Geochemistry

Wrinkles of Life

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Science  05 May 2006:
Vol. 312, Issue 5774, pp. 659
DOI: 10.1126/science.312.5774.659c

A variety of geochemical evidence implies that life evolved on Earth roughly 3.5 billion years ago, yet more direct evidence—specifically, fossils or fossil-derived structures such as stromatolites or alteration pits—is still sparse or disputed in rocks dating several hundred million years closer to the present. Most of the evidence has been found in siliceous oozes or sediments, carbonate rocks, or altered basalt from deep oceanic or hydrothermal settings.

Noffke et al. have discovered fossil microbial mats in another environment, South African tidal sandstones, dated to ∼3.2 billion years ago. These rocks display wrinkles, layered roll-up structures, and carbon-rich laminations that resemble features seen in modern intertidal sandstones and commonly preserved in much younger rocks. Such structures form as ductile microbial mats are buried. Analysis of the carbon isotope compositions of the laminations further supports their bacterial origin. Concentration of these features at the top of sedimentary sequences formed in shallow water environments suggests that the microbes in the mats may have derived their energy through photosynthesis. — BH

Geology 34, 253 (2006).

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