Paleoanthropologists have discovered as many as 27 different human species (the experts tend to debate where to draw the line between groups). These hominids diverged after our lineage split from a common ancestor we shared with chimpanzees 7 million years ago, give or take a few hundred millennia.

Many of these species crossed paths, competed, and mated. Populations ebbed and flowed in tight little tribes, at first on the expanding savannahs of Africa, later throughout Europe, Asia, and all the way to Indonesia. Just 100,000 years ago, there were several human species sharing the planet, possibly more: Neanderthals in Europe and West Asia, the mysterious Denisovan people of Siberia, the recently discovered Red Deer Cave people living in southern China, Homo floresiensis (the Hobbits of Indonesia), and other yet unknown descendants of Homo erectus who left indications that they were around (the DNA of specialized body lice, to be specific). And, of course, there was our kind, Homo sapiens sapiens (the wise, wise ones), still living in Africa, not yet having departed the mother continent. At most, each species consisted of a few tens of thousands of people hanging on by their battered fingernails. Somehow, out of all of these struggles, our particular brand of human emerged as the sole survivor and then went on, rather rapidly, to materially rearrange the world.

If there once were so many other human species wandering the planet, why are we alone still standing? After all, couldn’t another version or two have survived and coexisted with us on a world as large as ours? Lions and tigers coexist; so do jaguars and cheetahs. Gorillas, orangutans, bonobos, and chimpanzees do as well (though barely). Two kinds of elephants and multiple versions of dolphins, sharks, bears, birds, and beetles—countless beetles—inhabit the planet. Yet only one kind of human? Why?