<P> There are a variety of ideas which promote a specific change in behaviour as the key driver for the evolution of hominid bipedalism . For example, Wescott (1967) and later Jablonski & Chaplin (1993) suggest that bipedal threat displays could have been the transitional behaviour which led to some groups of apes beginning to adopt bipedal postures more often . Others (e.g. Dart 1925) have offered the idea that the need for more vigilance against predators could have provided the initial motivation . Dawkins (e.g. 2004) has argued that it could have begun as a kind of fashion that just caught on and then escalated through sexual selection . And it has even been suggested (e.g. Tanner 1981: 165) that male phallic display could have been the initial incentive, as well as increased sexual signaling in upright female posture . </P> <P> The thermoregulatory model explaining the origin of bipedalism is one of the simplest theories so far advanced, but it is a viable explanation . Dr. Peter Wheeler, a professor of evolutionary biology, proposes that bipedalism raises the amount of body surface area higher above the ground which results in a reduction in heat gain and helps heat dissipation . When a hominid is higher above the ground, the organism accesses more favorable wind speeds and temperatures . During heat seasons, greater wind flow results in a higher heat loss, which makes the organism more comfortable . Also, Wheeler explains that a vertical posture minimizes the direct exposure to the sun whereas quadrupedalism exposes more of the body to direct exposure . Analysis and interpretations of Ardipithecus reveal that this hypothesis needs modification to consider that the forest and woodland environmental preadaptation of early - stage hominid bipedalism preceded further refinement of bipedalism by the pressure of natural selection . This then allowed for the more efficient exploitation of the hotter conditions ecological niche, rather than the hotter conditions being hypothetically bipedalism's initial stimulus . A feedback mechanism from the advantages of bipedality in hot and open habitats would then in turn make a forest preadaptation solidify as a permanent state . </P> <P> Charles Darwin wrote that "Man could not have attained his present dominant position in the world without the use of his hands, which are so admirably adapted to the act of obedience of his will". Darwin (1871: 52) and many models on bipedal origins are based on this line of thought . Gordon Hewes (1961) suggested that the carrying of meat "over considerable distances" (Hewes 1961: 689) was the key factor . Isaac (1978) and Sinclair et al. (1986) offered modifications of this idea, as indeed did Lovejoy (1981) with his "provisioning model" described above . Others, such as Nancy Tanner (1981), have suggested that infant carrying was key, while others again have suggested stone tools and weapons drove the change . This stone - tools theory is very unlikely, as though ancient humans were known to hunt, the discovery of tools was not discovered for thousands of years after the origin of bipedalism, chronologically precluding it from being a driving force of evolution . (Wooden tools and spears fossilize poorly and therefore it is difficult to make a judgment about their potential usage .) </P> <P> The observation that large primates, including especially the great apes, that predominantly move quadrupedally on dry land, tend to switch to bipedal locomotion in waist deep water, has led to the idea that the origin of human bipedalism may have been influenced by waterside environments . This idea, labelled "the wading hypothesis", was originally suggested by the Oxford marine biologist Alister Hardy who said: "It seems to me likely that Man learnt to stand erect first in water and then, as his balance improved, he found he became better equipped for standing up on the shore when he came out, and indeed also for running ." It was then promoted by Elaine Morgan, as part of the aquatic ape hypothesis, who cited bipedalism among a cluster of other human traits unique among primates, including voluntary control of breathing, hairlessness and subcutaneous fat . The "aquatic ape hypothesis", as originally formulated, has not been accepted or considered a serious theory within the anthropological scholarly community . Others, however, have sought to promote wading as a factor in the origin of human bipedalism without referring to further ("aquatic ape" related) factors . Since 2000 Carsten Niemitz has published a series of papers and a book on a variant of the wading hypothesis, which he calls the "amphibian generalist theory" (German: Amphibische Generalistentheorie). </P>

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