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Paleobiology

Human Arrival in Australia Dated at about 65,000 Years Ago

By Guest Author Chris Stringer inPaleobiology |
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Human arrival in Australia marks a watershed in human evolution, because the journey required ocean-going watercraft and multiple island-hops. In terms of technology and planning it clearly demonstrates the high capabilities of the people who first accomplished it. New excavations at Madjedbebe rock shelter in Arnhem Land in northern Australia, reported by Clarkson and colleagues in Nature, have uncovered thousands of artefacts concentrated in three dense bands, with hearths, grinding stones, ochre pigments, shiny minerals, and the oldest known ground-edge hatchet heads. Although there is possibly even more ancient material in lower levels, the oldest well-dated assemblages are placed at more than 61 ka by luminescence methods, and most probably date to around 65 ka. The authors of the study claim ‘This evidence sets a new minimum age for the arrival of humans in Australia, the dispersal of modern humans out of Africa, and the subsequent interactions of modern humans with Neanderthals and Denisovans’.

Arnhem Land in the Northern Territory, Australia. It was most likely here that the very first humans entered the Australian continent approximately 65,000 years ago. At least that is the age of the oldest known artefacts, which have recently been found in this northern region of Australia (Image credit: Jon Connell, flickr).

The generally accepted arrival date in Australia of about 50 ka has been estimated using genetic data from recent populations, and while there have been previous claims for an arrival earlier than 50 ka, these have not gained general acceptance. An arrival date of ~50 ka is also consistent with genetic estimates of ~60 ka for the main exit of anatomically modern humans (AMH) from Africa, allowing about 10 ka for the transit across southern Asia to Australasia.

If valid, these new dates certainly set an older minimum age for the arrival in Australia, together with early evidence of complex technology, and minerals that were probably used for symbolic purposes.

However, there were evidently earlier dispersals of AMH from Africa. We know early AMH were in Israel at >100 ka, and there are claims of AMH teeth from Fuyan Cave, China dated to at least 80 ka. But the fact that mtDNA and Y-chromosome DNA give comparable estimates of arrival into Australia at ~50 ka implied that the earlier waves never reached Australia, died out, or were largely or completely replaced by later waves.

The oldest known Australian burial: Mungo 3 from southern Australia at dry Lake Mungo in New South Wales, dated at 42,000 years (Image credit: Colin Groves).

Thus any inference about the timing of the incorporation of Neanderthal and Denisovan DNA into extant Australasians would only be valid if we knew that these earliest inhabitants were direct ancestors of the extant Australasians who have evidence of this ancient hybridization in their genomes - and we don't know that as yet. As already mentioned, there could have been extinctions, or replacements or swampings of these earliest pioneers by later AMH arrivals. And of course, while the assumption that this early colonization was by AMH seems very likely, it does need to be confirmed by supporting fossil evidence.

The apparent survival of archaic humans in Eurasia and S.E. Asia (Neanderthals, Denisovans, Homo floresiensis and perhaps also H. erectus) after this date of ~60 ka may indicate that the effect of these early dispersals was less than the inferred major wave(s) after 60 ka. Perhaps the numbers were smaller, they moved faster and in narrow zones (maybe coastal?), or their ability to replace the other populations was more limited at this earlier date. Thus the potential for interaction and intermixture between dispersing AMH and the archaic species may also have varied across the different waves of dispersal.

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Responses

Claus Schroeder

That's an interesting finding. It has far-reaching implications as to the cause of the extinction of the Australian megafauna approx. 40,000 years ago.

In what has become known as the "blitzkrieg" or "overkill" hypothesis it has generally been assumed that humans wiped out the Australian megafauna soon after first setting foot on the continent at about 45,000 years ago. It's now evident that humans co-existed with the megafauna for at least 25,000 years without having a major negative effect on diprotodonts and giant kangaroos etc.

The "overkill" hypothesis is thus off the table, at least as far as the Australian mainland is concerned.

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Thomas Lang

That's an interesting development implying that modern humans co-existed with archaic human species much longer than previously thought.

This finding is of particular relevance for the discussion about a potential co-existence with Homo floresiensis. After the H. floresiensis remains had been dated back last year, chances of an overlap were low; with this new finding, however, the chances that we encountered the dwarf hominin on our way to Australia appear to be quite intact again.

This also raises the question to which extent these early Homo sapiens populations interbred with Homo neanderthalensis, the Denisova hominin, and possibly even Homo floresiensis, when migrating through Asia. I'm wondering whether it once will be possible to reconstruct such early interbreeding events from ancient DNA (rather than from our extant gene pool which presumably has become diluted over the millenia).

