Aboriginal Colonisation of Australia Along Drainage Systems or the Coast

Australia: The Land Where Time Began

During the Pleistocene anatomically modern humans (AMH) were dispersing globally which required the crossing of arid and semiarid regions where a primary constraint on the pathways of dispersal was provided by the distribution of potable water. In this paper Bird et al. have provided a spatially explicit assessment on a continental scale of the opportunities for the occupation by AMH of Australia, which is the driest inhabited continent on Earth. The location and connectedness of persistent water in the landscape was established by Bird et al. by the use of the Australian Water Observations from Space dataset in combination of small permanent water bodies, such as springs, Gnammas, native wells, waterholes and rock holes. A high degree of directed landscape connectivity was demonstrated by the results during wet periods and high density of permanent water points that were distributed widely, though unevenly, across the landscape across the interior of the continent. It was shown that there was a connected network that represents the least cost distance between water bodies, and graded according to terrain costs, that 84% of archaeological sites that are more than 30,000 years old are within 20 km of modern permanent water. It was further shown that there were multiple routes into the interior that were well-watered and available throughout the period of occupation by humans. Bird et al. suggest that a wave of dispersal further along the routes that were well watered, to patches where the returns from foraging were higher, were potentially driven by the depletion over time of high-ranked resources in these palaeohydrological corridors.

There has been considerable debate surrounding the timing, routes and mechanism of colonisation of the continent of Australia by early humans. Prior to 47 Ka (O’Connell, & Allen, 2015; Bird, Hope &Taylor, 2004; Clarkson et al., 2015; David et al., 2013; Roberts, Jones & Smith, 1990; Roberts et al., 1994; Summerhayes et al., 2010; Veth et al., 2009) initial occupation from the north appears to have begun, and with movement that was relatively rapid thereafter; e.g., the Willandra Lakes region in the southeast of the continent may have been occupied within 1,000 years after the arrival of humans (O’Connell & Allen, 2015; Bowler et al., 2003; Heupink et al., 2016). It was considered by Birdsell (Birdsell, 1957) the dispersal was rapid and throughout the continent, whereas it was considered by Bowdler (Bowdler, 1977) that early dispersal occurred along the coastlines, with initial occupation of the interior being limited.

The postulates that following arrival in the northwest, Horton (Horton, 1981), or north Tindale (Tindale, 1951), were added that when the colonists arrived in the northwest and north, respectively, humans dispersed through the woodlands of the northern and eastern interior along riverine corridors and eventually to the coasts. Subsequently, these “end-member” scenarios have been reworked in order to include an understanding that is more nuanced of the “filling of the continent” as dependent variably on a matrix of biogeographic (Veth, 1989), ecological/climatic (Veth, O’Conner & Wallis, 2000), and sociological/technological (Hallam, 1977; Lourandos & Ross, 1994) facilitators of-or barriers to-dispersal from an initial entry point in the north (Kealy, Louys & O’Connor, 2015). According to Bird et al. the interior of the continent is now viewed as a mosaic of potential oases, corridors and barriers, which has the viability of a specific region for occupation or transit that also depends on the trajectories of environmental change (Smith, 1993; Smith, 2013; Hiscock & Wallis, 2005; Veth, 1993).

Building on previous work and drawing on optimal forging theory, O’Connell and Allen (O’Connell & Allen, 2015), propose a model of human dispersal throughout the interior of the continent that was driven by availability/depletion of resources, in which the major rivers of the interior/river basins representing the most attractive environments to human foragers; for short periods these environments extended into other areas, at time of rain related resource “flushes.” Smith (O’Connell & Allen, 2012) attributes Human dispersal through the desert in order to access food resources provided by stepping stones in the form of small and variable water features, and not the resources themselves. An explicit spatial dimension is lacking in all treatments to date of the dispersal of humans in the Pleistocene. The questions were, which potential dispersal routes were available, where they were, and under what circumstances. Discussion of the patterns of human colonisation in Australia, without spatial information, have usually been framed in terms of aridity – the lack of water – though it has been well known that periodically even the driest deserts are flooded periodically (Smith, 1993; Rouillard et al., 2016; Peterson, 1976). It has been noted, Peterson (p.65 in ref. 33) that the population disperses widely to the most ephemeral sources far out on the plains in the Western Desert, e.g., after substantial rain events. The people retreat back to the more permanent water supplies as the water supplies out on the plains dry up, where they may be trapped for a period (Cane, 1987). It is possibly for this reason that it was observed by Gould (Gould, 1969) that Aboriginals prioritise their foraging near satellite water holes, and then eventually settle closer to the main water hole. Veth noted (Veth, 2003) that in the Western Desert there is a positive correlation between the number of extractive artefacts and the permanency of the water.

