by Dan Ariely
In the 1950s, a pair of anatomists, Williams Straus and Alexander Cave, decided to reexamine the skeleton from La Chapelle. What Boule had taken for the Neanderthal’s natural posture, Straus and Cave determined, was probably a function of arthritis. Neanderthals did not walk with a slouch or with bent knees. Indeed, given a shave and a new suit, the pair wrote, a Neanderthal probably would attract no more attention on a New York City subway “than some of its other denizens.” More recent scholarship has tended to support the idea that Neanderthals, if not quite up to negotiating the IRT, certainly walked upright, with a gait we would recognize more or less as our own. The version of Neanderthals offered by the Neanderthal Museum—another cartoon—is imbued with cheerful dignity. Neanderthals are presented as living in tepees, wearing what look like leather yoga pants, and gazing contemplatively over the frozen landscape. “Neanderthal man was not some prehistoric Rambo,” one of the display tags admonishes. “He was an intelligent individual.”
Pääbo announced his plan to sequence the entire Neanderthal genome in July 2006, just in time for the hundred-and-fiftieth anniversary of the Neanderthal’s discovery. The announcement was made together with an American company, 454 Life Sciences, which had developed a so-called high-throughput sequencing machine that, with the help of tiny resin spheres, could replicate tens of thousands of DNA snippets at a time. Both inside and outside the genetics profession, the plan was viewed as wildly ambitious, and the project made international news. “A Study with a Lot of Balls,” the headline in The Economist declared.
By this point, a complete version of the human genome had been published. So, too, had versions of the chimpanzee, mouse, and rat genomes. But humans, chimps, mice, and rats are all living organisms, while Neanderthals have been extinct for 30,000 years. The first hurdle was simply finding enough Neanderthal DNA to sequence. The chunk of the original Neanderthal that Pääbo had received had yielded shreds of genetic information, but nowhere near the quantities needed to assemble—or reassemble—an entire genome. So Pääbo was placing his hopes on another set of bones, from Croatia. (The Croatian bones turned out to have belonged to three individuals, all of them women; the original Neanderthal was probably a man.)
Toward the end of 2006, Pääbo and his team reported that, using a piece of Croatian bone, they had succeeded in sequencing a million base pairs of the Neanderthal genome. (Just like the human genome, the full Neanderthal genome consists of roughly 3 billion base pairs.) Extrapolating from this, they estimated that to complete the project would take roughly two years and six thousand “runs” on a 454 Life Sciences machine. But later analysis revealed that the million base pairs had probably been contaminated by human DNA, a finding that led some geneticists to question whether Pääbo had rushed to publish results that he should have known were wrong. Meanwhile, subsequent bones yielded a much lower proportion of Neanderthal DNA and a much higher percentage of microbial DNA. (Something like 80 percent of the DNA that has been sequenced for the Neanderthal Genome Project belongs to microorganisms and, as far as the project is concerned, is useless.) This meant the initial estimates of the labor involved in finishing the genome were probably far too low. “There were times when one despaired,” Pääbo told me. No sooner would one problem be resolved than another materialized. “It was an emotional roller coaster,” Ed Green, the biomolecular engineer from Santa Cruz, recalled.
About two years into the project, a new puzzle arose. Pääbo had assembled an international team to help analyze the data the sequencing machines were generating—essentially, long lists of A’s, T’s, G’s, and C’s. Sifting through the data, one of the members of this team, David Reich, a geneticist at Harvard Medical School, noticed something odd. The Neanderthal sequences, as expected, were very similar to human sequences. But they were more similar to some humans than to others. Specifically, Europeans and Asians shared more DNA with Neanderthals than did Africans. “We tried to make this result go away,” Reich told me. “We thought, This must be wrong.”
