In August 1856, in the German valley of Neander—Neanderthal in German—men cutting limestone for the Prussian construction industry stumbled upon some bones in a cave. Looking vaguely human, the bones—a piece of a skull, portions of limbs, and fragments of shoulder blades and ribs—eventually made their way to an anatomist in Bonn named Hermann Schaafhausen. 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 two 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.

The youngest Neanderthal fossils date to 28,000 years ago.

Within the human species, European anatomists divided people into races. They often ranked Europeans as the noblest race, considering the others barely better than apes. To justify this racist view of humanity, anatomists searched for clear-cut differences between the skeletons of different races—the size of skulls, the slopes of brows, the width of noses. Yet their attempts to neatly sort people into groups were bedeviled by the blurry variations in our species. Within a single so-called race, people varied in color, height, and facial features. Schaafhausen knew, for example, about a skull dug up from an ancient grave in Germany that "resembled that of a Negro," as he wrote. A barbarian (with sword) attacking a Roman legionary in a second-century relief. Neanderthals wouldn't have been out of place amongst such savage sorts, Schaafhausen believed. © The Gallery Collection/Corbis European savages On this confusing landscape Schaafhausen tried find a place for the Neanderthal Man. He decided that its heavy brow didn't disqualify it as a human. To back up this diagnosis, he relied on stories of ancient European savagery. "Even of the Germans," Schaafhausen wrote in his 1857 report on the Neander Valley bones, "Caesar remarks that the Roman soldiers were unable to withstand their aspect and the flashing of their eyes, and that a sudden panic seized his army." Schaafhausen searched historical records for other clues of Europe's monstrous past. "The Irish were voracious cannibals, and considered it praiseworthy to eat the bodies of their parents," he wrote. In the 1200s, ancient tribes in Scandinavia still lived in the mountains and forests, wearing animal skins, "uttering sounds more like the cries of wild beasts than human speech." Surely, in such a savage place, this heavy-browed Neanderthal would have fit right in. A distinctive creature When Schaafhausen published his report, many other naturalists tried to make sense of the bones for themselves. After Darwin published his theory of evolution in 1859, new possibilities arose: Perhaps humans evolved from Neanderthals, or perhaps they were both descended from a common ancestor. Thomas Huxley, Darwin's great champion in England, argued that Neanderthals were human, pointing to the thick foreheads of living Australian Aborigines. William King, an Irish geologist, disagreed. In an 1864 paper, "The Reputed Fossil Man of the Neanderthal," he pointed to a long list of traits that separated it from living humans—from its tightly curved ribs to the massive sinuses in its skull. Its braincase was so ape-like that it could not house a human-like brain. Australian Aborigines have a prominent brow ridge, a fact that helped lead Thomas Huxley to argue that Neanderthals were indeed human. © Kerrie Kerr/iStockphoto "I feel myself constrained to believe that the thoughts and desires which once dwelt within it never soared beyond those of a brute," King wrote. From all this evidence, King concluded that the Neanderthal Man was not simply an ancient European, as Schaafhausen had thought. It was a separate species. He even gave that species a name: Homo neanderthalensis. Mounting evidence King was certainly right that Neanderthals were distinct from living humans. Subsequent generations of fossil-hunters have found remains of Neanderthals from Spain to Israel to Russia. The youngest Neanderthal fossils date to 28,000 years ago. The oldest ones date back over 200,000 years. Like the original Neanderthal Man, they were stocky, with a heavy brow ridge and other singular traits. We can't know exactly what thoughts and desires soared in their heads, but they certainly left behind some telling clues—carefully engineered spear blades and stone knives; painted shells that might have been used as jewelry. Neanderthals endured the comings and goings of ice ages in Europe and Asia, hunting for reindeer, rhinoceroses, and other big game. As the fossils have emerged, paleoanthropologists have revisited the question of whether Neanderthals are part of our own species—call them Homo sapiens neanderthalensis—or a separate Homo neanderthalensis. Some researchers argued that Neanderthals belonged to a single species of humans stretching across the Old World, one that evolved over the past million years from small-brained hominids into our big-brained form.

Europeans and Asians carry a small portion of DNA inherited from Neanderthals.

