On 23 September 1832, Charles Darwin made an important discovery at Punta Alta, near Bahia Blanca in Argentina. He recorded the find in his Beagle diary:
Sunday Sept: 23rd [1832]
A large party was sent to fish in a creek about 8 miles distant; great numbers of fish were caught. — I walked on to Punta alta to look after fossils; & to my great joy I found the head of some large animal, imbedded in a soft rock. — It took me nearly 3 hours to get it out: As far as I am able to judge, it is allied to the Rhinoceros. — I did not get it on board till some hours after it was dark. —
The bones were eventually shipped back to Britain, where the great anatomist Richard Owen identified them as belonging to an extinct giant ground sloth, Megatherium. Megatherium‘s close relatives, the tree sloths, still live in South and Central America.
The phenomenon of extant species being found in the same part of the world as closely allied extinct species was one of the major clues that got Darwin thinking about evolution. Indeed, he put this observation front and centre in the opening paragraph of On 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 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.
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