A genetic study of unprecedented rigour has given a big boost to the theory that all modern humans trace their origins to Africa. The analysis traces our ancestry back to a small group of women who lived in Africa about 170,000 years ago and whose descendants migrated about 50,000 years ago.
The so-called “out of Africa” hypothesis has relied partly on the genetic analysis of a small region of DNA from mitochondria, cellular structures that provide energy and are inherited solely from your mother.
While the theory is widely accepted, even its supporters admit that genetic idiosyncrasies of this tiny DNA region may have skewed results and left room for some doubt.
The new work by Swedish and German researchers looked at all 16,000 DNA letters of the mitochondrial chromosome (mtDNA) in 53 people originating from areas around the globe. The result provides the first statistically robust support for an African migration about 50,000 years ago.
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“The weakness of mitochondrial data has been a nagging doubt for a long time,” says study leader Ulf Gyllensten, a geneticist at the University of Uppsala. He says that researchers had previously only examined 7 per cent of the mtDNA. “No one can contest that we’ve pushed this as far as we can.”
“It’s a big leap from what we had before,” says evolutionary biologist Blair Hedges of Pennsylvannia State University, University Park. “The evidence for out-of-Africa is so overwhelming now.”
Rapid mutation
Previous studies had relied on a region of mtDNA known as the D-loop or control region. Researchers focused efforts there because it was known to mutate rapidly, giving them a greater payoff for a small DNA sequencing project. The trouble was that overlapping mutations often obscured the results.
But Gyllensten and his colleagues realised it was now possible to examine all the mtDNA thanks to advances in DNA sequencing technology accelerated by the human genome project. In fact, a single graduate student completed all the sequencing in a two-month period.
“If I had asked someone to do this 10 years ago, they would have laughed or cried and then walked out of my lab,” says Gyllenstein. His team now plans to examine mtDNA from more individuals to help track the evolution and migration of humans with even greater precision.
More at: Nature (vol 408, p 708)
