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Australian site yields early human dates.

Australian site yields early human dates

Scientists have identified Australia's oldest known human settlement, dating to approximately 50,000 years ago. The finding leads them to suggest that the initial peopling of Australia occurred around 60,000 years ago.

Until now, the oldest date for a human occupation site in Australia was nearly 40,000 years. In New Guinea, researchers had similarly dated a site containing stone hand axes to at least 40,000 years ago -- a time when a land bridge connected New Guinea and Australia (SN: 12/13/86, p.374).

The new dates for humans in Australia arise from an archaeological site, initially excavated in the 1970s, that lies at the foot of a sloping cliff leading to the Arnhem Land plateau in the northern part of the continent. On returning to the site in 1988, Richard G. Roberts of the University of Wollongong, Australia, and his colleagues unearthed more than 1,500 artifacts of human design from the lowest occupation levels. Artifacts included stone flakes, red and yellow ochres, a grindstone and pieces of human-altered quartzite or white quartz.

The team applied the thermoluminescence (TL) dating technique to quartz grains taken from nine progressively deeper locations in the sediment. A pair of TL samples that sandwiched the deepest, earliest occupation level dates to between 45,000 and 61,000 years ago, with an average margin of error of 11,000 years. A TL sample from within that occupation level dates to 52,000 years ago, with the same margin of error, the team reports in the May 10 NATURE.

TL dating compares the decay of radioactive elements in buried objects with radioactive decay in the ground that surrounds them. Researchers heated quartz grains from artifacts at the Australian site and measured the radioactive energy emitted in the form of light. They also determined the TL dates of sediments lying within 1 foot of each TL-measured artifact.

Although some scientists have questioned the accuracy of the technique at other sites, Roberts and his co-workers defend their TL dates as reliable. Not only do the nine TL dates become older in deeper sediment layers, but TL dates of up to 40,000 years agree with previous carbon-14 dates for the same site. Moreover, they say, the artifacts show no signs of having been moved or otherwise disturbed by streams in prehistoric times.

The investigators conclude that humans from southeast Asia probably reached northern Australia about 60,000 years ago and arrived at the base of the Arnhem Land plateau a few thousand years later.
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Author:Bower, Bruce
Publication:Science News
Date:May 12, 1990
Words:426
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