Controversial claim for earliest life on Earth
2 August 2006
Researchers claim to have found ‘compelling’ new evidence for the ‘earliest’ forms of life on Earth.
Australian and Canadian scientists describe, in a paper in Nature,1 seven varieties of stromatolites along a 10-km rock formation in the Pilbara region of Western Australia. Known as Strelley Pool Chert, the formation is supposedly 3.43 billion years old.
Much caution is needed when making claims about the earliest signs of life. In rocks of this great age we must assume the hypothesis of a non-biological origin.
There has been an ongoing controversy about the origin of these stromatolites. If, as many have argued, their finely laminated sedimentary structures are the result of non-living chemical processes, then there is nothing particularly remarkable about the find.
But, lead author, Abigail Allwood, from Sydney’s Macquarie University, says the stromatolites formed a ‘reef’ and that the reef was built by microbial organisms.2
This makes the find highly significant—like finding the ‘Holy Grail’3 as she describes it. At an age of 3.43 billion years, the stromatolites would represent evidence for some of the oldest life forms on Earth.
‘We’re seeing evidence not just of life’s existence,’ Allwood said, ‘but that it was probably well established and already biodiverse, which suggests it could have emerged much earlier in Earth’s history.’
So why did the team claim that the laminated sedimentary structures were biogenetic stromatolites? Because they say they are similar to younger stromatolites that have been described by others, and because they say that abiotic processes capable of producing such laminated structures are ‘unknown and unlikely in the natural world’.
However, even within their interpretive framework, the Archaean was a most unnatural period of geological history compared with today. There was no multicellular life, and there was supposed to be a reducing atmosphere of methane and ammonia. And chert horizons, although common in the geologic record, are not known to be forming now.
The incredibly old dates assigned to the rocks create some peculiar consequences, too. Although the Strelly Pool Chert formation varies from only 23 to 102 metres in thickness, it occupies a time period of at least 80 million years.4 Where is all the time?
And it is even more curious when we consider the rocks themselves. The basal member of the formation is a boulder conglomerate, deposited rapidly. It contains soft clasts (pieces) of mudstone derived from the underlying formation, evidence that not much time elapsed between the two.
The underlying and overlying formations were deposited from volcanic eruptions. How could microbes build these ‘reefs’ amid volcanic eruptions, rapid sedimentation and geological catastrophe? Perhaps soft sediment deformation rather than bacteria may explain some of the unusual shapes of the laminations.
But, even if we accept the billions of years and the evolutionary scenario, was there enough time for life to have formed from scratch so quickly after the Earth’s formation? If this new discovery finds acceptance, theories about the origin of life will probably need revision. Or perhaps the age of the earth will need to be pushed out.
With such big implications, some remain unconvinced of the claims.
Martin Brasier is one who has long argued against the idea that the stromatolites are of biogenetic origin. Professor of paleobiology at the University of Oxford, he is reported to have said ‘Much caution is needed when making claims about the earliest signs of life. In rocks of this great age we must assume the hypothesis of a non-biological origin.’2
Note that it’s not because of the evidence that he rules out a biological origin, but the assumed ‘great age’—i.e. the rocks are presumed to be older than life itself, therefore any evidence to the contrary is automatically dismissed.
- Allwood, A.C. et al. Stromatolite reef from the Early Archaean era of Australia, Nature 441, 714–718, 8 June 2006. Return to text
- Skatssoon, J., Pilbara rocks show early signs of life on Earth, <www.abc.net.au/news/newsitems/200606/s1658283.htm>, 15 June 2006. Return to text
- Stromatolites confirmed as ancient fossils, <www.abc.net.au/rn/scienceshow/stories/2006/1658194.htm>, 14 June 2006. Return to text
- Allwood, A.C. et al., Ref. 1, Supplementary text and figures. Return to text