Darwin’s fossil mammals: discoveries that sparked the theory of evolution | Digital Collections Programme

1) book cover
Adrian Lister’s book, Darwin’s fossils: discoveries that shaped the theory of evolution

Guest blog by Adrian Lister

When I first joined the Museum as a fossil mammal researcher in 2007, I received a set of keys that gave me access to much of the museum’s huge collection.  Browsing one day, I opened an unremarkable cupboard and was startled to find six shelves of fossil bones with a sign reading ‘Charles Darwin, Beagle Voyage’.  Labelled and arranged by the then curator of fossil mammals, Andy Currant. Here were numerous fossils collected by Darwin in South America and sent back to England, where they had been studied and named by the Museum’s founder, Richard Owen.  One specimen in particular, a beautiful jaw bone of an extinct giant sloth, caught my eye – it had been named by Owen Mylodon darwinii after its finder, and here it was – a direct link to one of the most important theories in the history of science.  Questions crowded in: why had Darwin taken such trouble to collect fossil bones? What did they tell us about the working relationship between Darwin and Owen, before they famously fell out? And most important of all, what (if any) was the significance of the fossils in leading Darwin to his theory of evolution? I soon discovered that there were other, equally remarkable Beagle fossils in other parts of the collection: ancient marine shells collected by Darwin high in the Andes; pieces of wood from a petrified forest he had encountered quite by chance on his travels; and fragments of coral rock from islands visited on the journey home.

I was not, of course, the first person to marvel at the Darwin fossils, and some of them had been the subject of specialist research over the years. However, no-one, it seemed, had surveyed the collections as a whole or brought out their significance for Darwin and the development of his ideas.  As my interest grew, I conceived the idea of writing a book on the subject and, after several years’ work, Darwin’s Fossils is the result.  In this endeavour I have benefited immensely from the curators of the various collections, who dug out obscure Darwin fossils for me and helped bring the identifications up to date.  I was also fortunate that almost all of Darwin’s copious diaries, notebooks and letters from the Beagle voyage had recently been transcribed and made available online by a group of dedicated scholars based in Cambridge and elsewhere.  In preparation for writing Darwin’s Fossils I read all these records, flagging every mention of fossils, and linking them where possible to the remaining specimens. It was thrilling to read Darwin’s on-the-spot excitement at finding the remains, and his initial scribbled thoughts about their meaning.

Darwin studied all aspects of natural history during his five-year voyage on H.M.S. Beagle, but by far the largest portion of his time was devoted to observing and recording rocks and fossils. Whenever the Beagle was in port, Darwin was off exploring the coast and the interior, on foot or on horseback. Fossils and geology were clearly paramount in these excursions, as he wrote: ‘My object in all this galloping was to understand the geology of those beds which so remarkably abound with the bones of large and extinct quadrupeds’. Darwin’s most successful hunting-ground was in a low cliff at a spot called Punta Alta, in the bay of Bahia Blanca, some 400 km south of Buenos Aires. There he discovered seven distinct genera of extinct mammal, while six more were collected from other sites.  Of these 13, only two were known at the time, and six were named on the basis of Darwin’s specimens. He wrote to his sister Caroline how former pastimes paled into insignificance: ‘The pleasure of the first day’s partridge shooting or first day’s hunting cannot be compared to finding a fine group of fossil bones, which tell their story of former times with almost a living tongue’.  Many of the species discovered by Darwin are now celebrated elements of the South American fossil fauna of the last ice age, some 100,000 to 12,000 years ago.

