Long Glacial

 

Only a small part of Australia was actually glaciated during the Last Glacial Maximum (LGM), in the Snowy Mountains and Tasmania.  On the Kosciuszko Massif, terminal moraine dates at Blue Lake are between 22.3 thousand rears ago (ka) and 19.3 ka. At Cradle Mountain - Lake St Clair, Tasmania's largest Glacier at 27 kilometres (kms) long and 350 metres thick, dates are from 20.5 ka to 18.8 ka. Calibrated dating of most of the mainland and Tasmanian glaciers has indicated their greatest extent to have been from about 22,000 to 19,000 years before present (YBP). This date range gives a reasonably precise time frame for the most severe period of the LGM in Australia.

Avoiding continental coverage by ice sheets was fortunate, but did not mean escaping a serious downturn in climatic conditions. Australia experienced a decline in average annual temperatures of up to 10⁰C and a 60% reduction in rainfall. Significant cooling and aridity produced a change in vegetation towards steppe-like and grassland dominated environments. There was widespread sand-dune activity and dust storms reached far out into the Tasman Sea. Desert expanded into semi-arid zones and temperate and rainforest areas shrank.

This climatic downturn was as prolonged as it was pronounced. Deterioration began around 32 ka when the Kozciuszko Glacier advanced. It did not relent until thousands of years after the LGM peak, because while temperatures began rising, rainfall stayed low. Particularly relevant to the South West Coast of Victoria were two momentous events which 'bookended' the 'long glacial'.

In the 1800 years between 32.6 ka and 29.8 ka, coastal sea level dropped 40 metres, from 65 to 105 metres below present (mbpsl). And in the even shorter interval between 14.6 ka and 14.3 ka, sea level rose 40 metres, from about 105 to roughly 65 mbpsl, in what is known as the Melt-Water Pulse (MWP).

The long period between 29.8 ka and 14.6 ka was therefore a time when the biggest area of additional landmass was exposed for occupation and/or exploitation by humans. However, this was probably of limited practical use for inhabitants of the south eastern corner of Australia. A considerable time lag is to be expected from first exposure of a saline seabed to that of a fertile landscape with edible plants and animals. This delay can only have been made longer by cold and windy weather with little rain. 

The relative abruptness of the emergence and then disappearance of big parts of the continental shelf did have a human impact though. In the Bass Strait region, the land-bridge connecting mainland Australia and Tasmania was directly affected, first broadened and then severed. Inundation of the corridor began with flooding of the west side about 15 ka and concluded with the island's complete isolation by 11 ka.

An important footnote to understanding the effect of the 'long glacial' is that the total number of hunter-gatherers occupying Australia from 35 ka to 13 ka averaged only 20,500 individuals. This was a small population in a very large continent and it had been like this for pretty much the whole human era. In geographic terms, there were probably plenty of options for taking shelter under adverse climatic conditions. In cultural terms, there may have been limits to their willingness to take advantage of all of them.

The prehistoric context of the Aboriginal settlement of Australia is one of rapid exploration followed by patterns of fixed regionalism, despite continuing low density. The population did not begin to 'take off' until after the Melt-Water Pulse signalled the end of the LGM. 

The date of first arrival by homo sapiens can be fairly confidently asserted at around 50,000 years ago. This chronology is attested to by archaeology and genetics but politically motivated claims for older dates are still made. As it remains a contentious area, the following authoritative summary is quoted:

     "...appropriately calibrated molecular clock studies of mitochondrial, Y chromosome, and nuclear DNA all converge on Sahul colonisation around or slightly after 50 ka, in close accord with the estimates for Denisovian interbreeding and a wide range of Sahul archaeological data

     "....a range of molecular clock estimates and archaeological data suggest the initial colonisation of Sahul by AMH [anatomically modern humans] occurred around 48-50 ka, matching estimates for an out-of-Africa movement 50-55 ka.

