.g. Friedmann and Koriem, 1989). Microtubular tunnels and galleries in olivine and pyroxene crystals from the Nakhla meteorites that are remarkably similar to bioerosion textures found in terrestrial iron-magnesium silicates have also been recently described by Fisk et al. (2006). These authors made cautious conclusions about their microtubular weathering textures: "though the tunnels found in Nakhla are similar to the biosignatures found in terrestrial minerals, their presence cannot be used to prove that the Martian alteration features had a biogenic origin.'" This conclusion underscores the need for further investigation into biotic and abiotic mechanisms of microtube generation and the refinement of criteria for establishing their biogenicity (cf. McLoughlin et al., 2007).
An alternative strategy for seeking life on other planets is the use of stable isotope ratios as a biomarker. Above we have reviewed the use of C and S isotopes in seeking evidence for biological processes on the early Earth. These isotopic investigations, both on Earth and other planetary surfaces, rely on several key assumptions, which we now remind the reader of before discussing whether these assumptions hold on other planetary surfaces: (1) biological processes generate an isotopic signature which is distinguishable from that resulting from abio-logical processes (but with regards to C isotopes, the growing appreciation of Fischer-Tropsch type reactions have questioned this assumption); (2) the isotopic signatures of the various reservoirs i.e. geo- hydro- atmos- and bio- spheres on the planet concerned have been characterized and have remained constant over geological time; (3) secondary geological processes have not modified the isotopic signature measured; and (4) the isotopic signature is syngenetic to the rock sample on which it is measured (see also Van Zuilen, 2007). On Mars, the study of C isotopes is hampered by our relatively poor understanding of how the different C reservoirs and the fluxes between them have changed over time (e.g., changes in volcanic out-gassing, loss of CO2 from the atmosphere) and also by the absence of a well characterized, inorganic carbonate reservoir to act as an isotopic standard (for further discussion see Van Zuilen, 2007; Grady and Wright, 2006). Alternatively, S isotopes could be employed to understand bacterial sulfur cycling given that there are evaporitic sulfates in sedimentary rocks on Mars (Squyres et al., 2004) which could provide an abiogenic standard - but here again relatively little is known about the evolution of the S cycle on Mars and so great care would need to be taken with the interpretation of any potential S isotopic measurements. These should be considered in concert with additional lines of morphological and chemical evidence.
In summary, extinct or extant life may await discovery on Mars and indeed on other planetary bodies such as Jupiter's moon, Europa, perhaps near subsurface hydrothermal vents (e.g. Fisk and Giovannoni, 1999). The review of the early terrestrial rock record presented here highlights the importance of volcanic and hydrothermal rocks between 3.5 and 3.0 Ga to the early history of life on our planet and is consistent with hypotheses which advocate a hydrothermal cradle for life (e.g. Russell and Arndt, 2005; Nisbet and Sleep, 2001). It also highlights the importance of siliciclas-tic rocks which may contain some of the best preserved fragments of the earliest rock record. However, studies of the 'Apex chert' and other early Archean hydrothermal systems caution that the unambiguous identification of fossil microbes may be hampered by abiotic carbon synthesis (cf. Brasier et al., 2005 and references therein). One thing is for certain. Improving our understanding of the earliest fossil record here on Earth is critical to our chances of confidently interpreting putative biological structures and signals found in the future on other planets.
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