Paabo decided to look for DNA in the original Neanderthal bones. Needless to say, the curators at the Rhineland Museum in Bonn, who are responsible for the fossilized bones, were not eager to let him take samples. Analyzing the bones would mean grinding up irreplaceable fossil material and dissolving it in chemicals. But Paabo persisted, and the curators finally agreed. A bone specialist sawed a half-inch chunk from the upper right arm bone of a 42,000-year-old Neanderthal fossil.
Paabo handed over the sample to graduate student Matthias Krings, who wasn't optimistic—extracting DNA from 3,000-year-old mummies had been hard enough. He focused on DNA from the mitochondria, which is much shorter and more plentiful than the DNA that dictates the workings of the rest of the body. Soon Krings began to find DNA sequences that were clearly different from those of any human beings living today.
The results, along with those of subsequent studies, indicated that Neanderthals contributed little, if any, DNA to modern humans. Instead, they appear to have been displaced by modern humans—the taller, more graceful creatures with round skulls and prominent chins who first appear in the fossil record in eastern Africa about 200,000 years ago. The Neanderthals retreated into more remote parts of Europe before going extinct. Paabo's work means that during the thousands of years that Neanderthals shared the continent with modern humans, there was probably little interbreeding between the two groups. The same thing happened in other parts of the world: archaic populations of humans in Africa and Asia gradually went extinct without leaving an obvious genetic trace.
The apparent lack of interbreeding between archaic and modern humans means that we are a very young species—brash upstarts that overran the older and more established species of humans. "In a sense, we are all Africans, though some of us have gone to live in exile," Paabo says. To be sure, physical appearances changed as groups of modern humans moved into different environments. For example, as they moved into northern climates, natural selection appears to have favored lighter skin colors—probably because lighter skin admits more sunlight and thereby allows the body to synthesize sufficient vitamin D to endure long, dark winters. As a result, over many generations, the occupants of northern Europe and Asia gradually developed lighter skin than their ancestors. But these superficial differences disguise a remarkable genetic similarity. "Different subgroups of chimpanzees, such as those in eastern or western Africa," says Paabo, "have a much longer history of genetic separation than do, say, Chinese and Africans."
The German government provided very little support for anthropological research after World War II, a response to abhorrent wartime activities of the Kaiser Wilhelm Institute for Anthropology, Human Genetics and Eugenics in Berlin. (The head of the institute supported Nazi racial policies, and his assistant, Josef Mengele, sent body parts from Auschwitz to be studied at the institute.) But following the 1990 reunification of Germany, officials began looking for neglected areas of science to support in the effort to build new ties between East and West. In 1997, the government invited Paabo to move to Leipzig, a university town in the former East Germany, to start a new institute on human evolution with three other prominent scientists: Christophe Boesch of the University of Basel in Switzerland, who studies wild chimpanzees; Bernard Comrie, a linguist from the University of Southern California in Los Angeles; and psychologist Michael Tomasello from the Yerkes Primate Center in Atlanta. In the summer of 1997, the four scientists set off for a hike in the Alps south of Munich to mull over the invitation. By the time they returned from the mountains, they had decided to accept it. "There’s no reason to let Hitler keep us from working on human origins anymore," says Paabo.
Originally housed in an old Leipzig publishing house, the Institute for Evolutionary Anthropology moved in 2002 into a new $30 million building south of downtown. The four directors collaborated on the design, with Paabo insisting that a four-story rock-climbing wall be installed in the lobby. The directors agreed to focus their efforts on one particular question: What makes human beings unique? And to avoid empty speculation, they decided to work only on questions for which data are available. "The kinds of questions we ask are ones where we can see how to go about finding answers," says Comrie.
One day, Tomasello and Paabo were talking in the institute's cafeteria about a family in England with a remarkable genetic defect. Some members of the family have a mutation in a gene known as FOXP2, which helps direct the development of the brain during infancy and childhood. Every family member with the mutation had great difficulty speaking. Paabo had been thinking about how to identify genes that had changed during human evolution to make speech possible, and FOXP2 seemed like a prime candidate. He and his co-workers sequenced the gene—that is, they figured out the order of the DNA bases that make up FOXP2—in six different species. They found that it was one of the most stable genes they had ever studied; from mice to rhesus macaques to chimps, the protein produced by the gene is almost exactly identical, suggesting that the gene itself plays a fundamental role in animal function. But in humans the gene had undergone a slight modification. About 250,000 years ago, according to the scientists' calculations, two of the molecular units in the 715-unit DNA sequence of the gene abruptly changed. That's not long before modern humans first appeared in the fossil record. Could the changes in FOXP2 have enabled modern humans to speak? And could articulate speech have given modern humans an edge over the Neanderthals and other archaic humans?
That's certainly what some newspaper stories implied, labeling FOXP2 a "language gene." But Paabo and other scientists are more cautious. FOXP2 "is one of who knows how many genes that affect language ability," says Ken Weiss, an expert on evolution and genetics at Pennsylvania State University. The change in FOXP2 might have been entirely coincidental. Or the gene may be related to language indirectly—for example, by influencing coordination. And some scientists argue that language evolved much earlier than our version of FOXP2, and that archaic humans also had speech.
Still, Paabo's work on FOXP2 has raised fruitful questions. Researchers are genetically engineering mice with "broken" FOXP2 genes, to see how disruptions in the gene might affect the animals. Also, researchers are splicing the human version of the gene into mice to see if it makes any difference. (So far, none of the mice have started talking.)
More recently, Paabo has taken an even broader view of the genetic changes responsible for our uniquely human traits. For example, mutations in individual genes like FOXP2 may not be the most important force in evolution. An even bigger factor may be changes in the genetic switches that turn on and off many genes at once. Paabo and his colleagues have been looking at the patterns of gene activity in humans, chimps and other species. As might be expected, the brain has been a particularly active site for recent human evolution. Paabo's team finds that genes in the human brain have undergone more changes in how they are turned on than similar genes in chimp brains.