‘Sterile’ cave discovery reshapes timelines for Australia’s first arrivals – again

A cave in Timor has added another piece to the puzzle that is the story of human migration to the Australian continent.

It’s all thanks to a ‘sterile’ layer of sediment.

To piece together ancient timelines, archaeologists will perform complex dating techniques on organic material such as decayed plant and animal matter, and the sediments surrounding non-organic artefacts – like arrowheads, pottery and other stone materials, depending on the site location.

Layers rich in these artefacts were found in the Laili rock shelter in the Manatuto district of Timor-Leste, among them hackberry, sea and land snail shells, crab and barnacle shells, rodents, lizards, snakes, frogs and bats. Stone tools and grindstones were recovered as well.

But crucial to the excavation was the finding of a ‘sterile’ layer – one with no signs of human occupation.

This provided the research group with a comparison – one dateable layer where humans were settled, and one occurring before their arrival.

The ‘arrival signal’ was calculated from this layer of human artefacts, suggesting people began occupying Timor no earlier than 44,000 years ago. The sterile layer was 54,000 to 59,000 years old.

“It’s the only site that we have found, which actually had human occupation,” says Sue O’Connor, an archaeologist at the Australian National University, specialising in Indonesian and Timorese sites.

O’Connor and her colleagues had previously excavated the Laili cave but returned to see whether more detail about the region’s human occupation could be, literally, unearthed.

She describes the human occupation sediment layer as “very dense” and suggestive of a deliberate, large-scale human occupation around 44,000 years ago.

The sterile layer beneath was a gift craved by archaeologists seeking the most precise confirmation of human activity possible.

“If we can date this sterile unit below this very, very dense human occupation layer, we’ll know when people weren’t there, which is just as important,” O’Connor says.

The finding is compelling evidence of the first occupation of islands in the east Wallacean archipelago.

But it also seems to remove a potential route for the earliest occupation of Sahul, the supercontinent that would eventually become Australia due to rising sea levels.

If Madjedbebe was inhabited 65,000 years ago, it wasn’t through the ‘southern route’

In 2022, archaeologists upended established knowledge by announcing a site in Arnhem Land had been occupied about 65,000 years ago.

For a long time, most sites had been dated between 45-40,000 years ago, with recent discoveries pushing first arrivals towards 55-50,000 years.

But the evidence of continuous stone tool usage at Madjedbebe was paradigm-shifting.

Several highly regarded archaeologists contest the timelines, while others agree the science behind that study is robust.

The Laila discovery doesn’t challenge the 65,000 settlement date directly, but it does make it harder to consider the continent’s first arrivals came via the ‘southern route’ – one of the suggested migration pathways from southeast Asia.

“This is the first time we can say people weren’t there [in Timor] 55,000 years ago,” O’Connor says.

“[On Madjedbebe], I don’t have an opinion one way or the other, but I do think that we’ve shown that if people did get to Australia 65,000 years ago, they probably didn’t use the southern route, or they didn’t go through Timor, which is the biggest, most prospective island to get to the Arnhem Land coast.”

Peter Veth, an archaeologist from the University of Western Australia who wasn’t involved in the research, says the findings are an important step in piecing together a patchwork of sites across the region that tell the story of human movement to Australia.

“It’s sharpened up something Sue and her team have been arguing for some time,” Venn says.

Like O’Connor, Veth specialises in archaeology through the Southeast Asian region.

“[Across] the lower southern island chain [of Indonesia and Timor], there’s really nothing coming back to suggest sites in that order of 50-60,000 range. So it’s either another colonising route through the top, through PNG, or this is a large wave of folk that overlay an earlier colonisation.”

While it now seems unlikely that Timor was the launch pad for the first wave of Australian migration, Veth says the discovery confirms the people who first colonised Timor had access to resources capable of supporting a large population and a vibrant seafaring culture.

“There’s no question from here [Laili] and Cape Range and Barrow Island and even parts of the Kimberley that people were using mangrove and coastal resources early on, and that coasts were productive and probably always were for the spread of modern humans.”

It’s a view echoed by Bruno David, an anthropological archaeologist from Monash University, also independent of the O’Connor research.

“Laila helps us begin to understand the cultures of that region at that time,” David says.

“Whether it represents a colonisation event or the local occupation of a cave and area by a population that already lived in the area is not certain. What it does mean is that now we can begin to understand what people were doing there and then, how they used their environments.”

The Laila excavation results are published in the journal Nature Communications.

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