A solution to the puzzle which came to be known as ‘Darwin’s Dilemma’ has been uncovered by scientists at the University of Oxford in a paper published in the Journal of the Geological Society

‘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’, Charles Darwin wrote in On the Origin of Species by Means of Natural Selection, or The Preservation of Favoured Races in the Struggle for Life in 1859, summarizing what came to be known as ‘Darwin’s Dilemma’ – the lack of fossils in sediment from the Precambrian (c. 4500 – 542 Mya).

The puzzle was that if Darwin’s theory of natural selection was right, life evolved gradually over millions of years but 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.' 

Darwin struggled to find evidence for fossils prior to the Cambrian, and the mystery continued to perplex palaeontologists.   A study carried out by Richard Callow and Martin Brasier from the Department of Earth Sciences at the University of Oxford focused on a rock formation from Shropshire, England, known as the Longmyndian Supergroup. These rocks had been examined in Darwin’s time by the Geological Survey geologist J. W. Salter, who suspected them of containing records of Precambrian life, although he was unable to identify anything beyond ‘trace fossils’: unusual markings which may have been left behind by organisms. Darwin even noted the significance of Salter’s discoveries in what is now colloquially called the ‘Origin of Species’.

The study used Salter’s original collections as well as new samples from the Longmyndian Supergroup, and for the first time have been able to reveal a variety of microscopic fossils of exceptional preservation.

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. Others were preserved in three dimensions and are thought to have undergone permineralization, a process where water containing minerals seeps into the spaces within an organism and evaporates, leaving behind mineral deposits which build up into a hard fossil. Some had also been preserved as impressions and moulds within layers of sediment, appearing as sharp ridges on bedding planes, or as their equivalent negative impressions. 

It is not clear how the microbes kept themselves alive. As they lived in shallow marine environments, they may have survived either by converting light into energy in a similar way to plants, or by converting organic substances into energy as animals and humans do. Suggestions as to what organisms they might be related to include algae, fungi or a wide variety of other filamentous bacteria. 

Darwin himself was confident that fossils from the Precambrian would eventually be found, believing it to be a time when ‘the world swarmed with living creatures’. Although the potential importance of the Longmyndian Supergroup in solving Darwin’s dilemma has been recognized since Salter’s pioneering discoveries, it is only now, with more sophisticated techniques for examining specimens, that the secrets of the Longmyndian rocks and their exceptionally preserved fossils can be uncovered.

Article: 'A solution to Darwin's dilemma of 1859: exceptional preservation in Salter's material from the late Ediacaran Longmyndian Supergroup, England', Richard H. T. Callow and Martin D. Brasier, Journal of the Geological Society, Vol. 166, 2009, pp 1-4