Dr. Clarkson said that the finding provides further insight into the complex capabilities of ancient humans as well as the chronology of when they migrated from Africa and spread across the world.
He also added that the findings provide evidence against a prevailing theory that people rapidly drove Australia’s largest animals to extinction shortly after arriving on the continent.
“It puts to bed the whole idea that humans wiped them out,” said Dr. Clarkson. “We’re talking 20,000 to 25,000 years of coexistence.”
To determine the age of the artifacts, the team had to date the sediment layer where they were buried. They first performed radiocarbon dating on sediment starting at the surface until they got to layers that were about 37,000 years old. They then shifted to a technique called optically stimulated luminescence dating for the deepest layers, which was used to measure the last time the sand in the rock shelter was exposed to sunlight.
Think of a grain of sand as an empty battery that slowly collects charge once it’s buried. As long as it remains in the dark it will continue gaining energy over time. If researchers can recover the grain of sand, and keep it dark, they can then use a laser to release the ‘charge’ within it. By measuring the amount of energy the grain of sand releases, and comparing that with the amount of radiation that the sand was exposed to while it was buried, researchers can determine when it was last in sunlight.
When it was dark, Zenobia Jacobs, a geochronologist from the University of Wollongong, and her colleagues used long tubes to bore into the sand layers where they found artifacts and collected 56 samples. Back at the lab she painstakingly measured more than 28,500 individual grains of sand and used a laser to determine their ages. After getting her results, the team sent several samples to independent labs to double check its work. The results came back verified.
“It was a great relief, I can tell you that,” said Dr. Jacobs.
She also helped confirm that the sand had not been significantly disturbed for tens of thousands of years. That meant that it could provide an accurate assessment for the age of the artifacts.
Jean-Luc Schwenninger, head of the Luminescence Dating Laboratory at the University of Oxford who was not involved in the study, said in an email that the team’s use of luminescence dating techniques provides a convincing case that humans came to Australia 65,000 years ago.
“However, the results of this thorough study also seem to suggest that this might be a rather conservative age estimate,” said Dr. Schwenninger, “and I would not at all be surprised if this date was pushed back even further in the future.”