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Fabulous Fossils

Darwin's search for fossils brought him back again and again to Punta Alta, on the coast of Argentina, where countless bones could be picked from the cliffs. His first visit produced a treasure trove of skulls and armor plates from six different animals. "I have been wonderfully lucky," he wrote to Henslow; "many of them are quite new." On a later visit he found "nearly an entire skeleton" of a giant ground sloth called Megatherium.

After two years of collecting, however, Darwin still had not heard whether any of his prized fossils had reached England. For all he knew, they were lying on the bottom of the ocean. Finally a letter arrived from Henslow. His fossils were safe in England, and Henslow urged Darwin to send back "every scrap of Megatherium skull you can set your eyes upon.- & all fossils." Darwin resumed collecting with renewed vigor, and with confidence that he was on the right track.

Three-toed sloth (Bradypus variegatus)

© AMNH Library

Sloths the Size of Elephants

The three-toed sloth lives in the forests of South America. Sloths spend most of their time in the treetops, hanging from their curved claws and eating leaves. Extinct ground sloths like the ones Darwin collected were far too big to climb trees—some were the size of elephants. Darwin assumed the giant ground sloths instead used their curved claws to pull leafy branches down to them.

"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."