RETURN TO THE HOMINID JOURNEY....
When opponents can't resolve a conflict in-house, a third party often has to break the stalemate. Enter the molecular geneticists. Many geneticists feel they are uniquely equipped to settle the matter of modern human origins. After all, they don't have to depend on a spotty assortment of remains to make sense of human evolution. All the information they need is available in the genes of living people. Mark Stoneking, a molecular anthropologist at Pennsylvania State University, contends, "The question of modern human origins is ultimately a genetics question."
In 1987, Stoneking, Allan Wilson and Rebecca Cann, all then at the University of California, Berkeley, tackled this knotty problem by reconstructing an evolutionary history of the modern human line. To accomplish this, they focused on the genetic material, or DNA, found in the parts of cells known as mitochondria. Mitochondrial DNA (mtDNA) appealed to the researchers for several reasons. First, unlike ordinary DNA from the nuclei of our cells, which we inherit from both our parents, mtDNA comes from the mother alone. This means it records a pristine family history from one mother to the next, not one that is shuffled every generation by mixing of genes from mother and father. Second, it is thought that mtDNA quickly and steadily piles up differences that result from random copying errors, or mutations. Thus, mtDNA offers a quick-ticking "molecular clock." By comparing the number of mutations that have collected in separate populations, geneticists can infer when the populations split from each other.
Wilson and his colleagues compared selected sequences of mtDNA from a group of people representing African, Asian, Australian, Caucasian and New Guinean ethnic groups. Among these people they found 133 variants of mtDNA. Next, they arranged these different mitochondrial types into an evolutionary tree. That tree showed a trunk splitting into two major branches. One branch consisted only of Africans, the other included some modern Africans and some people from everywhere else. According to the researchers, a population of Africans, the first modern humans, forms the trunk and longest branch of the tree. The second branch represents a subgroup that left Africa and later spread out to the rest of the world.
The researchers also found that all of the mtDNA, even the samples from far-flung regions of the world, was strikingly similar. This suggested that the molecular clock has not been ticking long enough to accumulate appreciable differences in our DNA. In other words, our species is young. But the African samples had the most mutations. This too implied that the African lineage is the oldest, that all modern humans trace their roots back to Africa.
But a critical question remained: When did modern humans arise? After all, if the Out of Africa camp is correct, our species emerged only within the last 200,000 years. If the multiregionalists are right, however, our species is essentially an updated version of Homo erectus, in existence for a million years or more.
To find out which date is correct, the researchers searched for a sequence of mtDNA that all the subjects shared. This snippet must have come from a common ancestor of all modern humans and must have given rise through mutation to the 133 variants in people today. Using the known rate of mutation among other primates (and assuming it pertains to us), the researchers calculated how much time that ancestral mtDNA would have taken to mutate into the 133 variants. From this, they traced the mtDNA back to a single, common female ancestor who lived in Africa between 140,000 and 290,000 years ago. The press dubbed her "Eve." Newsweek put her on the cover.
The fact that "Eve" existed was no surprise in itself. All snippets of DNA will eventually trace back to a single common ancestor. And despite her biblical name, Eve was not the the sole mother alive in her generation. Since mitochondria are inherited through the mother, mitochondrial genes are snuffed out whenever one generation in a family line fails to have a daughter. So Eve was simply the only one of a group of the earliest modern humans whose lineage managed to survive to the present.
The molecular clock constructed by the Berkeley team also showed that the splitting of the human evolutionary trunk into two branches occurred 90,000 to 180,000 years ago. This, the researchers argued, was when Eve's descendents left Africa.
To the supporters of the Out of Africa theory, the genetic evidence added up to a grand story: A new species called Homo sapiens split off from Homo erectus in Africa somewhere between 140,000 and 290,000 years ago. Eve was born among this pioneering group of modern humans. At some point, one of her mitochondrial genes mutated. She then passed it on through her lineage to other modern humans. Thousands of years later, people carried the gene out of Africa. Eventually, their descendants replaced all other humans on the planet, including the Neanderthals.
Not surprisingly, Eve quickly became the poster girl for the Out of Africa view. Since that first study, the Out of Africa camp has gained still more scientific support. A year ago, a Japanese team published the results of its own search for Eve, this time after analyzing all of the mitochondrial DNA from three people, an African, a European and a Japanese. The researchers again found scant differences, but the African mtDNA had mutated the most. Using more precise techniques than the Berkeley team had, they estimated that Eve lived 143,000 years ago, a date consistent with the Berkeley findings.
Two other studies using DNA from the nuclei of cells have also given Out of Africa a recent boost. The first, led by Robert Dorit of Yale University, ferreted out Eve's male counterpart, the common male ancestor of modern humans. They studied a portion of the Y chromosome that is transmitted only by the male line. "Adam," by their estimate, lived some 270,000 years ago, a date relatively close to that for Eve. The second study, led by David Goldstein of Pennsylvania State University, examined other portions of nuclear DNA to calculate where and when modern human populations first diverged. "Where" turned out to be Africa, and "when," 156,000 years ago. One more point for Out of Africa. Strike three for the multiregional view.