relics : Humans did come out of Africa, says DNA

JEREMY THOMSON

Brushed aside: Homo sapiens neanderthalensis

Archaeologists are still not sure when and where modern humans first appeared. Some believe that Homo sapiens evolved independently in several places around the globe. But research revealed in this week’s Nature¹ lends support to the idea that we appeared in one location in sub-Saharan Africa and spread from there, replacing Neanderthals and other early humans as we went.

Researchers led by Ulf Gyllensten of the University of Uppsala in Sweden have found evidence that we are all descended from a single ancestral group that lived in Africa about 170,000 years ago1. And they suggest that modern humans spread across the globe from Africa in an exodus that took place only around 50,000 years ago.

Gyllensten’s team didn’t scrutinize fossils to come up with these results — instead the group examined DNA from living people around the world.

The genetic material in our chromosomes is a combination of genes from our parents. But each cell also contains structures called mitochondria, and these house DNA that is independent of that found in chromosomes.

Mitochondrial DNA (mtDNA) gives researchers a window into history because it is only transmitted along the female line. There is no mixing between generations, and the DNA sequence only changes as a result of random mutations or copying errors. If these mutations appear at a fairly constant rate, then comparing the mitochondrial DNA of two populations reveals roughly when they had a common ancestor.

This approach is not new, but in the past researchers have concentrated on only 7% of the total mitochondrial DNA sequence, known as the control regions. The mutation rate for these regions might be different from the rest of the sequence, so Gyllensten and co-workers sequenced the entire mitochondrial DNA of 53 people from diverse ethnic backgrounds. They excluded any parts of the DNA that mutated unusually quickly, then compared the data to produce a sort of human family tree.

They discovered that the most recent common ancestor of everyone in the sample group lived in Africa 171,500 ± 50,000 years ago. They also found a significant branch in the tree that separates most Africans from non-Africans. This genetic divide probably represents an exodus of people from Africa that took place 52,000 ± 27,500 years ago.

This evidence favouring the ‘recent African origin’ theory of human evolution is compelling, but the researchers hope for better data in the near future. "Mitochondrial DNA is only one locus, and only reflects the genetic history of females," they write. "With the human genome project nearing fruition, the ease by which such data may be generated will increase, providing us with an evermore detailed understanding of our genetic history."

1. Ingman, M., Kaessmann, H., Pääba, S. & Gyllensten, U. Mitochondrial genome variation and the origin of modern humans. Nature 408, 708 –713 (2000).
2. Blair Hedges, S. A start for population genomics. Nature 408, 552–553 (2000).