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Helga Vierich

That is a viable suggestion you make here about an early, fast, trajectory along a narrow pathway; and indeed a coastal route would fit. Aside from access down the Nile and along the Mediterranean coasts into Eurasia, the most obvious route of expansion was a coastal ecosystem highway (“narrow” despite being a wider during periods of glacial advance, when sea levels were 70-120 feet lower).

We are probably looking for a single culture group who spoke a single language. It appears to have represented only one of a semi-continuous chain of coastal demes/culture/language groups located in refuge areas along the Eastern and Southern African seacoasts during periods of extreme drought from 134 ka. to 74 ka. Afterward, the bulk of humanity could expand inland as rivers began to run and lakes refilled - and probably constituted 9-10 times the number that trickled into Eurasia.

New evidence has come to light that tells of some profound challenges that were faced by early modern humans in Africa during their first 100,000 years, challenges that only began permit the full re-occupation of the interior since about 72,000 years ago.

This data comes from climatic interpretation of deep core samples drilled in deep Lake Malawi bottom sediments. Severe interior droughts happened in four huge waves from 134,000 to 74,000. In the paper reporting these findings, these were referred to as “Mega-droughts”. which suggests the coasts were refuge areas for the species Homo sapiens during a span of nearly 80,000 years.

Each humid period during this time was followed by these severe droughts. This Malawi core data lists humid periods in East Africa at 1)148 and 132 ka, 2) 124-110 ka, 3) 85-76 ka, and 4) 70-65ka.

As each mega-drought ended and a wetter climate began, these population in these refuge areas would have experienced accelerated rates of population growth and expansion. Until the final drought ended around 70,000 years ago, each expansion into interior regions would have been knocked back as soon as the next five, ten or even 20 years of Mega-drought set in. But, through wet and the dry millennia, if their numbers were going up, the only place people could go and persist, was up and down the coast, or, possibly, north along the Nile.

It seems likely that at least some of these cultures made boats, hunted fish at sea, were intimate with fish, marine mammals, seabirds, island-hopping, and tidal dynamics; a kind of culture where people grew up with knowledge of both fresh and salt water, use of both hunting bows and harpoons, ate shellfish and fish but sought mammalian skins for clothing and blankets. This culture was capable of using three ecological areas - 1) the seacoast and 2) marine areas just off shore, and 3) the mixed savanna and forests long rivers perhaps as much as a hundred miles inland from the coast. It seems likely then, that these people were, in fact the keepers of encyclopedic knowledge about three different ecosystems - the shore, the sea and the land, and they used them all.

A culture adapted to this triple ecosystem might well have expanded preferentially along coastal environments. It is roughly 20,000 km along the coastline from the northeast coast of Africa to the north coast of Australia.

Assuming 1) a “starter” language group/deme of 1000 people, living along the coast of SE Africa, and assuming - based on my own Kalahari data - that such a group requires a territorial range of roughy 120 x 120 km and 2) an average population growth rate of .05%/year, thus a doubling time of about 1400 years*, it would take such a population, just over 12,000 years to reach the area of these Chinese sites you mentioned. They might even make it there faster, if their birth rate went up even enough to bring population growth to ~.08% a year. Another 5000 years would get them to Australia. What the new Australian date tells us is that this steady expansion along a coastal route likely started before 70 ka. But this does not mean that most of the people who undertook this expansion were not part of a relatively small genetic founder group.

*Doubling time is calculated by the formula: Td = log(2) / log(1 + r) Where: Td = doubling time and r = a constant growth rate.

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Bob Keeter

If we have bones that credibly tell one story and DNA analysis that tells another, do we discard the bones or question the DNA assumptions? The DNA dating is based on assumptions regarding the rate of coding errors in "dead" DNA.

What if the DNA includes sections subject to selection but not currently recognized as such?

What if the error rate is not a constant?

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RS
Rüdiger SchweigreiterE

Hi Bob,

I don't think we have to discard either bones or DNA, they rather complement each other decently well. It's just a matter of interpretation. If genetics indicates an arrival at about 50 ka, this implies that earlier arrivals did not contribute, or not visibly contribute, to the extant gene pool of Indigenous Australians. Please note that we have not yet found any human fossils from the time of 65 ka, let alone DNA that one could use to compare with present-day DNA.

Molecular phylogenetics (DNA dating) has become an elaborate methodology that does not rely on one or a few markers but can essentially cover the whole genome (also depending on the quality of the source material). As with physical bone and artefact dating methods, there are certain weaknesses but that's exactly the reason why it's imperative to support a scientific claim with more than one method.

Rüdiger