The man from down under
Ancient DNA strikes at "out-of-Africa" theory
(13/01/2001)
By Leigh Dayton
A MAN who died about 60,000 years ago in Australia may be about to turn our theory of human origins on its head. Researchers in Australia have accomplished the extremely difficult feat of extracting DNA from his skeleton, and were astonished to find that it looks like nothing they have ever seen before.
The DNA, which is the oldest ever recovered from human remains, shows that while the man is completely anatomically modern, he came from a genetic lineage that is now extinct. This finding challenges the prevailing theory that all modern humans are descended from a group of people who migrated from Africa around 100,000 years ago. "It's remarkabletotally unpredicted," says anthropologist Alan Mann of the University of Pennsylvania. "What it says is that the more we know [about human origins], the more confusing the picture becomes."
Mungo Man's remains were found on the shores of Lake Mungo in south-eastern Australia in 1974. They were originally radiocarbon-dated to about 30,000 years old, but in 1999 a reassessment using three different techniques showed the bones to date from around 60,000 years ago.
In 1995, a team led by anthropologist Alan Thorne of the Australian National University in Canberra began an attempt to extract genetic material from the remains. Doctoral student Gregory Adcock and his colleagues at CSIRO Plant Industry managed to replicate and sequence a single gene from Mungo Man's mitochondria, the powerhouses of cells whose small genome is passed down the female line.
Simon Easteal, an evolutionary geneticist at ANU, then set about analysing the sequence and comparing it with sequences of the same gene from nine other early Australiansranging in age from 8000 to 15,000 yearsas well as 3453 contemporary people from around the world, chimpanzees, bonobos (pygmy chimps) and two European Neanderthals.
Easteal looked for patterns of descent and worked out which "genetic tree" fitted the data best. According to this evolutionary tree, chimps and bonobos were first to branch off the trunk leading to modern people. Neanderthals split off next, then Mungo Man's line and finally the line that led to the most recent common ancestor of contemporary people, including the ancient Australians but excluding Mungo Man. "We can say with a high degree of confidence that modern people arrived in Australia before the new lineage [of the most recent common ancestor] arrived," Easteal says.
According to Thorne, the findingsdue to be published next week in the online edition of the Proceedings of the National Academy of Sciences threaten to topple the leading theory of human origins, the "out-of-Africa" model. This proposes that all living people are descended from a group of modern Homo sapiens who left Africa roughly 100,000 to 150,000 years ago. Their descendants spread around the world, replacing existing populations of "archaic" people, such as Neanderthals and the more ancient Homo erectus.
But if anatomically modern humansfrom a lineage that emerged before the most recent common ancestor of people todaywere living in Australia 60,000 years ago, "a simplistic out-of-Africa model is no longer tenable", says Thorne.
Thorne is one of the founders of the rival "regional continuity" model, which postulates that H. erectus began migrating from Africa over 1.5 million years ago, and from these migrants H. sapiens evolved at the same time in various regions around the world. Those early people remained on the same evolutionary path by sharing their genes through interbreeding.
In Thorne's scenario, Mungo Man's ancestors probably evolved in Asia. They gradually migrated to Australia, where the lineage vanished. Because the lineage is based on a single mitochondrial gene, it is too early to know exactly what happened. The clan could have been wiped out by newcomers, or the gene may, for some reason, not have been passed from mother to daughter.
That is why out-of-Africa proponents, including ANU physical anthropologist Colin Groves, argue that the new data does not knock their model from the top of the theoretical pile. The genetic evidence is equivocal, he says. "The African-origin model stands or falls by the fossil evidence. In my opinion, it stands."
But Groves praises Adcock's technical achievement. Retrieving such old DNA is a "real coup", he says.
From New Scientist Magazine 13 January 01
http://www.newscientist.com/hottopics/dna/article.jsp?id=22730500&sub=Ancestry