Neanderthal DNA may refute 65,000-year-old date for human occupation in Australia, but not all experts are convinced

Humans did not arrive in Australia 65,000 years ago, and likely didn't reach the land down under until around 50,000 years ago, a controversial new paper reports.

The reasoning behind the finding is that modern humans didn't mate with Neanderthals until around 50,000 years ago, but Indigenous Australians have a small percentage of Neanderthal DNA. So, the first Australians could not have arrived until after humans mated with Neanderthals.

But we can't yet rule out archaeological evidence that places humans on the continent much earlier than genetic models do, other experts say.

In a research report published Sunday (June 29) in the journal Archaeology in Oceania, archaeologists Jim Allen of La Trobe University in Australia and James O'Connell of the University of Utah used recently published Neanderthal DNA evidence to suggest that Australia was not occupied by humans until 50,000 years ago.

Allen and O'Connell's new theory is based on two recent DNA studies that revealed Neanderthals and humans likely interbred in Europe during one long "pulse" between 50,500 and 43,500 years ago. Since all living humans outside Africa have at least 2% Neanderthal DNA, including Indigenous Australians, this means the earliest Homo sapiens in Australia had some Neanderthal roots — and those roots can't go back much earlier than 50,000 years ago.

Researchers interested in the earliest humans in Australia have focused primarily on archaeological sites in southeast Asia and Oceania, broadly across the modern borders of Indonesia, Australia and various islands, also known as the paleocontinent Sahul.

"The initial colonization of Sahul is important because it occurs in the Late Pleistocene [129,000 to 11,700 years ago], which is coincident with a major expansion in the distribution of anatomically modern human populations out of Africa," O'Connell told Live Science.

Archaeological evidence of human occupation in Sahul largely lines up with the genetic evidence, Allen and O'Connell wrote in their study. All archaeological sites except one have been dated to between 43,000 and 54,000 years ago, meaning humans could have mixed with Neanderthals in Eurasia and then headed east.

Related: Australia's oldest rock painting is an anatomically accurate kangaroo

Archaeology versus genetics

But archaeological evidence at one site called Madjedbebe in the far north of Australia's Northern Territory suggests the area may have been occupied much earlier — at least 65,000 years ago.

Archaeologists recovered human-made artifacts, including stone tools and ocher "crayons," from the Madjedbebe rock shelter and published their findings in a 2017 study. One difficulty in dating the artifacts, however, was the copious amount of sand on the floor of the rock shelter, which can move easily and cause artifacts to fall farther down, making them look older than they are.

Although the research team took steps to counteract this issue and landed on a 65,000-year-old date, Madjedbebe's occupation timing is still uncertain because it is by far the oldest archaeological site in Australia, making it an outlier.

"It doesn't necessarily mean that the data is wrong," O'Connell said, "but it does mean that if the data is right, the people responsible for Madjedbebe are not ancestral to any significant degree to modern Sahul populations."

But Allen and O'Connell's new theory relies heavily on assumptions in the DNA model and in early human behaviors, several researchers suggested in a commentary, also published Sunday in Archaeology in Oceania.

"Both archaeological and molecular dating of Sahul are still in an early stage of development," wrote Peter Veth, an archaeologist at the University of Western Australia, so "can we rely on current assumptions underlying these molecular clocks to test Australian archaeological evidence?"

Adam Brumm, an archaeologist at Griffith University in Australia, wrote that Southeast Asian archaeological sites, such as on Sulawesi in Indonesia, actually have compelling evidence for early rock art that dates back to at least 51,200 years ago.

"Remembering that we only have minimum ages for rock art, I think there is a very real possibility that the people who created the earliest artworks from Sulawesi were a part of the same broader cultural group that went on to colonise Sahul some 65,000 years ago," Brumm wrote.

O'Connell and Allen, though, think that this sort of artwork, intensive seafaring and the creation of complex artifacts are all connected to a shift in human behavior that began around 50,000 years ago, sometimes called the Paleolithic Revolution. In a narrow window of time, they wrote, these early humans "began the process of displacing archaic hominins and occupying diverse environments in Europe and Asia."

Related: When did modern humans reach each of the 7 continents?

But in their commentary, archaeological scientists Huw Groucutt and Eleanor Scerri questioned this idea of a "revolution" in behavior that occurred around the time humans met Neanderthals.

"In Africa, decades of research now clearly show the presence of complex behaviours tens of thousands of years earlier than the supposed revolution, and arguably occurring in a gradual and piecemeal fashion," Groucutt and Scerri wrote.

While genetic and archaeological evidence are currently at odds, it is important to remember that there are major gaps in both data sets, meaning there is no strong evidence favoring either the pre- or post-50,000 year date for the first occupation of Sahul, Groucutt and Scerri wrote.

But even though archaeological evidence does not currently refute Allen and O'Connell's theory, Brumm wrote, "I think this evidence is coming, however, and it will have big implications for our understanding of ancient Sahul."

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