and wondered if there was a connection between animals like the giant ground sloths of prehistory and the continent's modern, arboreal sloths. If sloths, armadillos and other mammals were so different in the ancient past, then obviously life was not static and could change over time.

But by 1859 when On the Origin of Species was published, Darwin had largely left paleontology behind. It was up to other researchers to find the fossil proofs of evolutionary change.

Many paleontologists agreed with Darwin that some sort of natural laws were behind the succession of different organisms through time discovered in the earth's layers. But many — if not most — were not convinced that natural selection was the driving force behind these changes.

Even so, Darwin brought the subject of evolution to the forefront of Victorian science. And with an eye toward evolution, his colleagues began to pick through the traces of ancient life for clues about how organisms changed. This is a gallery of some of the key fossil species that have both confounded and inspired scientists in their efforts to understand the history of life and, placed in context of what we know today, have confirmed Darwin's vision of a branching tree of life produced by natural selection.

Image: During his 1876 American lecture tour, T.H. Huxley used this diagram to illustrate how the limbs of dinosaurs (middle) were more similar to those of birds (left) than crocodiles (right)./Written in Stone

Authors: Brian Switek

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