Therefore, aridity in isolation is not necessarily a barrier to either habitation or transit. The time period for which the Aboriginal groups could occupy a particular site, or cross most of the Australian interior, is dictated by the duration of inundation, the location in the landscape of permanent water, and the connectedness of water during times of inundation. It was noted by O’Connell and Allen that “terrestrial patch rank was determined primarily by the presence of freshwater, as measured by volume and reliability of precipitation and/or local stream flow” (pp.7-8 in ref. 25).

The assessment of the spatial distribution and permanency of standing water in the Australian landscape of the present is allowed by The Water Observations from Space (WOfS) dataset (Geoscience Australia, 2014). In this study Bird et al. used this information, together with the distribution of small natural water bodies, such as springs, gnammas, native wells, waterholes and rockholes, compiled from 1:250,000 topographic sheets (nationalmap.gov.au) to provide an assessment that was spatially explicit of the opportunities for the occupation during the Pleistocene by human occupation of, and dispersal throughout Australia. A connected network was produced that represents the least-cost distance between water bodies and graded according to terrain cost.

Discussion

A high degree of directed landscape connectivity during wet periods and a high density of water points that are distributed widely, though unevenly, across modern semiarid and arid areas of Australia, is demonstrated by the spatial analysis of the 112,786 individual natural water points (Dataset S1). In the past inundation events may have been both more common and less common, and in the past permanent water points may have shifted location, some permanent water may not be potable, and small isolated water points may not have been accessible to dispersing groups that were not familiar with the area into which the population was moving. The broad scale patterns that are observable at present, nevertheless, are likely to have held in the past, particularly during the wetter periods as the initial dispersal was underway. According to Bird et al. all the archaeological sites that are earlier than 30 Ka are associated with pathways of least cost that were identified on the basis of the distribution of modern permanent water.

Conclusions

In order to understand the dispersal of humans through water-limited environments around the Earth, the location and connectivity of water in the landscape is critical (Finlayson, 2013; Oppenheimer, 2013). For Australia during the Pleistocene these considerations are particularly important, given the broad extent of arid and semi-arid environment and the comparative rapidity which which these environments were colonised. Strong evidence that, at the continental scale, these factors have always been important is provided by the clear relationship between archaeological sites dating to more than 30 Ka and permanent water of the present. The clear linkage of routes that are well watered from northern Australia, through the eastern semiarid and arid zone, to southeastern Australia and to the rocky arid centre of the continent, is a feature of the spatial analysis that is presented in this paper. The apparently rapid dispersal through much of the Australian interior is likely to be focused similarly along those routes that are well-connected as defined by permanent water points, given that permanent water points act as a focus for potential prey and other resources, and potentially, dispersal was driven further along these routes by the progressive depletion of local resources. Some regions that are apparently well-watered lie along potential dispersal routes, though they have not yet yielded evidence of early (or in some places, any) occupation in prehistory, is a corollary of this analysis. Among these regions are the Channel Country in southwest Queensland and the route identified in this study that runs south through the Northern Territory into the arid centre. The reasons for the lack of evidence of early occupation could be the result of poor preservation potential, from a limited archaeological survey, or from the identified route not actually being used. It is suggested by these alternatives, in turn, that the dataset, combined with other attributes of the terrain, to predictively identify areas that would be worth investigating for their potential to produce archaeological sites that have been unrecognised to date in order to delimit the routes of dispersal better.

According to Bird et al. the approach that s presented in this paper represents a progression from abstract ideas with regards to the filling of the continent to hypotheses that are testable grounded in data that is spatially explicit. It is suggested by this analysis that the permanent water that is provided during periodic inundation events-and provides- effective conduits for the movement of humans over thousands of kilometres through much, though not all, of the interior of the continent. The Australian continent currently is the only continent covered by the WHfS dataset. However, it is based on satellite imagery that has global coverage. Similar products elsewhere can be derived by the use of this imagery, which in turn enables progression beyond the identification of drainage networks and basins of potential routes of dispersal (Breeze et al., 2015; Breeze et al., 2016; Field, Petraglia and Lahr, 2007) to an interpretation that is more nuanced that considers the permanency of the water across this landscape. Therefore, the same approach could be used to develop and test models that seeks an explanation of the rapid dispersal of humans out of Africa (Finlayson, 2013; Oppenheimer, 2013; Breeze et al., 2015; Breeze et al., 2016; Field, Petraglia and Lahr, 2007) by way of similar hydro ecological networks that are interconnected periodically (O’Connell & Allen, 2015; Finlayson, 2013; Oppenheimer, 2013; Williams et al., 2013; Boivin et al., 2016).

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