Ageing fast

 作者:周圯钋     |      日期:2019-03-08 04:06:08
By Leigh Dayton in Sydney ABORIGINAL people have lived in Australia for more than 60 000 years, claim researchers in Canberra. They have used three dating techniques on bones found at Lake Mungo in New South Wales to provide the most convincing evidence so far for an early date for human colonisation. This date has been the subject of much debate among archaeologists. For years, most researchers thought that the first people arrived in Australia 40 000 years ago. But in 1990, Bert Roberts of La Trobe University in Melbourne dated some red ochre-a manufactured pigment-from Malakunanja in Arnhem Land to between 50 000 and 60 000 years ago. And in 1996, David Price of the University of Wollongong in New South Wales claimed that rock art at Jinmium in the Northern Territory was 176 000 years old. The second claim has since been disputed by scientists who say that bits of ancient bedrock in the samples would have produced an erroneously high age (see “Australia’s date with destiny”, New Scientist, 7 December 1996, p 28). Archaeologists also really want dates based on human remains, rather than artefacts. That’s what researchers at the Australian National University (ANU) have now provided. They say that “Mungo Man”, a skeleton excavated from the Willandra Lakes area of New South Wales in 1974, is between 56 000 and 68 000 years old. Previous radiocarbon dating had put the skeleton’s age at around 30 000 years. However, radiocarbon dating becomes increasingly unreliable for samples this age or older. So the ANU researchers dated the skeleton and the sediment under it using three different techniques. The researchers report their findings in the Journal of Human Evolution (vol 36, p 591). One team, led by Rainer Grün, dated bone shavings and the skull using techniques based on the radioactive decay of uranium isotopes. They also used electron spin resonance analysis to date Mungo Man’s teeth. This relies on the fact that electrons in buried teeth become excited over time to higher energies by radioactive elements in the surrounding sediments. Grün estimates the skeleton’s age at 62 000 years, plus or minus 6000 years. Another ANU researcher, Nigel Spooner, used a technique called optically stimulated luminescence, which also measures the cumulative exposure to radiation of a buried sample. In the same paper, he reports a age of 61 000 years with an error of 2000 years either way. Most prehistorians believe that Australia’s earliest inhabitants arrived by boat from Indonesia or China. Alan Thorne of the ANU, who excavated Mungo Man, believes it would have taken these seafaring people several thousand years to adapt to conditions in the Australian interior. “The absolute beginning of colonisation was well in excess of 60 000 years ago,