News

Climate change, new dates may solve pre-history riddle

Media Release, Friday 9 January 2004

New dating techniques have nearly doubled the age of possibly Australia’s most significant indigenous remains, shedding light on one of the biggest riddles of Australian pre-history.

Two University of Melbourne geochronologists used a relatively new dating technique that measures the trapped energy in mineral grains to boost the age of indigenous remains from Kow Swamp in northern Victoria, from 9,000-15,000 years to 19,000-22,000 years The Kow Swamp remains are the world’s largest single population of human remains from the late Pleistocene era (120,000 -10,000 years).

The riddle surrounds the robust physical characteristics of the Kow Swamp people that some experts suggest links them to earlier more ‘archaic’ humans such as Homo erectus that have been found in Indonesia. But the question has always been how could people with such archaic traits exist only 9-15,000 years ago when more modern-looking and gracile people had been at Lake Mungo in south-west NSW 40,000 years ago?

The geochronologists from the University’s School of Earth Science, PhD candidate Tim Stone and Dr Matthew Cupper, say the new dates place the Kow Swamp people in the region at the time of Last Glacial Maximum (LGM), a time that saw indigenous Australians battling an increasingly arid environment and colder climate. This included advancing glaciers in the Snowy Mountains and drying, saline lakes in the interior.

At Kow Swamp, the shellfish population became extinct about 19,000 years ago. They suggest the lake was abandoned soon after.

“Such conditions would have forced indigenous populations to survive in small groups. The robust skeletal form of the Kow Swamp people probably arose from the increasing physical and hence genetic isolation, accentuated by the severity of the LGM,” says Mr Stone.

“It may even be that robust traits were better adapted to the extreme cold and aridity,” he says.

“The Mungo and Kow Swamp people were most likely part of a single evolving population that had to adapt to a period of climatic stress lasting more than 20,000 years.

“When environmental conditions improved, populations could travel further and begin interacting with other groups. This would have increased gene flow between groups, diluting and finally eliminating the robust form. In Australia, this morphology is rarely encountered after the glacial maximum.”

Their findings appear in Journal of Human Evolution (August 2003).

Stone and Cupper used the one of the latest dating techniques called Optically Stimulated Luminescence (OSL) to date sediments related to where the remains had been found.

Stone and Cupper suggest the differences between the previous carbon dates and the OSL dates are the result of contamination of some of the carbon samples by younger carbon.

Optically Stimulated Luminescence
OSL is a technique that can be used to date the burial age of common minerals such as quartz or feldspar that are found in sedimentary deposits. The mineral grains act as tiny clocks and they do this by accumulating energy from ionizing radiation that occurs naturally in the surrounding sediment.

The grains trap this energy only when they are buried in the sediment, away from sunlight. Exposure to sunlight resets their clocks to zero.

By measuring this energy as light signals emitted by the grains, along with the background radiation of the sediments, scientists can calculate the age of burial."