Before Darwin's time, humans were not considered part of the natural world. People saw that we resembled other animals, especially other primates like the orangutan and the chimpanzee. Still, despite the undeniable similarities between "us" and "them," only a handful of early naturalists classified humans, too, as animals.

In England during the 1700s and early 1800s, few questioned the Biblical story of creation. The prevailing view was that people were created to rule over animals, "over the fish of the sea and over the birds of the sky." Naturalists could see that, particularly beneath the skin, a chimpanzee looks a lot like a human—but the idea that we might somehow be related to apes was almost unthinkable.