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David Le Page
12 Jan 2001 00:00
Were the first Australians related to the rest of humanity, or a breed apart? This is the question raised this week by Dr Alan Thorne, of the Australian National University, and lead author of a paper describing the analysis of mtDNA—mitochondrial DNA extracted from the bones of Mungo Man, a 60 000-year-old Australian fossil.
According to Colin Menter of the Wits University department of palaeoanthropology, there are two competing theories of the origin of man.
The first—the “Out of Africa theory”—is that successive waves of Homo sapiens modern humans—evolved—in Africa around 200 000 years ago, and migrated out of Africa in successive waves during the Pleistocene ice ages, when sea levels were lower. Homo sapiens‘s predecessor—Homo erectus—and neanderthal man in Europe and Asia.
In favour of this theory, according to Menter, is the fact of considerably more genetic variation among Africans than among European or Asian populations, suggesting a common African origin. Thorne points to the fact that the degree of mutation in the mtDNA of Mungo Man—named after the lake where his bones were discovered suggests that his most recent common ancestor with African Homo sapiens lived far longer than 200 000 years ago, perhaps a million years or more.
MtDNA is present in greater quantities in living remains than nuclear DNA, and is easier to extract and analyse. The multi-regional theory (or regional variation theory), of which Thorne is a proponent, suggests that Homo sapiens evolved simultaneously from Homo erectus in Africa, Europe and Asia. Mungo Man therefore might be related to African Homo sapiens through his Homo erectus great-great-great grandfather, but not through his Homo sapiensgrandfather.
Most scientists hold that we all share that same Homo sapiens “grandfather”. Thorne argues that theories placing modern man in Australia as a results of more recent migrations between 100 000 and 200 000 years are incorrect.
Dr Lee Berger of the Wits palaeoanthropological unit for research and exploration is sceptical of Thorne’s conclusions. “It’s a wonderful discovery,” he says, “but to make the next [theoretical] leap may be squeezing too much blood out of that DNA.” He believes there are a number of possible explanations for the find, and that all need to be explored before proper conclusions can be drawn.
“The positive evidence for ‘Out of Africa’ is so strong ... I’m not convinced that negative evidence such as not sharing genes with living humans is sufficient to throw it out of the window.” Strangely, the skeleton of Mungo Man supposedly has an “aboriginal” morphology yet shares no DNA with modern aborigines. Other scientists believe it would be worth reassessing assumptions about ancient human genetic variation before Thorne’s discovery will justify ditching “Out of Africa”.
Already, Thorne’s claim of an age of 60 000 years for Mungo Man has been disputed by Australian geologist Jim Bowles, who discovered the skeleton in 1974 and reckons it’s 20 000 years younger. This means, he says, that Thorne’s team is incorrect in saying they have analysed the oldest DNA ever extracted from human remains.
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