For the last few decades, researchers have debated about when humans reached the continent of Australia. Dating of archaelogical sites has given a wide range, stretching from around 47,000 years ago to as far back as 80,000 years ago. Now, rigorous dating of tools and geologic layers found in a north Australian rock shelter could help solve the mystery, suggesting that humans were living on the continent around 65,000 years ago, reports Ann Gibbons at Science.
As Gibbons reports, the Madjedbebe rock shelter, located near Kakadu National park, has been at the center of the controversy since 1989, when researchers dated sediment layers in the shelter that contained stone tools and artifacts. Their result? The tools were roughly 50,000 to 60,000 years old.
But critics argued that the artifacts could have slipped downward in the sandy soil over time or been shoved down by burrowing animals. This would mean that they weren’t as old as the sediment layers in which they were located.
So to sort out the controversy, archaeologist Chris Clarkson from the University of Queensland obtained permission from the Aboriginal Mirarr people, who control the shelter, to re-excavate the site. And in 2012 and 2015 Clarkson and his colleagues did just that, using state-of-the-art methods to date sediment layers.
It was a painstaking process, writes Tony Wright at the Sydney Morning Herald. They used a method called Optically Stimulated Luminescence, which measures the last time sunlight struck a grain of sand. But that means any exposure to light would reset the signal. The team worked in the dead of night, examining individual grains of sand every five centimeters down under a red light. In total, Gibbons reports, they dated 28,500 grains of quartz from 56 samples.
The results of this work suggest that the sediment layers had not shifted or intermingled much over time. Thus, the more than 10,000 artifacts they recovered during the digs, including ax heads, seed grinding tools and red ochre used to paint the body, were the same age as the sediment layers in which they were found. The researchers concluded that the human occupation of the shelter occurred by 65,000 years ago. They published their results this week in the journal Nature.
“Previous excavations, they didn’t have the access to the dating methods that we do these days to actually confirm that the deposits and the archaeology really were that old,” Andy Herries, geoarchaeologist at La Trobe University in Melbourne, who was not involved in the study tells Tom Westbrook at Reuters. “The problem previously was that there was some old dates and stones but it was just a couple of them, whereas this research shows a significant occupation.”
The new dates rewrite the timeline of human history, especially the story of how humans left Africa and slowly spread across Asia, eventually reaching Australia. “People got here much earlier than we thought, which means of course they must also have left Africa much earlier to have traveled on their long journey through Asia and south-east Asia to Australia,” Clarkson tells Helen Davidson and Calla Wahlquist at The Guardian.
According to The Conversation, modern humans evolved in Africa earlier than 200,000 years ago then made their way to the area of China by 80,000 years ago. It was believed that their venture into Australasia was what led to the extinction of the hobbits, Homo floresiensis, as well as the collapse of megafauna, which researchers suggest were hunted to extinction some 45,000 years ago. But the new date means that humans, hobbits and giant wombats co-existed for around 20,000 years.
The study jibes with a study from earlier this year that has put the timeline for when exactly humans left Africa is in flux. A recent find in Morocco suggests that humans had evolved and were on the move as early as 300,000 years ago.
While the evidence is strong that humans made it to Australia 65,000 years ago, they may not be the direct ancestors of Aboriginal Australians. Geneticist David Reich of Harvard University tells Gibbons that Aboriginal DNA shows pretty clearly that they intermingled with Denisovans and Neanderthals, humanity's hominin cousins, in Asia 45,000 to 53,000 years ago. “If these [new] dates are correct,” he says, “they must be from a human population that was largely replaced by the people who are the primary ancestors of today’s Australians and New Guineans.”
The researchers plan to survey the surrounding area to see if they can find more and even older evidence of human occupation.