CRITICALLY TESTING THE EARLIEST SIGNS OF LIFE

Establishing reliable evidence for the earliest signs of life on Earth is both complex and difficult. New claims for the earliest life require a well-constrained geological context, including age; unambiguous evidence for biology-like morphology, i.e. morphospace mapping; ultra-high resolution geochemical evidence for metabolic cycling; and falsification of plausible non-biological origins. Happily, these challenges can be seen as exciting and timely thanks to new technologies being harnessed by the astrobiological community.

In our view, early Archean marine environment were very different from those of the later Archean and Proterozoic.The role of biology in producing simple stromatoloids from the Strelley Pool chert (c. 3430 Ma) is questionable, with little evidence for either phototropism or photoauthrophy. Yet more problematic are the Apex 'microfossils' (c. 3460 Ma), at one time regarded to contain cyanobacteria-like prokaryotes but these are now seen to be indistinguishable from coexisiting diagenetic artefacts. Even the microtubes from the shallow water Strelley Pool sandstone (c. 3430 Ma)formed during burial, and the role of biology awaits demonstration.

We find no evidence consistent with oxygenic photosynthesis in the surface waters of the early Archaean Earth. Early shorelines bore little resemblance to modern Sharks Bay nor to Caribbean lagoons. Our studies of the Strelley Pool sandstone indicate near shore marine conditions more akin to a vast toxic mine waste site, with evidence for rounded pyrite clasts, arsenopyrite and pyrite placers, and silicate etching. We infer that the pore waters of Earth's earliest 'beach' sediments were sulfidic, arsenic-rich, acidic and anoxic- more like Hell than Eden...