DNA backs African origin of man
Analysis supports archaeological and fossil evidence
NEW YORK - An analysis of human DNA from around the world has provided the strongest evidence yet that modern humans originated in Africa, according to a study published Thursday in the journal Nature.
Although scientists familiar with the latest salvo say the research likely won't end the debate about when and where we arose as a species, "it's somewhat of a turning point," said Blair Hedges, an evolutionary biologist at Pennsylvania State University. "I think this study really puts the ball in the court of the 'out of Africa' theory and it's going to be really, really difficult for the proponents of the other theory to come up with convincing evidence to support their model."
The "out of Africa" theory, also called the "recent African origin" hypothesis, proposes that our ancestors arose on the African continent sometime between 130,000 and 465,000 years ago and subsequently migrated to other continents within the past 100,000 years. An alternative hypothesis, sometimes called the multiregional model, proposes that modern humans arose simultaneously in Africa, Europe and Asia from evolutionary predecessors such as Neanderthals or Homo erectus, who had left Africa much earlier.
Previous analyses have provided circumstantial evidence for the "out of Africa" theory. But since they included only small mitochondrial or chromosomal DNA segments, they have often been criticized as statistically weak by the model's gainsayers, said Ulf Gyllensten, a professor of medical molecular genetics at the University of Uppsala in Sweden and a coauthor of the new study.
"This is the first study that has been performed on a complete genome to address questions of our origins," he said. "It shows the future in that sense."
To arrive at the story of our past, Gyllensten's team of Swedish and German scientists harnessed the unique genetic power of mitochondria, the maternally inherited energy factories that reside within our cells and supplement the DNA of our much larger chromosome-based genomes.
The researchers sequenced the mitochondrial genomes 16,500 letters of DNA in all from 53 volunteers representing 14 of the world's major language groups. By aligning the DNA sequences, the researchers were able to identify subtle differences in the genetic code. Since genetic sequences are thought to diverge at a constant rate through mutations, the differences in the 53 sequenced genomes allowed the researchers to construct a family tree depicting how and when those sequences diverged from a common ancestor.
What the tree reveals, Gyllensten said, is that "the deepest branches all go to Africa."
Of the four main branches, the oldest three lead exclusively to individuals of African descent. The fourth branch includes both Africans and non-Africans, likely representing both the contemporary survivors of early humans who migrated to Europe and Asia and their counterparts who stayed behind in Africa. Gyllensten and his collaborators have dated this exodus to about 52,000 years ago, a number that Hedges says is "in the ballpark" of dates determined by archaeological, fossil and other genetic evidence.
But Henry Harpending, an anthropologist at the University of Utah, said the research, while admirable, provides only a narrow view of our past.
Harpending said family trees constructed from different genetic markers often tell different stories. The challenge now facing scientists, he said, is to gather data from separate genetic, fossil and other archaeological sources to gradually build a complete history of the human race.
Researchers have used DNA comparisons to advance the theory that humans first evolved in sub-Saharan Africa, then migrated to other parts of the world, rather than evolving simultaneously in different regions. The DNA of Africans is more genetically diverse and thus older than other groups of people.