ANH close to dating Aboriginals' arrival
By Damon Shorter
The controversy surrounding the date of Aboriginal arrival in Australia may soon be resolved with the help of dating techniques being refined by ANU researchers.
At the Horizons of Science forum on the origins of life in Sydney last month, Professor Rhys Jones, from the ANU's Division of Archaeology and National History (ANH), predicted advances in dating technology will soon help resolve the question of Aboriginal antiquity.
The breakthrough has been acheived by improvements in a dating technique called optically stimulated luminescence (OSL) which can accurately date deposits up to 250,000 years old. A research team led by Dr Nigel Spooner, also from ANH, has been refining the process for several years and can date sites from single grains of sand.
Prof Jones told the forum that limitations of conventional radiocarbon dating have made it difficult to know exactly when humans first arrived in Australia. Although the technique can place the earliest artefacts as "older than 35,000 years", determining precisely how much older had depended on guesswork.
Now, Prof Jones and his colleagues, who include Dr Tim Flannery from the Australian Museum and Dr Bert Roberts from La Trobe University in Melbourne, are revisiting some of Australia's most important sites in Western Australia and Arnhemland to more accurately determine their age.
"We have dated two sites in Arnhemland that show human occupation between 50,000 and 60,000 years," Prof Jones said. At another site, the group was able to narrow the date where human artefacts first appeared in a perfectly preserved sand column to around 60,000 years.
"We have put forward twin hypotheses," Professor Jones told the forum. "People were here 50,000 to 60,000 years ago and they were not here before 60,000 years."
Apart from pinning-down when people first walked the continent, Prof Jones says their new measurements may help solve one of the other great mysteries of Australia's past - the mass extinction of Australia's so-called megafauna. Many centuries ago, huge marsupials and reptiles roamed the continent. Predatory flesh-eating marsupials, grazing wombat-like animals the size of cows, flightless birds as big as giraffes and massive goannas and snakes up to seven metres long coexisted, filling the same ecological niches that the big game in modern Africa do today. But at some point more than 35,000 years ago, the megafauna vanished.
The cause of this cataclysmic prehistoric event, which also wiped-out a third of Australia's marsupials, has intrigued archaeologists since the first megafauna fossils were discovered 150 years ago. Prehistorians are divided on the issue. Some argue a natural shift in climate probably drove the megafauna to their death. The alternative, controversial suggestion, presented by Dr Flannery in The Future Eaters, is that human arrival, and especially the arrival of fire, killed-off the Australian megafauna.
"To resolve the why we need to know when," Prof Jones said. If his group can show the megafauna died out around the same time that Aboriginal people arrived, the coin-
cidence of the two events would suggest a human-driven extinction. Alternatively, if the extinction occurred before human habitation, humans can be absolved of responsibility, he said.
"I am convinced we now have the appropriate methods of dating to tell us when these animals became extinct," Prof Jones said.
The OSL dating process measures electrons trapped in imperfections of minerals such as quartz. Over time, the numbers of electrons trapped in the crystals accumulate through exposure to natural radiation.
Direct sunlight "resets the clock" by releasing the trapped electrons. By measuring the proportion of electrons trapped in a sand grain the scientists can determine how long the grain has been buried since it was last exposed to light. The technique was used earlier in the year to date rock art from a site in the Kimberley region of Western Australia at over 17,000 years old.
Ironically, the ANH Division, of which Prof Jones and the dating facility are both a part, is threatened with closure at the end of the year due to cuts within the Research School of Pacific and Asian Studies.
"I was recently invited by the British Academy to present an overview of the revolution in our understanding of Australian and Pacific prehistory, the bulk of which has been generated by past or present members of this Division," Prof Jones said. "Yet I still don't know whether this work will continue at the ANU." Internal negotiations aimed at retaining the Division at the ANU are continuing.