A THOROUGH analysis of the burial site of Australia鈥檚 oldest human remains could settle a long and acrimonious debate over when humans first colonised the continent.
Experts have been arguing for decades about the age of the skeleton, dubbed Mungo Man. Now its discoverer says he has confirmed that it is 40,000 years old. That date will reassure many palaeontologists, because if it were significantly older, Mungo Man would have challenged the dearly held 鈥淥ut of Africa鈥� hypothesis. This states that Homo sapiens spread quickly around the world from its Rift Valley heartland. The new date also supports the idea that early humans drove Australia鈥檚 large mammals to extinction.
鈥淎ustralia鈥檚 colonisation is one of the keys to our understanding of how Homo sapiens evolved and spread around the world. It is critical we get the story correct,鈥� says Jim Bowler at the University of Melbourne, who discovered the remains at Lake Mungo in south-eastern Australia in 1974 (see Map).
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In 1976, Bowler and his colleague Alan Thorne estimated the remains to be 30,000 years old. But in 1999, Thorne and his team published a sensational new paper claiming Mungo Man was 62,000 years old. That earlier date, along with mitochondrial DNA evidence from remains gathered in 2001 by another team led by Thorne, at Australian National University in Canberra, fuelled speculation that modern humans evolved in Australia as well as Africa.
鈥淭hat date was out of kilter with a lot of other dating and archaeological evidence,鈥� says Bert Roberts of the University of Wollongong in New South Wales. Thorne鈥檚 team estimated Mungo Man鈥檚 age by analysing how much uranium had decayed in a sample from the skeleton. But some consider this an unreliable technique, as porous bone can take up uranium-rich minerals at uneven rates. However, Thorne also confirmed that the sand strata in which the skeleton was buried was of a similar age, using a more trustworthy technique called thermoluminescence, which measures how long it has been since quartz crystals were exposed to sunlight.
Now researchers at four separate laboratories, including Bowler, Roberts and a member of Thorne鈥檚 original team, have claimed that the burials of Mungo Man, and other remains known as Mungo Woman, in fact took place 40,000 years ago. They have also dated stone tools 鈥� the earliest evidence of human occupation at the site 鈥� to 50,000 years ago (Nature, vol 421, p 837).
The researchers determined the new dates using optically stimulated luminescence, an improved version of thermoluminescence. And crucially, they sampled sand from the actual burial sites, whereas it has emerged that Thorne鈥檚 team had sampled sand 400 metres away. The revised dates are consistent with archaeological evidence that humans occupied sites in northern and western Australia around 50,000 years ago, the team says.
鈥淭hey could have an impact on another contentious area of archaeology 鈥� the disappearance of the Australian megafauna,鈥� says Don Colgan, head of evolutionary biology at the Australian Museum in Sydney. Evidence that people spread rapidly across Australia around 50,000 years ago would support the 鈥渂litzkrieg鈥� theory of the extinction of the continent鈥檚 large mammals soon after humans arrived. Other researchers have suggested that climate change could be to blame.
The latest dates for Mungo Man are also compatible with the Out of Africa theory, which says that Homo sapiens evolved in Africa 100,000 years ago, and then spread around the world displacing its hominid relatives. If Mungo Man were around 60,000 years old, as Thorne suggested, then these early humans must have spread at a remarkable rate, quicker than some palaeontologists believe was possible.
But if they arrived just 10,000 years later than this, then the archaeological evidence of their long migration begins to make more sense.