French and Australian scientists have modelled the evolving landscape of the ancient supercontinent Sahul to work out how quickly humans were able to populate the landmass that would eventually become Australia.
It turns out these first people to arrive via entry points at West Papua and the Timor Sea more than 65,000 years ago were able to occupy much of the continent within 30,000 years – at rates quicker than other ancient groups around the world.
During the last glacial maximum, lower sea levels meant modern-day Australia and New Guinea were united by a land bridge that enabled human movement south from Asia.
The earliest records of human occupation of Australia had been dated by one research group at 65,000 years ago, from a site in Arnhem Land. The site is one of many along proposed “migratory routes” along which it was thought the Australian landmass was populated over thousands of years.
But the new work led by deep-time climate modellers and archaeologists, overlays simulations of human movement over the continent’s changing landscape and suggests a relatively quick process, and not merely along previously described “superhighway” migration routes. Their findings are published in the journal Nature Communications.
“From what we see in our models … it’s not really the case,” says Tristan Salles, an earth process researcher at the University of Sydney who led the study.
“There are obviously some places where [migration] seems to follow these superhighways, but there are also other places where, basically, you’ve got high probability, according to our model, of finding some people moving around these places which were not part of the superhighway routes that were described previously.”
Instead of simply long migratory corridors, the modelling performed by Salles’ team suggests waves of migration also occurred along water sources like rivers and coasts, following drainage patterns rather than a defined route.
Time machining the first trailblazers
To understand human movement patterns, the group simulated the shifts in the Sahul landmass across 40,000 years during the Late Pleistocene period.
With changes in sea level and rainfall believed to be the main drivers of geographic change throughout this period, they simulated the transformation of the continent’s temperate and rain forests; desert, savanna and alpine regions; and grasslands.
They then considered how shifts in environmental productivity and water geography would have helped – or hindered – ancient human movement.
At an average, they predict humans were able to populate the southern continent relatively quickly – between 6-10km each year, depending on simulated entry points of West Papua or off the Timor Sea coast.
This is quicker than hunter-gatherer groups in Europe and North America, though they note similar population speeds were described when continents were first occupied.
“When you put it back to the number of kilometres per year, it doesn’t seem super rapid,” Salles says. “But if you compare it to other people immigrating in different places around the world, it’s quite rapid.
“This rapid migration has also been inferred by other people in previous studies before, either with models or looking at genetics and genome sequencing – people have managed to see it was happening, relatively quickly compared to other places.”
Charting a map for archaeological use
While the research doesn’t delve into the complexity of the cultures developed by the Sahul peoples that may have influenced their movement across Australia’s predecessor continent, their modelling does provide a ‘heat map’ of where Australia was likely to be populated by 35,000 years ago.
Parts of Australia’s red centre, the Nullarbor Plain, modern-day Tasmania and the south-eastern part of the continent were modelled as having a lower probability of occupation by the cutoff point.
The benefit of the study, Salles’ team says, is that with a better understanding of likely pathways for the ‘Sahul walkers’, archaeologists may now be in a better position to locate future study sites.
Rather than trying to connect known archaeological sites to determine human movement through time and space, this modelling begins from two northern ‘entry points’ and watches how groups move across the continent.
“You can use your map and look at where in the regions that we’ve defined the model is predicting high probability of the presence of humans, and that could be a first way of targeting some specific location,” Salles says.
From there, the model can be run repeatedly to try and refine possible study sites for human occupation. It won’t land on exactly where human remains and artefacts might be buried, but it will consider the geography of Australia over thousands of years, combined with the anticipated arrival of people to various places on the continent, to determine when and where the Sahul walkers might arrive.
