Locomotor decoupling and the origin of hominin ..

rather than increasing from low to high as predicted by the arid-bipedalism hypothesis
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Bipedalism in Jerboas (Rodentia: Dipodoidea): ..

As an example, the appearance of some predator should no longer have reminded those transitional “bipeds–quadrupeds” to escape using all four limbs in a galloping manner: if this would still have been faster than a bipedal flight, full bipedalism would certainly never have evolved. The question a promising hypothesis must explain is which combination of selective pressures overcame the threshold, so that habitual standing upright and continuous bipedal walking started to pay off.

the arid-bipedalism hypothesis
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Definition of bipedalism in the English dictionary

Except for the orang-utan terminal branch-feeding locomotion, which happens in a rather slow fashion, bipedality has always been combined with a terrestrial way of life. However, all terrestrial monkeys—as well as Proconsul without regard for its degree of terrestriality—have front and hind limbs of similar lengths. This shows that terrestrial quadrupedalism does not favour longer legs, which is in agreement with Witte et al. (). On the other hand, although the largely bipedal australopithecines still possess rather short hind limbs, the Amphibian Generalist Theory or Shore Dweller Hypothesis offers several functional, i.e., selective, factors for the evolutionary beginnings of longer hind extremities from the very start of upright locomotion and posture (c.f. Niemitz , ). For some time, there must have been a trade-off, for longer bipedal wading hind extremities and relatively shorter ones for terrestrial quadrupedalism in combination with the possibility of a fast vertical escape up into the branches. The specific power of each of these selective factors is hitherto unknown. In any case, wading may offer advantages for longer legs that may well have contributed somehow in this process (Fig. ).

Recommended Citation. Sylvester, Adam David, "The Decoupling Hypothesis: A new idea for the origin of hominid bipedalism. " PhD diss., University of Tennessee, 2006.
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Wading was an appropriate trigger not only to stand up but also forced the primate to walk on. It seems likely that habitual bipedalism began not long after the separation from the gorilla and chimpanzee clade(s). From that time onwards, throwing could be evolved with free upper extremities much more successfully than before. Selective factors related to the reduction of incoming solar radiation became effective. Endurance running and adaptations to carry tools (like weapons) started their evolutionary improvements. If these processes took about 4 Ma, the wading hypothesis is consistent with a rather perfect bipedal anatomy as shown, e.g., in Homo ergaster (WT 15000), about 1.6 Ma ago. In this way, many of the hypotheses competing in the past may be harmonised, as some of them have yielded important contributions to the understanding of the evolution of the human habitual upright gait.

The Evolution of Bipedalism in Jerboas (Rodentia: Dipodoidea): Origin in ..
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The Decoupling Hypothesis The considerations ..

During the last 4 years, two articles appeared which discussed the evolution of the upright human posture and locomotion from a perspective different from the one chosen here. While Preuschoft () concentrates on biomechanics, Crompton et al. () focus on paleoanatomy. Moreover, two new explanations for the evolution of human bipedalism appeared (Sylvester ; Skoyles ), which will also be discussed later in this text. But still, most of the recently emerged theories did not automatically devalue former theories in part or as a whole, and many of them produced respectable grounds, or at least partly, uncontested arguments. These hypotheses may serve as constructive elements for the synthesis of a conciliatory, updated theoretical construction. After a theoretical outlook and a discussion of some Miocene and later hominoid and hominid fossils, this article presents an evaluation and discussion of the more influential, and finally, the more recent hypotheses, aiming to reconcile quite a number of aspects of the various theories.

The evolution of the upright posture and gait?a review ..

During the last century, approximately 30 hypotheses have been constructed to explain the evolution of the human upright posture and locomotion. The most important and recent ones are discussed here. Meanwhile, it has been established that all main hypotheses published until the last decade of the past century are outdated, at least with respect to some of their main ideas: Firstly, they were focused on only one cause for the evolution of bipedality, whereas the evolutionary process was much more complex. Secondly, they were all placed into a savannah scenario. During the 1990s, the fossil record allowed the reconstruction of emerging bipedalism more precisely in a forested habitat (e.g., as reported by Clarke and Tobias (Science 269:521–524, ) and WoldeGabriel et al. (Nature 412:175–178, )). Moreover, the fossil remains revealed increasing evidence that this part of human evolution took place in a more humid environment than previously assumed. The Amphibian Generalist Theory, presented first in the year 2000, suggests that bipedalism began in a wooded habitat. The forests were not far from a shore, where our early ancestor, along with its arboreal habits, walked and waded in shallow water finding rich food with little investment. In contrast to all other theories, wading behaviour not only triggers an upright posture, but also forces the individual to maintain this position and to walk bipedally. So far, this is the only scenario suitable to overcome the considerable anatomical and functional threshold from quadrupedalism to bipedalism. This is consistent with paleoanthropological findings and with functional anatomy as well as with energetic calculations, and not least, with evolutionary psychology. The new synthesis presented here is able to harmonise many of the hitherto competing theories.

Study 69 Evolution of Flight flashcards from Anna W

Our ancestors would have probably become extinct if they were not adapted to their specific temporal environment, where they developed their bipedal habits including the corresponding transitional behavioural constraints. This is an important statement, because the change from a quadrupedal to a bipedal forerunner of extant humans has most often been considered from the perspective that an upright stance might have offered specific favours or positive selective values. Hunt (), however, discussing “less than optimal adaptation to bipedal locomotion” in australopithecines, summarises: “Locomotor inefficiency (my italics) supports the hypothesis that bipedalism evolved more as a terrestrial feeding posture than as a walking adaptation.” Other authors emphasise that, in the first place, the adoption of an erect posture for a quadrupedal primate would reduce speed and agility (Taylor and Rowntree ; Lovejoy ; Niemitz ).