At the dawning of Homo sapiens, our ancestors were born into a world we would find utterly surreal. It’s not so much that the climate and sea levels or the plants and the animals were different, although of course they were—it’s that there were other kinds of humans alive at the same time. For most of H. sapiens’ existence, in fact, multiple human species walked the earth. In Africa, where our species got its start, large-brained Homo heidelbergensis and small-brained Homo naledi also roamed. In Asia, there was Homo erectus, a mysterious group dubbed the Denisovans and, later, Homo floresiensis—a hobbitlike creature, tiny but for its large feet. The stocky, heavy-browed Neandertals, for their part, ruled Europe and western Asia. And there were probably even more forms, as yet undiscovered.