Darwin himself made clear, that the fossil finds – especially the fossil mammals – played a key role in triggering his early thinking about evolution. As he later wrote in the famous first line of the Origin of Species: ‘When on board H.M.S. Beagle as naturalist, I was much struck with certain facts in the distribution of the organic beings inhabiting South America, and in the geological relations of the present to the past inhabitants of that continent’ (italics added).  Therein lies the clue: it was not just the existence of the extinct mammals, but their evident relationship to those still living in the same area, that became a key line of evidence for evolution. Darwin’s finding of the metre-wide domed carapace of the extinct mammal later named Glyptodon made a particular impact; as he later wrote: ‘I shall never forget my astonishment when I dug out a gigantic piece of armour like that of the armadillos’. Darwin was not a man to be astonished by something without asking himself why it should be so, and in the case of Glyptodon, it was the evident similarity to the living armadillos that he had seen and studied (and eaten) during the course of the voyage: both shared a similar carapace of bony tiles under the skin.  Another example was the sloths, of which Darwin collected fossils of at least four extinct genera (three of them new to science); even in the field he recognised, principally from their simple, peg-like dentition,  that they fell within the group including the much smaller living sloths that hang from branches in tropical forests. The key point was the congruence of geographical distributions between the extinct and living forms: both sloths and armadillos today have their core distributions in South America, and here was Darwin finding remains of related, but evidently different, extinct forms in the same continent.

 

 

Back in England, this relationship between the past and present occupants of a region was formalized by Darwin as the ‘Law of Succession of Types’.  His senior colleagues, such as Richard Owen and the geologist Charles Lyell, could subscribe to it too, but for Darwin it held a deeper meaning:  the living and extinct forms were similar not just because of similar habitats or some idea of continuity in the mind of the Creator, but because they were linked by genealogical descent (in other words, evolution).  By the time Darwin came to publish his book Voyage of the Beagle in 1845 he was already a committed evolutionist, and although he took care in his writings to conceal the fact, in retrospect we can see hints at his thoughts on the matter.   After detailing the evidence in support of his law of succession of types, he writes: ‘This wonderful relationship in the same continent between the dead and the living, will, I do not doubt, hereafter throw more light on the appearance of organic beings on our earth, and their disappearance from it, than any other class of facts’.

As for the celebrated skull of Toxodon,  Richard Owen, after intensive study, admitted that it bore no close resemblance to anything known. Yet comparisons had to be made, and Owen’s famous conclusion was that Toxodon had affinities to mammals as diverse as rodents, rhinos, sloths and sea-cows. By ‘affinities’ Owen meant simply shared aspects of form, not common descent.  But Darwin was enthralled, writing in 1845:  ‘How wonderfully are the different Orders, at the present time so well separated, blended together in different points of the structure of the Toxodon!’ For him, anything that broke down the perception of rigid boundaries between species or groups of species was grist to his evolutionary mill. And what are we to make of his suggestion that ‘at the present time’ the Orders were well separated, if not a hint that at some time past there might have been a common ancestor in which the features of now disparate species were merged or combined?

Darwin soon began to consider evolution not just as a series of individual lineages governed by the ‘law of succession’, but as a branching tree of life. Here again, it is clear that his fossil mammals were guiding his thinking, as his first notes on the tree-like relationships of species (illustrated by his famous diagram, below) are linked to the fossil sloths and glyptodonts in relation to their living relatives. He wrote: ‘We may look at Megatheria, armadillos and sloths as all offsprings of some still older type.’ In other words, while the glyptodonts and giant sloths such as Megatherium have no direct descendants, they and the living sloths and armadillos had all descended from a common ancestor.

darwin's tree evolution
Darwin’s famous evolutionary tree, from his 1837 ‘transmutation notebook’. It was inspired, at least in part, by a consideration of his South American fossil mammals. (Wikimedia Commons)

Darwin had arrived at these insights before his 1838 discovery of natural selection – the mechanism that explained how evolution could occur.  The fossils did not contribute directly to that discovery. But before even considering the problem, Darwin had to be convinced that evolution had occurred in the first place. In that, the fossils from South America, discovered years before he arrived in the Galápagos, played a critical role.  They were the foundation for the edifice of evolutionary theory that Darwin subsequently built.

Adrian Lister’s book, Darwin’s fossils: discoveries that shaped the theory of evolution is published by the Natural History Museum.

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