Population Curve

 

The discipline of numbers can give stunning insights into early human experience. This is true of the Aboriginal occupation of Australia.

Demographic work by a group of archaeologists has shown the probability of a perilously small population of homo sapiens inhabiting this large continent for about 80% of its 'human era'. Beginning with 2-3,000 migrants from South East Asia 50,000 years ago, total numbers may not have exceeded 20,000 until exponential growth took off in the Holocene. Before that the prehistory of Aborigines seems to be a story of extreme vulnerability to climate change and their bare survival as a viable gene pool.

The underlying assumption of work by Alan Williams and others (2013) is that dateable archaeology is evidence of human activity, and by extension, the level of population. An increase in radiocarbon dates over time demonstrates a rise in people doing things, and implies that more people are involved in the doing.

The idea of using proxies in paleo-science is not new. Best known is the use of pollen counts from drill cores to estimate climate conditions in a particular time frame. An increase in the pollen sample of treed species over grass is interpreted as warmer and wetter conditions, while the reverse suggests less abundant biological activity in cooler and drier times.

For this study, the authors have collated 4,575 radiocarbon dates from 1,750 sites across Australia. They have also attempted to "correct" the raw data for "taphonomic error", because "time-decay progressively removes older evidence". Their composite graph of uncorrected and corrected information is reproduced here:

The correcting process of 'equalising' the lower amounts of older data with the higher quantities from the more recent period creates a lot of graphical 'noise', which is why the 'smoothing' median line has been added. The corrected data exaggerates or subdues the numbers of dates, but also 'brings out' the rate of change as distinct from just the relative quantities. A connection between the proxy and population becomes clearer in the next couple of composite graphs.

The lead up to Last Glacial Maximum (Ice Age) conditions, and the slow recovery from these effects up to the Terminal Pleistocene and Early Holocene spurt in numbers, is  reflected in both corrected and uncorrected datasets. Together they produce very similar growth rate patterns. 

The uncorrected data set shows an annual growth rate of 0.01% over the last 50,000 years with a range between 0.07% and -0.03%. Overall average growth rate remains the same for the corrected dataset at 0.01%, but the range of extreme values is broader at 0.09% to -0.05%. The plausibility of plotting the proxy to represent population growth and decline becomes apparent. The next step is to 'match' the shape of the curve to start and end points that are realistic:

A founding migrant group of 50 or 500 individuals is too low, ending in a pre-European population of 300,000 or less. 5,000 is too high, giving a peak of more than 3,000,000.

The authors conclude that around 2,000 produces the 'best fit', in line with what is generally accepted ― a top of 1.1 to 1.2 million people about 500 years ago, shortly before the introduction of pandemic disease by Macassan traders in the north of Australia reduced the Aboriginal population to 6 or 700,000.

"This analysis implies a founding population size of greater than 1,000 and less than 3,000...Taphonomic correction of the data suggests a significant number of sites by 40ka...but it does not support saturation of the continent to ethnographically observed population densities during the Late Pleistocene...Using the uncorrected dataset, population estimates during this time are still low ― less than 20,000 ― and suggest widespread settlement of the continent would probably have been unfeasible prior to the Last Glacial Maximum."

This is an important inference to make. Essentially, a sparsely populated continent is a partially occupied one.

Areas of continual land-use and concentrated population are considered to be best represented by central places people retreated to when the weather was at its worst. These 'nodes'  have been called LGM "refugia". In Victoria these included the Murray River and Port Phillip Bay. Excavations at Lake Tyrell in the northwest of the state (2007) and Keysborough in the southeast of Melbourne (2010) support the idea of "refugia" during climatic reversals.

At Box Gully, an eroded clay lunette on the north end of the Lake Tyrell salt pan, 10 radiocarbon dates of hearth-charcoal span from 37,220 to 26,420 years before present (YBP). Bone and shell remains describe a diet of small mammals (betongs and bandicoots), shingle-back lizards, emu eggs and freshwater mussels. Stone artefact scatters demonstrate "low intensity, early stage reduction of material brought in from elsewhere in a partially processed form." 

Together with the charcoal from open fireplaces and ovens, the evidence was thought "consistent with small-scale, short-term, seasonal visits to Box Gully that occurred repeatedly over thousands of years." In other words, the Lake Tyrell area was part of an extensive hunter-and-gatherer range in the Mallee for River people. This interpretation is borne out by the site's eventual abandonment towards the approach of the LGM, when foraging outside of the Murray Valley became untenable.

At Bend Road, a large sand deposit jutting into the north of the Carrum Swamp, 9 dates indicate human activity in the period from 35,300 to 15,100 YBP. This timespan includes dates from the LGM. Lithics distribution, with densities in "hotspots" like the "Tardis flaking floor", and the successful re-matching of fragments into "conjoined sets", was considered characteristic of "residential base camps" rather than "temporary encampments".

The likelihood of a persistent population cluster in the Melbourne Basin is strengthened by a similarly long series of dates from Keilor and the Maribyrnong River. These terraces have received a lot of archaeological attention since the discovery of the Keilor Skull (dated 14,500 YBP). Human occupation and stone tool manufacture  are recorded from before the LGM in 24,000 YPB to the Mid-Holocene at 6,460 YBP.

Of course, none of this information helps establish the early presence of people along the South West Coast of Victoria. It may, however, explain the absence of archaeological evidence to that effect. The known facts are that reliable signs of Aboriginal occupation do not appear before the Terminal Pleistocene about 13-12,000 YBP. The oldest archaeology from caves and shell-middens around Discovery Bay dates from the time that people started to move out from LGM refugia. 

"The growth during this time appears to be primarily driven from southeastern Australia, and demonstrates increasing spatial use of the landscape and diversification of economic activities...By the Early Holocene, there is evidence for increasing population ― with activation of new sites..."

It is quite possible that the South West Coast was an unoccupied region throughout pre-glacial and glacial times, when "high amplitude environmental fluctuations kept population low", only becoming inhabited some time after the LGM, when temperatures rose and rainfall increased. 

                                                                                                                                                      

REFERENCES

AN Williams, 2013, 'A new population curve for prehistoric Australia', Proceedings of the Royal Society B, 280, 1-9

Williams, Ulm, Cook, Langley, Collard, 2013, 'Human refugia in Australia during the Last Glacial Maximum and Terminal Pleistocene: a geospatial analysis of the 25-12ka Australian archaeological record', Journal of Archaeological Science, 4-, 4612-4625

Williams, Veth, Steffen, Ulm, Turney, Reeves, Phipps, Smith, 2015, 'A Continental Narrative: Human Settlement Patterns and Australian Climate Change over the Last 35,000 years', Quaternary Science Reviews, 123, 91-112

G Hewitt & J Allen, 2010, 'Site Disturbance and Archaeological Integrity: The Case of Bend Road, an Open Site in Melbourne Spanning Pre-LGM Pleistocene to Late Holocene Periods', Australian Archaeology, 70, 1-16

T Richards & R Johnston, 2007, 'Box Gully: new evidence for Aboriginal occupation of Australia south of the Murray River prior to the last glacial maximum', Archaeology in Oceania, 42, 1-11