They say you can choose your friends, but you can't choose your relatives. Unless you're a paleontologist tracing the lineage of the human race, in which case choosing your relatives is a full-time job. And the most vexing choice of all concerns our ancestral relation to the Neanderthals, the craggy-browed cave dwellers who vanished from Europe some 30,000 years ago.
Homo neanderthalensis was the last hominid species to rival our own. Like Homo sapiens, its members had big brains, used tools, lit fires, and buried their dead. They thrived for 200,000 years in severe ice age climates, from Britain to Uzbekistan. When H. sapiens began to arrive from the south, the two species dwelled alongside each other for thousands of years. But experts disagree about how they got along. Did they make love or war? Were the indigenous tribes killed or coddled by the newcomers? Why didn't the Neanderthals last?
"There is a short answer to that: modern humans," says paleoanthropologist Ian Tattersall of the American Museum of Natural History. Tattersall belongs to a faction that claims the Neanderthals were driven to extinction by the technical savvy and social cooperation of H. sapiens. "We did it by direct economic competition, or we did it by direct conflict," he insists. "It was not a happy interaction."
That stance is called the "out of Africa" theory, which holds that all of contemporary humanity derives from a small population with a single source on the African continent. It's based in part on fossil evidence that hominids evolved in Africa and peopled other continents in migratory waves that began nearly 2 million years ago. The first humans with modern features appeared in Africa more than 100,000 years ago; 40,000 years ago, H. sapiens called Cro-Magnons made it to Europe. Cro-Magnons had stone-shaping techniques and symbolic art that were dramatically different from anything found in previous Stone Age cultures. They adorned themselves with necklaces, bracelets, and beads, painted cave walls, and played drums and flutes. Their campsites and graves became more elaborate. Judging from the artifacts of the Upper Paleolithic, our ancestors had discovered life beyond the next mastodon brisket.
In contrast, Neanderthal culture was more about grim subsistence. Neanderthals were the sole survivors of hominids that moved into Europe half a million years ago. They didn't have bone needles or shell beads, they didn't paint or play music, and their burials were no-nonsense affairs. It's easy to imagine that they didn't stand a chance once the Cro-Magnons showed up. The last Neanderthal fossils come from sites in southern Spain and Portugal; they're about 28,000 years old.
"Of course, there are no Neanderthals left today," says Milford Wolpoff, an anthropologist at the University of Michigan in Ann Arbor. "And people like to use that as a demonstration that they went extinct like the dodo birds. But that ignores the process of evolution."
Wolpoff is the most vocal advocate of the multiregionalism theory, which submits that the Cro-Magnons who left Africa got on rather well with the natives they encountered in their travels. In this view, Neanderthals weren't so much driven to extinction as seduced. "Over time, more and more genes came into Europe and mixed with Neanderthal genes," says Wolpoff. "And the proportion of Neanderthal genes became lower and lower."
Even today, features thought to be Neanderthal are as familiar as the portraits in a grandparent's home: the sloping forehead, the heavy brow, the stocky, big-boned physique. Because their anatomy is so distinctive, Neanderthals have been classified as a separate species. That implies they didn't breed with other hominids. But Wolpoff believes they could and did get it on with the Cro-Magnons. Last year he published an analysis of 25 fossil skulls from Europe and Australia that he says hints at mixed ancestry between migrating moderns and local Neanderthals. And he believes many Neanderthal features persist in European visages today: a unique hole in the jawbone, the shape of a suture in the cheek, a highly angled nose, "like Jimmy Durante or Charles de Gaulle.
"The Neanderthals became extinct like the Cro-Magnons became extinct: because of mixing with other populations," says Wolpoff. "Their physical form is gone, their culture is gone, but their genes are still among us."
In 1999 Erik Trinkaus, a physical anthropologist at Washington University in St. Louis, described the 24,500-year-old skeleton of a child from Portugal that he claims is a product of interbreeding. Most of the child's features are modern, but Trinkaus points to a number of Neanderthal influences, including short lower legs, massive cheekbones, shovel-shaped incisors, and a pitting at the base of the skull.
Single-origin theorists say the bones are all within the normal limits of H. sapiens variation. Meanwhile, archaeologists are questioning their assumptions about the Neanderthal lifestyle. In particular, it has become less clear exactly who invented the Upper Paleolithic. One assemblage in France, dated between 39,000 and 34,000 years ago, has bone and shell pendants, carved teeth and beads, as well as finely worked tools like the Cro-Magnons used. But the only bones found with this technology are Neanderthal. Archaeologist Steve Kuhn of the University of Arizona in Tucson says a confusing array of transitional technologies is now emerging from sites in Eastern Europe, Turkey, and the Middle East. "It looks like some very complex changes in the local cultures, with some other cultures moving in as well," says Kuhn.
And researchers have never found any signs of warfare between the groups. "If there was a lot of fighting, you'd think there would be evidence of violent death," says Kuhn. "But there's not." In the Middle East, Neanderthals and Cro-Magnons seem to have occupied much of the same territory for 60,000 years. So far, their cultures there are indistinguishable. That could mean relations between our relations were more stable and more equitable than anyone had envisioned. Kuhn has even suggested that interactions between Neanderthals and early moderns may have inspired the flowering of Upper Paleolithic culture in both groups, either by competition or by collaboration.
But the out-of-Africans remain unpersuaded. Tattersall calls multiregionalism "wishful thinking." "Quite simply, Homo sapiens appears to be inherently—and probably inherently savagely—intolerant of competition from its relatives," he writes in The Monkey in the Mirror. "It's always possible that some hanky-panky may have occurred," he allows. "But it wasn't evolutionarily significant."
Tattersall says studies that use DNA from contemporary populations to reconstruct human genealogy support the idea of a single, small source of Homo sapiens. Unfortunately, the DNA that's best preserved in ancient remains is from cellular components called mitochondria that aren't representative of the larger human genome. The mtDNA extracted from Neanderthal bones doesn't match anything in the modern world. But last year, when geneticists compared mtDNA from an early modern Australian with contemporary mtDNA, it didn't match either.
Archaeologists say they need to refine the dating techniques that establish their chronologies. "We're right at the limits of radiocarbon dating, which may be one of the reasons this is such a knotty issue to sort out," says Kuhn. Anthropologists are hoping to find more fossils from the critical transition period, especially in central Asia, where the Upper Paleolithic tool kit may have originated. The fate of the Neanderthals has global ramifications as well, because whatever happened to them probably happened to the local populations of east Asia as early modern humans advanced. Eventually, the children of those interactions would enter the Americas, planting the seed for another rancorous debate. That's just what happens when paleontologists choose your relatives.