JF O'Connell, J Allen, MAJ Williams, AN Williams, CSM Turney, NA Spooner, J Kamminga, G Brown, A Cooper, 2018, 'When did Homo sapiens first reach Southeast Asia and Sahul?', PNAS 115.34, p 8486

The initial landing of the founding population was followed by separation of the New Guinea settlers from those heading south into Australia. No further interbreeding occurred between these two populations. They formed unique gene pools and lineages, continuing their isolation from one another until the present day.  Both populations retained their migratory impulse, however, quickly radiating out into their respective territories, as the map shows:

These radiocarbon dates imply a single rapid migration by Aborigines around the east and west coasts, reaching southern Australia between 45 ka and 49 ka. The 45-49 ka date range from Warratyi rock shelter (Flinders Ranges SA) and the 47.8 +/- 1.5 ka mid-dated sequence from Devil's Lair (Southwestern WA) are close in age to the earliest sites in northern Australia, affirming that original migration was very fast, only a few thousand years at most.

This evidence is supported by recent genetic studies using hair samples from Aboriginal groups who were distant from one another at the time of European occupation. This research argues that, "initial Sahul colonisers spread across northern Australia, and then south along the east (haplogroups P, S, M42a) and west (haplogroups O, R) coasts in parallel clockwise and counter-clockwise movements."

R Taylor, A Rorlach, J Soubrier, P Bover, B Llamas, J Tuke, N Bean, A  Abdullah-Highfold, S Agius, A O'Donoghue, I O'Loughlin, P Sutton, F Zilio, K Walshe, AN Williams, CSM Turney, M Williams, SM Richards, RJ Mitchell, E Kowal, JR Stephen, L Williams, W Haag, A Cooper, 2017, 'Aboriginal mitogenomes reveal 50,000 years of regionalism in Australia', Nature, pp 1-16 [doi:10.1038/nature21416]

What happened next is a bit surprising. According to the geneticists, "After continent-wide colonisation, strong regional patterns developed and these have survived despite substantial climatic and cultural change during late Pleistocene and Holocene epochs."

The scientific argument is compelling. The age of the most recent ancestor, calculated by the molecular clock method, gives a narrow window of time from approximately 43 to 47 ka. This 'fits' the radiocarbon and optically stimulated luminescence (OSL) ages from the earliest archaeological sites and a derivative estimate of initial colonisation of Sahul at 48.8 +/- 1.3ka. 

The five main lineages ("mitochondrial haplogroups") identified for Aboriginal Australia are apparently S, O, M, P, and R. The relevance of these markers is their geographical distribution, in fact their 'rootedness' in particular locations, that seems to have lasted 50,000 years.

For example, none of lineage O was found in eastern Australia, which is instead dominated by P, S, and M24a. The two main Australian P-clades, based around P5 and P4b1, are another example, showing a clear split between north-eastern Australia and Riverine/South Australia. Similar patterns observed in other haplogroups indicate that  these lineages have had very little dispersal over time and neighbouring individuals were closely related genetically. 

These and other findings reinforce the idea that "the deeply diverged mitogenomes of Aboriginal Australia" date back to the original arrival of people on the continent circa 50 ka ― "the basal lineages within each major haplogroup were mostly in northern Australia, presumably reflecting early divergences as members of the founding populations remained while others moved south where more derived lineages were observed."

It is also evidence that contemporary expressions of 'attachment to country' are not a recent cultural invention. The situation of Aboriginal society noted at the time of European settlement, of "localised, collective and stable land/language ownership groups within a broad environmental territory", has a pre-glacial origin.

The geographic diversity and intense grouping of mitogenome data further indicates that indigenous people survived momentous environmental changes like the LGM and MWP without further large scale migrations. If this is the case, then there should be some sort of archaeological footprint on the South West Coast of Victoria, from human activity before these events. That there is none suggests that this coastline and its immediate hinterland was not occupied by Aborigines until after the Melt-Water Pulse (14,600 to 14,300 years ago).