But some researchers challenged this view. They pointed out that for thousands of years, Europe was home to the burly Neanderthals as well as slender humans. Neanderthals didn't give rise to living Europeans, these scientists argued; they were replaced by immigrants expanding out of Africa—perhaps even outcompeted into extinction. Over the past 15 years, Svante Pääbo, a geneticist at the Max Planck Institute of Evolutionary Anthropology, and his colleagues have uncovered an entirely new source of evidence about the nature of Neanderthals: their DNA. Starting with those fossils from the Neander Valley, they extracted bits of genetic material that had survived tens of thousands of years. Eventually, they were able to assemble the fragments into the entire Neanderthal genome. Populations of the same species that a river or other barrier divides can become unable to breed successfully with each other. Such an inability never occurred between Neanderthals and humans, who bred successfully at least once. © thobo/iStockphoto It's clearly different from the genome of any human alive today, sprinkled with many distinctive mutations. These mutations accumulated in a clock-like way, and by tallying them up, Pääbo and his colleagues estimate that Neanderthals and humans share a common ancestor that lived 800,000 years ago. It's possible that the ancestors of Neanderthals expanded out of Africa then, while our own ancestors stayed behind.

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A question of breeding That's a long time—long enough to reasonably ask if humans and Neanderthals are indeed two separate species. Old species split into new ones when some of their members get isolated from the rest. If a river cuts the range of a species of frog in two, for example, the frogs on one side of the river may only be able to mate with one another. Each population will evolve along its own path. If they are isolated long enough, they will have trouble interbreeding. They may even be unable to interbreed at all. From these facts of evolution, the biologist Ernst Mayr developed what came to be known as the Biological Species Concept in the 1940s—namely, a species is made up of members of populations that actually or potentially interbreed in nature. Experiments on living animals have shown that barriers to this interbreeding can arise in tens of thousands, or even just thousands, of years. Once the Neanderthal lineage left Africa 800,000 years ago, did humans and Neanderthals have enough time to become unable to interbreed? Pääbo's research provides an answer: no. Does the late Ernst Mayr's notion of what constitutes a species, which held sway for many decades, need to be scrapped or substantially revised? Many biologists believe so. © Rick Friedman/Corbis Europeans and Asians carry with them a small portion of DNA inherited from Neanderthals—while Africans do not. The best explanation for our mixed genomes is that after humans expanded out of Africa, they encountered Neanderthals and interbred. Comparing the different Neanderthal-derived genes in different people, Pääbo and his colleagues estimate that this encounter occurred around 40,000 years ago. The tiny amount of Neanderthal DNA has been interpreted by some scientists as evidence that Neanderthals rarely mated with humans—perhaps just once, in fact. But as scientists sequence more genomes from more human populations, they're exploring the possibility that our ancestors mated with Neanderthals several different times. A matter of survival The presence of DNA from Neanderthals in human genomes is compelling evidence that humans and Neanderthals could mate and produce fertile offspring. If we stick to the Biological Species Concept, then we are a single species, as Schaafhausen originally thought. But some scientists reject this argument. They think that Mayr's Biological Species Concept has worn out its usefulness.

Homo neanderthalensis and Homo sapiens endured—at least until the Neanderthals became extinct.

With the advent of gene sequencing, scientists have found that many animal species regularly interbreed. It's easy for any safari tourist to tell the difference between olive baboons and yellow baboons that live in Kenya, for example. And yet the two species regularly produce hybrids in the places where their species overlap, and they've been doing it for a long time. What will it take for experts to agree on whether Neanderthals (foreground) and modern humans are one and the same species? © Frank Franklin II/AP/Corbis So why haven't the two baboon species merged into a single hybrid olive-yellow species? The baboons produced by interbreeding may not survive as well as purebred ones. They produce fewer offspring of their own, and so the genes from one species don't spread easily in the other. Thus, despite interbreeding—breaking Ernst Mayr's rule, in other words—the olive and yellow baboons endure as separate species. Perhaps humans and Neanderthals were the same: They only interbred rarely, and when they did, the hybrid children couldn't fuse the two kinds of humans together. That may be why human and Neanderthal fossils remained so different. William King would probably have been horrified at the notion of human beings having sex with Neanderthal "brutes." But despite this intermingling, Homo neanderthalensis and Homo sapiens endured—at least until the Neanderthals became extinct, and we survived.

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