For the past twenty-five years or so, the study of human evolution has been dominated by the theory known in the popular press as “Out of Africa” and in academic circles as the “recent single-origin” or “replacement” hypothesis. This theory holds that all modern humans are descended from a small population that lived in Africa roughly 200,000 years ago. (Not long before he died, Pääbo’s adviser Allan Wilson developed one of the key lines of evidence for the theory, based on a comparison of mitochondrial DNA from contemporary humans.) Around 120,000 years ago, a subset of the population migrated into the Middle East, and by 50,000 years ago a further subset pushed into Eurasia. As they moved north and east, modern humans encountered Neanderthals and other so-called archaic humans who already inhabited those regions. The modern humans “replaced” the archaic humans, which is a nice way of saying they drove them into extinction. This model of migration and “replacement” implies that the relationship between Neanderthals and humans should be the same for all people alive today, regardless of where they come from.
Many members of Pääbo’s team suspected another case of contamination. At various points, the samples had been handled by Europeans; perhaps their DNA had gotten mixed in with the Neanderthals’. Several tests were run to assess this possibility. The results were all negative. “We kept seeing this pattern, and the more data we got, the more statistically overwhelming it became,” Reich told me. Gradually the other team members started to come around. In a paper published in Science in May 2010, they introduced what Pääbo has come to refer to as the “leaky replacement” hypothesis. (The paper was later voted the journal’s outstanding article of the year, and the team received a $25,000 prize.) Before modern humans “replaced” the Neanderthals, they had sex with them. The liaisons produced children, who helped to people Europe, Asia, and the New World.
The leaky-replacement hypothesis—assuming for the moment that it is correct—provides further evidence of the closeness of Neanderthals to modern humans. Not only did the two interbreed; the resulting hybrid offspring were functional enough to be integrated into human society. Some of these hybrids survived to have kids of their own, who, in turn, had kids, and so on to the present day. Even now, at least 30,000 years after the fact, the signal is discernible: all non-Africans, from the New Guineans to the French to the Han Chinese, carry somewhere between 1 and 4 percent Neanderthal DNA.
One of Pääbo’s favorite words in English is “cool.” When he finally came around to the idea that Neanderthals bequeathed some of their genes to modern humans, he told me, “I thought it was very cool. It means that they are not totally extinct—that they live on a little bit in us.”
The Leipzig Zoo lies on the opposite side of the city from the Institute for Evolutionary Anthropology, but the institute has its own lab building on the grounds, as well as specially designed testing rooms inside the ape house, which is known as Pongoland. Since none of our very closest relatives survive (except as little bits in us), researchers have to rely on our next closest kin, chimpanzees and bonobos, and our somewhat more distant cousins, gorillas and orangutans, for live experiments. (The same or at least analogous experiments are usually also performed on small children, to see how they compare.) One morning I went to the zoo, hoping to watch an experiment in progress. That day a BBC crew was also visiting Pongoland, to film a program on animal intelligence, and when I arrived at the ape house I found it strewn with camera cases marked ANIMAL EINSTEINS.
For the benefit of the cameras, a researcher named Héctor Marín Manrique was preparing to reenact a series of experiments he’d performed earlier in a more purely scientific spirit. A female orangutan named Dokana was led into one of the testing rooms. Like most orangutans, she had copper-colored fur and a world-weary expression. In the first experiment, which involved red juice and skinny tubes of plastic, Dokana showed that she could distinguish a functional drinking straw from a nonfunctional one. In the second, which involved more red juice and more plastic, she
showed that she understood the idea of a straw by extracting a rod from a length of piping and using the pipe to drink through. Finally, in a Mensa-level show of pongid ingenuity, Dokana managed to get at a peanut that Manrique had placed at the bottom of a long plastic cylinder. (The cylinder was fixed to the wall, so it couldn’t be knocked over.) She fist-walked over to her drinking water, took some water in her mouth, fist-walked back, and spat into the cylinder. She repeated the process until the peanut floated within reach. Later, I saw this experiment restaged with some five-year-old children, using little plastic containers of candy in place of peanuts. Even though a full watering can had been left conspicuously nearby, only one of the kids—a girl—managed to work her way to the floating option, and this was after a great deal of prompting. (“How would water help me?” one of the boys asked, just before giving up.)
