Charles Darwin and the Origin of Species

In September 1832, a young naturalist named Charles Darwin was exploring the coast of Argentina as part of the voyage of HMS Beagle. At a place called Punta Alta, he found something extraordinary embedded in a cliff face: a massive fossil skull. For three hours, Darwin worked to dig the giant specimen out of the rock. It was later identified as a Megatherium, an extinct ground sloth the size of an elephant. Although he initially thought the fossil was related to a rhinoceros, he eventually recognized that these giant extinct creatures bore a striking resemblance to the small tree sloths still living in South American forests. He also found fossil armour plates similar to those of living armadillos nearby. This was the first hint of what he called the 'law of succession'—that extinct local species are replaced by similar, related living ones.