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How Darwin’s rats revealed evolution

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Charles Darwin, in his most famous book “On the origin of species“, almost doesn’t mention the fossils that he discovered in South America, apart the brief reference in the introduction:

WHEN on board HMS ‘Beagle,’ as naturalist, I was much struck with certain facts in the distribution of the inhabitants of South America, and in the geological relations of the present to the past inhabitants of that continent. These facts seemed to me to throw some light on the origin of species-that mystery of mysteries, as it has been called by one of our greatest philosophers.

Fig.1. Young Darwin’s encounter with the ghosts of ancient beasts, as imagined by fellow-blogger & science-artist Glendon Mellow on his blog “The Flying Trilobite” (image used with permission of the author). The bones emerging from the sediments seemed those of ancient monsters and yet show a surprising resemblance to animals still living. For Darwin this was more than a coincidence, it was an observation worth investigating…

During the first years of his voyage aboard the “HMS Beagle” Darwin collected a considerable number of fossils of mammals from different localities in Argentina and Uruguay. He found the first fossils at Punta Alta September 23, 1832 and the last in 1834 at Puerto San Julián. The fossils were then sent to England to his former mentor, the botanist John Stevens Henslow, and deposited in the Royal College of Surgeons at London: Here in 1837 to 1845 the bones were studied and classified by the famous Victorian palaeontologist Richard Owen.
On the basis of this fossil material Owen will describe various new species of the Pliocene and Pleistocene of South America, including Equus curvidens (a horse species), Glossotherium sp., Mylodon darwini and Scelidotherium leptocephalum (all species of giant sloths), Macrauchenia patachonica (an endemic ungulate), and the strange Toxodon platensis (a rhinoceros-like animal). Unfortunately in April 1941 the paleontological collection of the Royal College was severely damaged by an air attack, nearly 95% of the collection was lost. After the war the surviving material was transferred to the Natural History Museum in London.

Fossils were known in South America since before the Spanish conquistadors, but had been interpreted as the remains of mythical creatures or giants, destroyed presumably by the gods in a remote time. Still in 1774 the English Jesuit Thomas Falkner notes:

On the banks of the river Carcarania … there are a large number of bones of extraordinary size, which seem human.

Only 32 years later the French naturalist George Cuvier will publish the first scientific publication on a fossil mammal of South America, the giant sloth Megatherium americanum, followed in 1806 by the description of the elephant-like genus Mastodon.

In 1838 Owen writes in the opening paragraph of his work on the fossil mammals collected by Darwin (The Zoology of the Voyage of H.M.S. Beagle; 1838-1843):

It may be expected that the description of the osseous remains of extinct Mammalia, which rank amongst the most interesting results of Mr. Darwin’s researches in South America, should be preceded by some account of the fossil mammiferous animals which have been previously discovered in that Continent. The results of such a retrospect are, however, necessarily comprised in a very brief statement; for the South American relics of extinct Mammalia, hitherto described, are limited, so far as I know, to three species of Mastodon, and the gigantic Megatherium.

The young and inexperienced Darwin identified many of the recovered bones wrong. He attributed discovered bony plates (so called osteoderms) to the Megatherium, following the reconstruction by Cuvier of the animal as an armoured sloth; Owen later attributed the fossils to the armour of the giant “armadillo” species Glyptodon.
The molars of Toxodon were interpreted by Darwin as the remains of a giant rodent, but even Owen later admitted that these teeth display a certain similarity to those of rodents (in fact there is a bit of truth in Darwin errors, Toxodon is now considered a peculiar form of South American ungulates, a group distant related to rodents).
But also Owen made mistakes, he misinterpreted the relationships of these fossil mammals to modern animals, attributing them or implying to them a close connection with certain animal groups still existing today. Observing fossils similar to bones of the modern tucutucu or tuco-tuco, a small rodent of the genus Ctenomys, Darwin realized that species were replaced in time by similar species.
Darwin summarizes that “The most important result of these findings is the confirmation of the law that existing animals have a close relationship with extinct species” (1839), an additional clue for Darwin that species are not isolated entities in time and evolution of species is not only possible, but really happened.

Bibliography:

ELDREDGE, N. (2009): A Question of Individuality: Charles Darwin, George Gaylord Simpson and Transitional Fossils. Evo. Edu. Outreach 2(1): 150-155
ELDREDGE, N. (2008): Experimenting with Transmutation: Darwin, the Beagle, and Evolution. Evo. Edu. Outreach 2(1): 35-54
FERNICOLA; VIZCAINO & DE IULIIS (2009): The fossil mammals collected by Charles Darwin in South America during his travels on board the HMS Beagle. Revista de la Asociacon Geologica Argentina. 64(1): 147-159
QUATTROCCHIO, M.E.; DESCHAMPS, C.M.; ZAVALA, C.A.; GRILL, S.C. & BORROMEI, A.M. (2009): Geology of the area of Bahia Blanca, Darwin’s view and the present knowledge: a story of 10 million years. Revista de la Asociacion Geologica Argentina 64(1): 137-146

David Bressan About the Author: Freelance geologist dealing with quaternary outcrops interested in the history and the development of geological concepts through time. Follow on Twitter @David_Bressan.

The views expressed are those of the author and are not necessarily those of Scientific American.





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