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But for many species, this adaptation was not a solution.
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Many of the species of large animals (megafauna) such as mammoths, mastodons, ground sloths, and giant deer that roamed the Earth tens of thousands of years ago are now extinct. A major question is what caused these relatively sudden extinctions. In Australia, where giant kangaroos and other local megafauna species disappeared in the Late Pleistocene era (probably around 46,000 B.C.E.), some scholars have suggested that the arrival of humans on the continent had something to do with it. Others point toclimate changes.
In the interval leading up to the coldest and driest phase of the last glacial cycle, environmental pressures on large mammals presumably increased as Australia became more arid. Many people have suggested that this change at least contributed to the Late Pleistocene extinctions. The effects of the deteriorating ice-age climate on megafauna have been conceived in two major ways. First, the major impact could have come from an increase in climate variability, as might well have accompanied the transition from one climate pattern to another. The biologist A.R. Main suggested that an unstable climate would have been to the disadvantage of large-bodied species because of their generally low rates of population growth. A population of a small-bodied species knocked down by an extreme climate event, like a severe drought, might be able to recover before the next one hit; populations of large- bodied species, unable to rebound so quickly, could be driven down to very small numbers and ultimately to extinction by a series of extreme events.Main saw dwarfing (the evolutionary process by which a species' average body size decreases) as an adaptation to climate variability, because reduction in body size would be associated with earlier maturity and therefore a shorter generation time and faster recovery of populations. He suggested that the megafauna species that went extinct either lacked the evolutionary potential or were simply too large to reduce body size far enough to allow them to ride out the environmental variability of the Late Pleistocene.
Second, a general reduction in rainfall would have reduced the availability of drinking water and the productivity and nutritional quality of vegetation. Supporters of climate- driven extinction see large mammals as being most vulnerable to those changes because of their large requirements for food and water. The archaeologist Josephine Flood noted of the megafauna that "The one thing they all had in common was large size and a gigantic thirst," and J. M. Bowler remarked that "The progressive deterioration of climate in approach to the Last Glacial Maximum ... would have imposed nearly impossible stresses on animals with large energy requirements."
The biologist D. R. Horton has provided the most detailed account of just how these stresses might have caused extinction of large mammals. He argued that most of the extinct megafauna were species of woodland rather than truly arid habitats. Arid conditions expanded from the center of the continent toward the coasts in the last glacial cycle, and in this process woodland habitats were compressed and fragmented around the margins of the continent. As a result, formerly large and widespread populations of megafauna were confined to small isolated refuges where they were vulnerable to local extinctions. Within these refuges declining rainfall meant fewer sites had permanent surface water, essential for large-bodied species that needed to drink regularly. As some water points dried up, the distances separating remaining water points increased until animals that depended on access to free water were unable to travel between them. Populations of megafauna thus became tied to restricted zones of habitat within range of water holes. These zones of habitat were degraded, food supplies were exhausted by animals who for lack of water could not move away to use other areas, and populations died out. The repetition of these events at many locations eventually resulted in the total extinction of species. If the intensity of seasonality or between-year variability in rainfall also increased under the harsh conditions of the period known as the Last Glacial Maximum, occasional very deep droughts would have increased the pressures on small isolated populations of large mammals.
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