One way to try to answer the question “What makes us human?” is to ask “What makes us different from apes?” or, to be more precise, from nonhuman apes since, of course, humans are apes. As just about every human by now knows—and as the experiments with Dokana once again confirm—nonhuman apes are extremely clever. They’re capable of making inferences, of solving complex puzzles, and of understanding what others are (and are not) likely to know. When researchers from Leipzig performed a battery of tests on chimpanzees, orangutans, and two-and-a-half-year-old children, they found that the chimps, the orangutans, and the kids performed comparably on a wide range of tasks that involved understanding of the physical world. For example, if an experimenter placed a reward inside one of three cups and then moved the cups around, the apes found the goody just as often as the kids—indeed, in the case of chimps, more often. The apes seemed to grasp quantity as well as the kids did—they consistently chose the dish containing more treats, even when the choice involved using what might loosely be called math—and also seemed to have just as good a grasp of causality. (The apes, for instance, understood that a cup that rattled when shaken was more likely to contain food than one that did not.) And they were equally skillful at manipulating simple tools.
Where the kids routinely outscored the apes was in tasks that involved reading social cues. When the children were given a hint about where to find a reward—someone pointing to or looking at the right container—they took it. The apes either didn’t understand that they were being offered help or couldn’t follow the cue. Similarly, when the children were shown how to obtain a reward, by, say, ripping open a box, they had no trouble grasping the point and imitating the behavior. The apes, once again, were flummoxed. Admittedly, the kids had a big advantage in the social realm, since the experimenters belonged to their own species. But in general, apes seem to lack the impulse toward collective problem solving that’s so central to human society.
“Chimps do a lot of incredibly smart things,” Michael Tomasello, who heads up the institute’s department of developmental and comparative psychology, told me. “But the main difference we’ve seen is ‘putting our heads together.’ If you were at the zoo today, you would never have seen two chimps carry something heavy together. They don’t have this kind of collaborative project.”
Pääbo usually works late, and most nights he has dinner at the institute, where the cafeteria stays open until seven P.M. One evening, though, he offered to knock off early and show me around downtown Leipzig. We visited the church where Bach is buried and ended up at Auerbachs Keller, the bar to which Mephistopheles brings Faust in the fifth scene of Goethe’s play. (The bar was supposedly Goethe’s favorite hangout when he was a university student.) Pääbo’s wife, Linda Vigilant, an American primatologist who also works at the institute, joined us. Pääbo and Vigilant first met in the 1980s in Berkeley, but they didn’t get together until both moved to Leipzig, in the late nineties. (Vigilant was then married to another geneticist, who works at the institute too.) Pääbo and Vigilant have a six-year-old son, and Vigilant has two older sons from her previous marriage.
I had been to the zoo, and I asked Pääbo about a hypothetical experiment. If he had the opportunity to subject Neanderthals to the sorts of tests I’d seen in Pongoland, what would he do? Did he think he’d be able to talk to them? He sat back in his chair and folded his arms across his chest.
“One is so tempted to speculate,” he said. “So I try to resist it by refusing questions such as ‘Do I think they would have spoken?’ Because, honestly, I don’t know, and in some sense you can speculate with just as much justification as I can.”
By now, scores of Neanderthal sites have been excavated, from western Spain to central Russia and from Israel to Wales. They give lots of hints about what Neanderthals were like, at least for those inclined to speculate. Neanderthals were extremely tough—this is attested to by the thickness of their bones—and probably capable of beating modern humans to a pulp. They were adept at making stone tools, though they seem to have spent tens of thousands of years making the same tools over and over, with only marginal variation. At least on some occasions, they buried their dead. Also on some occasions, they appear to have killed and eaten each other. Wear on their incisors suggests that they spent a lot of time grasping animal skins with their teeth, which in turn suggests that they processed hides into some sort of leather. Neanderthal skeletons very often show evidence of disease or disfigurement. The original Neanderthal, from Mettmann, for example, seems to have suffered and recovered from two serious injuries, one to his head and the other to his left arm. The Neanderthal whose nearly complete skeleton was found in La Chapelle endured, in addition to arthritis, a broken rib and kneecap. Both individuals survived into their fifties, which indicates that Neanderthals had the capacity for collective action, or, if you prefer, empathy. They must—at least sometimes—have cared for their wounded.
