With quadrupedalism and the slow rate of food acquisition, an equilibrium would have resulted balancing off the benefits of inter-group competition and intra-group scramble competition. In other words, the evolutionary trend would slide into a log jam where groups would fare better with more members, but they would not have ready access to the resources to support any additional members.

In order to blast their way out of this log jam, our hominoid ancestors resorted to two basic strategies. Firstly, they could maintain quadrupedalism and evolve a fission-fusion system, or alternatively, they could evolve bipedalism and also larger group size. The former route was taken by the ancestors of chimpanzees. The latter option was selected by our ancestors. The tendency to aggregate into larger groups under certain circumstances has been observed among other primates as well, albeit resulting from a trait other than bipedalism. For example, a population of vervet monkeys developed larger group sizes in response to a shift toward a drier habitat, and subsequently outcompeted their surrounding groups through inter-group scramble competition. (Cheney & Seyfarth, 1987; Isbell et al., 1990; cited in Isbell & Young, 1996).

The Isbell and Young Hypothesis has been criticized on a number of grounds by such paleoanthropologists as Kevin Hunt, who developed the Postural Feeding Hypothesis.⁶ He argues that while a fully bipedal individual may gain a locomotory advantage over a quadruped, a facultative biped will not. He points out that the intermembral index of approximately one hundred percent for chimpanzees and proto-hominid ancestors would not have facilitated efficient bipedalism. However, according to an American Journal of Physical Anthropology paper, “These and other results from the recent literature suggest that increased lower limb length provided no selective advantage in locomotion and other explanations should be sought.” (Webb, 1996). Therefore, fore- to hindlimb proportions as a major factor in early hominid evolution should best not be accepted.⁷

Notes:
6a       In a nutshell, Hunt’s Postural Feeding Hypothesis states that the australopithecine bipedal adaptation is derived from a chimp-like tendency to assume an erect posture while performing a repetitive feeding motion. It would be much more energetically efficient for an organism to maintain such a posture than to pick a fruit while standing and then sit back down on the branch to ingest it and subsequently repeat the process. Owen Lovejoy has pointed out that despite the inordinate amount of time chimpanzees spend feeding bipedally in trees, they never acquire obligatory bipedalism (pers. comm.). Furthermore, Brian Richmond and David Strait have argued in a relatively recent Nature article that the last common ancestor was a knuckle-walker based on analyses of Australopithecus anamensis wrist fossils.
6b       Other objections have been raised, for example, that the behavior of extant chimpanzees cannot be used as an adequate basis for inferring paleoprimate behaviors. However, Isbell and Young have made sufficiently clear in their paper that they have taken several primate behavioral models into consideration for drawing their behavioral sketch of the last common ancestor.
7       In fact, Isbell and Young pointed out that a locomotory adaptation in proto-hominids equally efficient asin Homo sapiens is not necessary for their hypothesis to be viable. It has also been argued that the australopithecine morphological configuration does not allow for efficient arboreal or quadrupedal behaviors and that they were chiefly or perhaps completely bipedal. This view has been championed by such paleoanthropologists as Owen Lovejoy, Bruce Latimer, Daniel Gebo, Tim White, and Donald Johanson.

       

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