Signs of earliest life

September 12, 2011 12:00 am | Updated 04:19 am IST

The latest fossil finds of microscopic life about 3.4 billion years ago in sedimentary rocks in Western Australia suggests that life began some 200 million years earlier than previously reckoned. Though a paper published recently in the journal, Nature Geoscience (“Microfossils of sulphur-metabolizing cells in 3.4-billion-year-old rocks of Western Australia,” by David Wacey et al .), does not claim them to be the oldest ones to have been ever discovered, the implications are clear. Assuming that the findings are correct, geologists might be a step closer to solving one of the greatest riddles — when did life first begin after the earth was formed some 4.5 billion years ago? They would also get a handle on the conditions that allowed primordial life to thrive. The authors have come across several lines of evidence that confirm the find to be truly microfossils. The foremost among them is the cell-like structures of similar size found in clusters. The microstructures also have uniform cell thickness, a hollow interior, and partially preserved partitions seen between adjacent cellular compartments. The presence of carbon, nitrogen, and sulphur in the cell walls clearly emphasises the biological nature. The presence of pyrite crystals associated with the microfossils indicates a possible sulphur metabolism by the life forms wherein sulphates are reduced to sulphides.

The compulsion to provide a wide range of supporting evidence arises because unearthing early life as old as a few billion years is a highly controversial field. For one thing, the number of sedimentary rock outcrops that retain the earliest traces of life intact is severely restricted. Even where these rocks have escaped metamorphism, most of the microstructures in them are deceptive. A controversial paper published in Science in 1993 reported the presence of filamentous microbes from Apex chert, just a few kilometres away from where the Nature Geoscience authors have found theirs. That the 3.4 billion-year-old microfossils from Apex chert belonged to the oxygen-breathing family of bacteria turned out to be the biggest sticking point. The fact that the identification of filamentous life forms were based solely on morphology proved to be its greatest failing. A 2002 paper in Nature based on re-analysis of the fossils using state-of-the-art techniques established that they are nothing but pseudofossils. This and a few other studies done in the recent past have brought to the fore the importance of undertaking geochemical analysis before declaring old microstructures as fossils. The current study has been well received but it would be unwise to conclude that the last word has been pronounced.

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