Schaafhausen pored over the fossils, observing their crests and knobs. He noticed that the bones had the overall shape you'd expect from a human skeleton. But some bones had strange features, too. The skullcap, for example, sported a heavy brow ridge, hanging over the eyes like a boney pair of goggles. It was, at once, human and not.

The Neanderthal Man challenged Schaafhausen with a simple yet profound question: Was it a human, or did it belong to another species?

It's been over 150 years since the bones first emerged from the Neander Valley—a time during which we've learned a vast amount about human evolution. Today, scientists can even scan the genomes of Neanderthals who died 50,000 years ago. And yet the debate still rages. It's a debate that extends beyond Neanderthals, forcing us to ask what it means to be a species at all.

VARIATIONS ON A THEME

The Neander Valley bones were a sensation as soon as Schaafhausen published his report on them in 1857, because nothing like them had been seen before. Earlier in the 1800s, cave explorers had found ancient human bones, sometimes lying next to fossils of cave bears and other extinct animals. Naturalists had a hazy sense from such bones that humanity had been around for quite a long time. But the idea that humans—or any other species—had evolved was scandalous. Darwin would not publish The Origin of Species for another three years. Instead, naturalists saw humans as a species distinct from chimpanzees, gorillas, and all other primate species. We were distinct today, and we had been distinct since creation.