From the archaeological record, it’s inferred that Neanderthals evolved in Europe or western Asia and spread out from there, stopping when they reached water or some other significant obstacle. (During the ice ages, sea levels were a lot lower than they are now, so there was no English Channel to cross.) This is one of the most basic ways modern humans differ from Neanderthals and, in Pääbo’s view, also one of the most intriguing. By about 45,000 years ago, modern humans had already reached Australia, a journey that, even mid–ice age, meant crossing open water. Archaic humans like Homo erectus “spread like many other mammals in the Old World,” Pääbo told me. “They never came to Madagascar, never to Australia. Neither did Neanderthals. It’s only fully modern humans who start this thing of venturing out on the ocean where you don’t see land. Part of that is technology, of course; you have to have ships to do it. But there is also, I like to think or say, some madness there. You know? How many people must have sailed out and vanished on the Pacific before you found Easter Island? I mean, it’s ridiculous. And why do you do that? Is it for the glory? For immortality? For curiosity? And now we go to Mars. We never stop.” If the defining characteristic of modern humans is this sort of Faustian restlessness, then, by Pääbo’s account, there must be some sort of Faustian gene. Several times he told me that he thought it should be possible to identify the basis for this “madness” by comparing Neanderthal and human DNA.
“If we one day will know that some freak mutation made the human insanity and exploration thing possible, it will be amazing to think that it was this little inversion on this chromosome that made all this happen and changed the whole ecosystem of the planet and made us dominate everything,” he said at one point. At another, he said, “We are crazy in some way. What drives it? That I would really like to understand. That would be really, really cool to know.”
According to the most recent estimates, Neanderthals and modern humans share a common ancestor who lived about 400,000 years ago. (It is unclear who that ancestor was, though one possibility is the somewhat shadowy hominid known, after a jawbone found near Heidelberg, as Homo heidelbergensis.) The common ancestor of chimps and humans, by contrast, lived some 5 million to 7 million years ago. This means that Neandert
hals and humans had less than one-tenth the time to accumulate genetic differences.
Mapping these differences is, in principle, pretty straightforward—no harder, say, than comparing rival editions of Hamlet. In practice, it’s quite a bit more complicated. To begin with, there’s really no such thing as the human genome; everyone has his or her own genome, and they vary substantially—between you and the person sitting next to you on the subway, the differences are likely to amount to some 3 million base pairs. Some of these variations correspond to observable physiological differences—the color of your eyes, say, or your likelihood of developing heart disease—and some have no known significance. To a first approximation, a human and a Neanderthal chosen at random would also vary by 3 million base pairs. The trick is ascertaining which of these millions of variations divide us from them. Pääbo estimates that when the Neanderthal Genome Project is completed, the list of base-pair changes that are at once unique to humans and shared by all humans will number around 100,000. Somewhere in this long list will lie the change—or changes—that made us human to begin with. Identifying these key mutations is where the transgenic mice come in.
From an experimental viewpoint, the best way to test whether any particular change is significant would be to produce a human with the Neanderthal version of the sequence. This would involve manipulating a human stem cell, implanting the genetically modified embryo into a surrogate mother, and then watching the resulting child grow up. For obvious reasons, such Island of Dr. Moreau–like research on humans is not permitted, nor is it necessarily even possible. For similar reasons, such experimentation isn’t allowed on chimpanzees. But it is allowed on mice. Dozens of strains of mice have been altered to carry humanized DNA sequences, and new ones are being created all the time, more or less to order.