That’s why the Y chromosome has become the chromosome of choice for anthropological genetics. Unlike all the others, it has no matching partner, and only at its tips does it swap bits with the X chromosome. Remember: Men inherit a Y chromosome from their father and an X chromosome from their mother; women inherit an X from each parent. As a result the Y passes on largely intact from father to son, ad infinitum, each man adding at most a new mutation or two. The Y chromosome in every man on Earth today is thought to be more than 99.99 percent the same as the one carried by a common ancestor who lived 50,000 or 60,000 years ago. The tiny differences are the markers that record the spread of the human species around the planet—and which Underhill and Oefner’s invention made much easier to identify.
HISTORY OF Y’S GUYS
The X and Y chromosomes carry the genes that determine sex. Men have one X, inherited from their mothers, and one Y, inherited from their fathers. Only 5 percent of the Y chromosome’s DNA mingles with the X chromosome. The Y thus provides an unadulterated record of inheritance from father to son over generations. By analyzing Y chromosome samples from around the world, geneticists infer how and when humans originated in Africa and how they colonized the globe.
In the 1990s the Stanford group and Michael Hammer of the University of Arizona showed that “Adam” lived in Africa: The Y chromosome tree has its trunk and roots there. Earlier work with mitochondrial DNA—a nonchromosomal kind that escapes recombination, passing intact from mother to daughter—had shown that “Eve” lived in Africa too. Beginning around 50,000 years ago, the genetic evidence suggests, modern humans first migrated out of Africa. In his book, The Journey of Man, Wells sketches what is known of the subsequent story, but a lot of it is pretty murky.
As early as 1991, Cavalli-Sforza proposed the Human Genome Diversity Project: an effort to collect DNA samples from hundreds of populations worldwide. To Cavalli-Sforza and other geneticists who joined him, the proposal was altruistic in creating a record for all humanity of its history at a time when many of the world’s smaller populations were facing absorption into a globalized culture. Some groups reacted with outrage at the suggestion that they donate their blood to Western science—it smacked of exploitation. Cavalli-Sforza’s idea became hugely controversial, and the U.S. government never funded it. The research hasn’t stopped, however. It has simply trickled on in a less organized way, driven in part by entrepreneurial scientists like Wells.
Cavalli-Sforza encouraged his young colleagues to pick an area of the world in which to do fieldwork. Wells picked Central Asia—“a black box—we knew nothing about it.” Central Asia, to Wells, means the region from the Black Sea in the west to Lake Baikal farther east. It includes all the former Soviet “stans,” from Turkmenistan to Tajikistan and on into Mongolia. It is a region of endless steppes cut by soaring mountains. It is, even today, an intimidating expanse of bad roads and many languages.
Wells’s first expedition was to Uzbekistan, where in 1996 he and Ruslan Ruzibakiev, an immunologist at the Academy of Sciences in Tashkent, sampled DNA from 550 Uzbeks. There are more than 100 different ethnic groups in Uzbekistan. The chief result, Wells recalls, was that they needed to survey a much wider region if they wanted to understand the diversity of Y chromosomes.
That wider survey took place in 1998, and though it covered a lot of ground, Big Science it wasn’t. It was five men crammed into a Land Rover, along with many boxes of syringes, tourniquets, and chemicals for extracting DNA from blood. A small research grant from the Alfred P. Sloan Foundation paid for the equipment, but the Land Rover itself was donated by the vehicle’s manufacturer. “We chipped in a little bit of our own money for living expenses,” says Wells. “We also had friends who were very interested in this, who would give us a few hundred dollars here and there, which we collected in a big pot.”
One morning in April they drove through the Channel Tunnel to France. They didn’t stop to collect samples until they hit Georgia, because Europe’s DNA is old hat. After that they didn’t stop collecting until they had been to Kyrgyzstan and back, a total of 25,000 miles. They slept in borrowed rooms or offices, and even in yurts; they bonded with their local facilitators over streams of vodka. They had small adventures. A potentate in Uzbekistan insisted on driving the Land Rover; he gunned it and, top heavy with gear, it promptly rolled over. The man then hailed a passing car and left Wells and his companions nursing their bruises. Later, in Kyrgyzstan, a policeman tried to shake them down on the pretense that the Land Rover’s color, red, was illegal. Wells stood firm.
One problem they did not have, he says, was getting blood donors. Local research contacts did a lot of the footwork, and the inmates of urban hospitals, both patients and staff, proved a rich source of blood. But Wells and his crew also visited factories and villages, sometimes going door to door. On occasion they found themselves staying for a dinner of, say, sheep intestines and koumiss, which is fermented mare’s milk. “It’s one of the worst things I’ve ever tasted,” says Tatiana Zerjal, a graduate student who joined the expedition for a month in Uzbekistan and Kyrgyzstan.
To each donor or group of donors, Wells gave what he calls his blood speech, explaining DNA, the purpose of the expedition, their role in it, and then asking for “informed consent.” On the television version of The Journey of Man, for which Wells traveled the world in 2002, retracing some of his earlier steps, he visits a man in southeastern Kazakhstan whose blood had been sampled on the 1998 expedition and who has turned out to have an important Y chromosome marker called M45. At a festive dinner, Wells gives him the blood speech again and concludes with a toast: “To your very important blood, which has brought us together.” The man seems happy and relieved: As Wells candidly explains, he thought Wells had come back to tell him he had cancer.
Scenes like that demonstrate that truly informed consent can be an elusive goal in anthropological genetics, and yet it seems clear that Wells has done no harm to the man and has done our knowledge of the past a lot of good. M45 is an important branching point on the human family tree. One branch leads to M173, which is a mutation shared by most people of Western European descent. The other branch leads to M3, which is shared by most Native Americans. European and Native American men also have M45, but in Central Asia there are men, like Wells’s Kazakh dinner companion, who have M45 but neither of the two later mutations—they have a large range of different ones instead. That indicates Central Asia is where M45 originated and where both Europeans and Native Americans originated, from a single source.
By counting the number of mutations that have happened since M45, Wells and his colleagues estimate that M45 is about 35,000 to 40,000 years old. The European marker, M173, happened roughly 30,000 years ago, which is when the first cave paintings appeared in France. M3 is present only in Native Americans, and so it must have happened after humans first crossed the Bering Strait and arrived in the Americas. Archaeologists have long debated the timing of that momentous event; most favor a date of around 13,000 or 14,000 years ago, but a few have held out for one as early as 30,000 years ago.