The great drying: 35,000-25,000 years ago
Following the widespread settlement of landscapes throughout Sahul the climate shifted, gradually but persistently towards cooler, drier conditions. The amount and reliability of rainfall gradually diminished, evaporation increased and, in many areas, there was a decline in the availability of permanent surface water, resulting in a progressively more arid interior and the expansion of desert areas.
The trend to cooler, drier climates began 45,000 years ago but the last glacial cycle intensified rapidly with the onset of a particularly cold, dry period approximately 30,000-35,000 years ago. At that time moisture became locked up as ice or snow at high latitudes and ocean levels lowered dramatically, revealing the continental shelf to a depth of almost 150 metres below the present sea level.[587] The extensive exposure of the continental shelf greatly increased the landmass available to humans and changed environments in which they lived. Reduced effective precipitation led to decreasing trees/shrubs in many regions and an increased distribution of grasslands.[588] Many inland areas were then located even further from the sea than they had been, creating increasingly dry, continental situations that compounded the effects of drying climates after 30,000-35,000 years bp. For instance, monsoonal rain was much reduced and consequently Lake Eyre, which had been more frequently filled prior to 35,000 years ago than it is today, dried and remained so until around 10,000 years ago.[589] At Lake Mungo and nearby lakes there were lower, fluctuating water levels and dunebuilding processes were activated.[590] Reduction and disappearance of lakes was a widespread pattern, though timing of drying varied geographically. Environmental reconfigurations during this period, specifically the reduced surface water and expanded deserts and grasslands, have been discussed as triggers of economic and social innovation.An example of the impact of these drying climatic trends was the development of economic and social systems suited to the extreme dry and cold conditions that emerged. It has been argued that the initial dispersion of people across inland Australia when conditions were comparatively good, especially rainfall and surface water availability, provided a fortuitous context in which economic and social systems could be adapted to inland resources.[591] Subsequent climatic shifts towards colder, drier landscapes were gradual and progressive, again providing a context that might facilitate human groups developing practices that suited the emerging conditions. There were multiple changes to cultural systems during this period, including amplification of the use of grinding technology, the development of more ecologically dedicated economic strategies and the strengthening of trade network systems.
In arid and semi-arid landscapes in southeastern Australia the archaeological record documents the gradual expansion of the variety of grindstones and the increasing emphasis on grinding technologies during this period of drying and enlargement of grasslands. This shift towards the use of plants such as grass seed and related processing technology that were labour intensive and expensive relative to nutritional returns has been thought to be a response to ecological stress and resource declines.[592] Although energy gains were low, processing seeds offered reliable returns that might have provided a buffer against deteriorating conditions.
As more extreme environmental conditions developed, specific foraging and social strategies were able to be modified, partly because groups resident in each environment were modifying economic practices based on an established, detailed knowledge of their local environment. For instance, in the increasingly severe desert landscapes created after 35,000 years ago resident foraging groups living in regions with no large water bodies employed dispersed patterns of settlement based on flexible but unspecific terrestrial economies, developing more desert-dedicated economic strategies.
Another example is the creation and gradual intensification of a dedicated economic system in the increasingly cold, alpine heath land of the Tasmanian uplands. Between 35,000 and 25,000 years ago Pleistocene hunters resident in those uplands targeted, almost exclusively, young and older Bennett's Wallaby, which were tethered to grassland patches, thereby exploiting a predictable meat resource. Hunters travelled to grassland patches, located and killed wallabies that they butchered or transported whole back to the limestone caves that have preserved archaeological traces of their activities.A different economic and social strategy that becomes visible at this time is long-distance trade of objects. Since the objects moved were typically small and of little or no practical use as food or tools, it is likely that the reason for transporting such things over many hundreds of kilometres was in order to maintain social relationships, as a token of connections rather than as a functional item. Some of the clearest examples of long-distance linkages signified by such imported objects are from the northwestern, Kimberley region. There objects such as crustacean carapace and marine shells, neither of which had dietary value, were transported/traded hundreds of kilometres to receiving groups far from the coast. Such specific trade goods, like marine shells, may have been sought after because they had value as symbols, perhaps particularly because as exotic items they were rare and costly. Additionally or alternatively the reciprocal relationships that underpin long-distance trade may have been maintained through or facilitated by public displays or gifts of ornaments that carry social meaning about the reciprocal commitment. This evidence indicates that broad inter-group political and economic networks were probably growing during this phase of climatic deterioration.
The construction and maintenance of inter-group political relationships may have been elaborated in particular ways during this period in response to relatively persistent climate change.
Widespread reductions of water availability and temperature, with accompanying decreases in environmental productivity, continued for millennia, and had the cumulative effect of destabilizing economic systems. One response to the changing resource base was the modification/expansion of resource use, as indicated by the greater emphasis on seed processing indicated by grindstones during this period. Invention, or greater employment of, technologies to exploit more marginal resources may have been, in some regions, an effective and successful economic strategy. However there were also some regions in which small shifts in processing technology and reconfiguration of foraging practices appear to have been inadequate in creating viable economies within the evolving, drying landscapes. In some instances the archaeological evidence indicates failure of the economic system and consequently human abandonment of local areas or even entire regions. For instance, human groups abandoned the long Cape Range Peninsula on the west coast, documented in the cultural hiatus in sites such as Mandu Mandu Creek, Jansz, and C99.[593]Local withdrawal from resource-poor or high-risk localities appears to have been the initial response to economic difficulties, and in instances where that did not produce viable economic conditions the entire region was abandoned. Geographic territorial contractions and abandonments became more frequent over time in this period, as climate change proceeded. While the loss of economic capacity was undoubtedly greater in the subsequent Last Glacial Maximum (see below), the onset of processes of substantial economic change, and of the struggle of economic systems to formulate viable foraging strategies, is evident more than 30,000 years ago. In this context the elaboration of inter-group social networks through the formalization of exchange patterns, signified in the exchange of rare/exotic items, reveals another response to changes in landscapes, one that perhaps exploits the advantages of maintaining relationships with adjoining groups in the expectation that access to neighbouring territory would help buffer resource fluctuations.