2009年02月27日

BIOGEOCHEMISTRY : Life before the rise of oxygen

The discovery of moleclar fossils in 2.7-billion-year-old rocks prompted a re-evaluation of microbial evolution, and of the advent of photosynthesis and rise of atmospheric oxygen. That discovery now comes into question.

Go Back to Archaean time, the interval of Earth's history between about 4 billion and 2.5 billion years ago, and we're in largely unknown biological territory. Attempts to identify a fossil record of life have produced meagre results, and controversy persists about whether cartain microfossil-like structures are of biological origin.(Schopf,J.W. phil. Trans.R. Soc.B 361,869-885/2006)(Brasier,M.D. et al. Nature 416,76-81/2002)

Almost a decade ago, however, Archaean palaeontology received a big boost with the discovery by Brocks et al. (Brocks,J.J. et al. Science 285,1033-1036/1999) of a diverse assemblage of lipid 'biomarkers' in 2.7-bilion-year-old geological samples from Western Australia.

Biomarkers, or molecular fossils, are natural products (often hydrocarbons) whose synthesis can be linked to a specific biological origin - and, by physiological proxy, to environmrntal canditions. Together, this report and a subsequent study hinted at a much richer biological diversyty than hadpreviouslybeen recognized.(Brocks,j.j.,Buick,R.,Summons,R.E.&Logan,G.A. Geochim.Cosmochim. Acta 67,4321-4335/2003)

On page 1101 of this isse, However, Rasmussen et al.(Rasmussen,B., Fletcher,I.R., Brocks, JJ& Kilburn,M.R. Nature 455,1101-1104/2008) provude a robust challenge to the age of these biomarkers, and thr palaenontological and palaeoenbironmental insights that they offered.

The suite of lipid biomarkers reported by Brocks et al. included specific hopane and sterane compounds, respectively interpreted as the membrene remnants of cyanobacteria (a group of organisms characterzed by axygenproducing photosynthesis) and of ekukaryotes (cells bearing a membrane-bound nucleus and a complex cytiskeleton).

This discovery was so remarkable because it pushed back the minimum time for the origin of those groups by more than 700 milion years.

The oldest unambiguous fossil cyanobacteria were found in tidal-flat sedimentary rocks, some 2 bilionyears old, from Canada's Belcher Islands.(Hofmann,H.J.J. Paleontol. 50,1040-1073/1976)

It is probable that the evolution of cyanobacteria occurred much earlier; they must have existed by 2.4 billion years ago (Bekker,A. et al. Nature 427,117-120/2004) , because their metabolism is required, at last in part, to explain the apperance and rise of environmental oxygen at that time. But this still leaves a gap of 300 mikkion years.


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### DataBace ###
nature Vol.456 1007-1148 Issue no.7216 23 October 2008
News & Vews p.1051 / BIOGEOCHEMISTRY : Life before the rise of oxygen / Woodward W. Fischer

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