Where We Come From
Recent advances in genetics are starting to illuminate the wanderings of early humans
While Wallace and others were finding remarkable stories in mitochondrial DNA, scientists seeking similar tales in the Y chromosome were met with silence. It was particularly frustrating because the Y--passed intact from father to son--seemed like an ideal tool for tracking human origins. But unlike mitochondrial DNA, the male chromosome shows little variation, and searching for markers was excruciating work. Michael Hammer, a geneticist at the University of Arizona who first identified key Y markers, started looking for a cohanim marker in 1995, after he got a call from Karl Skorecki, an Israeli physician. Skorecki was wondering if the very different looking men he saw reading the Torah in shul could possibly all be sons of Aaron, as the Bible said. Intrigued, Hammer started searching the DNA of Skorecki and other Jewish men who according to oral tradition were cohanim, the priest caste. Hammer identified markers that are often shared by men who think they are cohanim, including Andy Carvin and Bill Swersky. By comparing the variations, Hammer determined that the cohanim had a common male ancestor 84 to 130 generations ago--which includes the time of the exodus from Egypt and the original cohen, Aaron.
Brothers and enemies. Since then, other researchers have used the cohanim markers to ascertain that the Lemba, a Bantu-speaking people in Southern Africa who have traditionally claimed Jewish ancestry, do indeed have Semitic roots. And last June, Hammer published results showing that although Palestinian and Jewish men may be political foes, they are also brethren, so closely related as to be genetically indistinguishable.
The Y chromosome is starting to yield other intriguing tales as well. Last November, Peter Underhill, a Stanford University researcher, published a list of 87 new Y markers, which he used to draw a tree that sorts all the world's men into just 10 branches. Indeed, men's lineages have much crisper divisions than women's, perhaps because men move into an area and kill or expel the men already there. "You get this alpha male effect," Underhill says.
Women, by contrast, move because they've married into a new family and village. Generation after generation, daughters marry and move out, while sons stay put, making women's DNA often more well traveled than men's. People living near Medellin, Colombia, have almost exclusively Native American mitochondrial DNA and European--specifically, Spanish--Y chromosome DNA. The story is familiar, and tragic: The Spanish colonists killed or supplanted the native men and married the native women.
For all its dazzle--or perhaps because of it--molecular anthropology is not without critics. "The molecular stuff has been very important," says Milford Wolpoff, an anthropology professor at the University of Michigan and a leading critic of the Out of Africa theory of human origins. "But in the end it has the same problem fossils have--the sample size is very small." Earlier this month, the journal Science published a Wolpoff study of early human skulls, which suggests that Africans may have mixed with earlier hominids rather than supplanting them. The small number of living humans sampled by geneticists, Wolpoff says, and the effects of natural selection over the millennia, make it foolhardy to say with assurance that Out of Africa is right. The geneticists, for their part, readily admit that they need more samples, more markers, and more precise calculations. But they also say that even with today's imperfect science, the DNA is right. And in places like India and China, where the fossil record is scanty, the genetic history will be the only history. "Genetics is moving so fast," says Chris Stringer, a paleoanthropologist at the Natural History Museum in London. "It's well ahead of the fossil and historical record."
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