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A solution to the puzzle which has come to be known as ‘Darwin’s Dilemma’ has been uncovered by scientists at the University of Oxford, in a paper to be published in the Journal of the Geological Society this week (January 12th).
‘To the question of why we do not find rich fossiliferous deposits belonging to these…periods prior to the Cambrian system, I can give no satisfactory answer’.
These words, written by Charles Darwin in The Origin of Species in 1859, summarise what has come to be known as ‘Darwin’s Dilemma’ – the lack of fossils in sediment from the Precambrian (c. 4500 – 542 Mya). If Darwin’s theory of natural selection was right, life evolved gradually over millions of years. However, the Cambrian period, which began around 542 million years ago, seemed to herald a sudden rapid increase in species diversity, an event which has come to be known as the ‘Cambrian explosion’.
The fossils represent a wide array of microbial life from the Ediacaran period, the period immediately preceding the Cambrian (630 – 542 Mya). They were preserved in a number of ways. Some had been compressed under layers of sediment until they formed a thin film of carbon residue on the surface of the rock.