Classification: Habitat Compulsion
Mnemonic: Hylobatian
Specific Model: Descent from 'Hylobatian' ancestors
Original Proponent(s): Keith (1923), Prost (1980)    
Basic Summary: Gibbons are largely bipedal when on the ground, perhaps our early bipedal ancestors were "hylobatian".    
Assessment: Popularity: This arboreal model of bipedal origins was broadly classified udner "non-wading habitat compulsion" and as such was found to be the 6th most popular of 9 categories with 25% of the reviewed texts referring to it.
Simple: #21 (=3) / 42 (52%)
Detail: #9 (=2) / 42 (62%)
Discussion: One of the earliest post-Drawinian ideas of the evolution of human bipedality is the idea that we evolved from gibbon-like apes that already had an upright posture through arboreal locomotion including brachiation and vertical climbing. The original proponent was Arthur Keith of the Natural History Museum, London, and he called it the "Hylobatian" theory. The idea was conceived when little was actually known about ape phylogeny. Darwin's view that humans were closer to chimpanzees and gorillas than other apes was respected but not universally accepted.

As the last century revealed more evidence in favour of a Pan-Homo-Gorilla clade and it was realised that Hylobates were more distant from humans that Pongo, the Hylobatian idea of human bipedal origins increasingly fell out of favour until iw was ressurected to some extent by Tuttle's "upwardly mobile hypothesis" and by Prost..
Origin of Bipedalism Prost (1980)
Prost compared in detail locomotor limb patterns of human and chimpanzee walking and climbing using a method which allowed limb displacements during typical locomotor patterns to be recorded cinematographically and then to be measured, represented and thus compared. It was concluded that “field patterns of human and ape bipedalism are so different that it is doubted whether the nonhuman type could ever have been a precursor of the human type.” Prost (1980:175.)

Prost however did find some similarities (more specifically, an overlap) in limb patterns in human bipedalism and chimpanzee vertical climbing. This led him to conclude that “the vertical climber, lifting against gravity, would have had the musculature to position and hold the bipedal NAC [human standing neutral action centre] arrangement, its ranges of motion overlapping that needed in bipedalism.” Prost (1980:188) This suggested that a predominance of "bipedal" traits is no guarantee of "bipedalism", Prost argued that whenever a fossil does not reproduce the exact morphological combinations found within extant taxa, it is likely to be an animal whose locomotor habits differed from those seen among the living. Hence it was concluded that as “the australopithecines look ca. 80% bipedal in their morphology but that their "bipedal' traits are probably climbing traits.” Prost (1980:186) Thus, Prost reconstructed the australopithecines “as pre-eminent climbers and facultative bipeds, adapted to life in the trees by climbing, but moving on the ground, in their silhouettes, just as humans do.” Prost (1980:187)

The paper did not include much discussion as to the possible adaptive reasons for a shift from this type of proto-bipedal locomotion towards human-like obligate bipedalism but it was speculated that “the shift from facultative bipedalism to pre-eminent bipedalism occurred when some australopithecines became exclusively terrestrial exploiters. With a shift to pre-eminent bipedalism, climbing adaptations would have been lost and bipedal refinements acquired” Prost (1980:187) and that “travel between trees, and between concentrations of trees around water resources, could have triggered a steady evolution of greater terrestrial exploration and increased terrestrial dietary supplements, an evolutionary path leading finally to an abrogation of tree living altogether” Prost (1980:188.)

This model overlaps considerably with Tuttle's "upwardly mobile" hypothesis and Thorpe et al's "Hand-assisted arboreal" bipedalism ideas.
 Strengths: Like the other arboreal models the Hylobation idea's strengths are in not being teleological, having good evidence from extant apes and being based on a paleoecology that has growing acceptance.    
Weaknesses: A number of criticisms of this model can be made.

In positing this specific putative locomotor pattern labelled ‘vertical climber’, nothing really new is being proposed. Most extant primates are vertical climbers. As the specific behaviour being cited is "vertical climbing" as opposed to just ‘climbing’, there is an implication that the locomotion being proposed was for a primate was rather large, indeed too large to climb smaller branches that would be more horizontal.

Prost’s model is also contradicted by the fossil evidence. His argument that australopithecines were likely to be vrtical climbers because their anatomy looked "only 80% bipedal" in their morphology is contradicted by the fact that no extant primate, including those that regularly practice vertically climbing, have anatomies anything like australopithecines. According to the logic of the argument, this alone should preclude vertical climbing from consideration.
1.1 Survival Value 3 (Poor) This model provides no extra component that improves survival value over ancestral forms of hominin.    
1.2 Sexual Selection 5 (Fair) This model was judged neutral with respect to this criterion.    
1.3 Not Teleological 9 (Good) This model is based purely on extant primate behaviour.    
2.1 Improved Food Acquisition 6 (Fair) This model was judged slightly higher than neutral with respect to this criterion because of it's close proximity to relatively food-rich woodland.    
2.2 Accounts for Predation 9 (Good) This model accounts for predation as well as any other.    
2.3 Why Apes are not Bipedal 1 (Poor) This model lacks any plausible argument as to why only humans adopted obligate bipedalism among the great apes.    
2.4 Extant Analogues 8 (Good) This model was derived from observations of chimpanzee climbing behaviour.    
2.5 Applies to Both Sexes 9 (Good) This model applies to both sexes.    
3.1 Hominid Anomalies 3 (Poor) Although Prost makes the case that australopithecine anatomy was only 80% bipedal and claims that the differences are explicable in terms of vertical climbing, the fact that no extant vertically climbing primate has a similar anatomy forced me to judge this model poorly here.    
3.2 Fits Paleoecological Record 9 (Good) The model is based on a wooded paleohabitat that is supported by a growing consensus.    
3.3 Precursor to Strider and knuckle Walker 6 (Fair) This model was judged slightly better than neutral with respect to this criterion because Prost makes a case that vertical climbing overlaps human bipedalism to an extent.    
4.1 Extended Explanatory Power 0 (Poor) The model offers explanations for no other human traits.    
4.2 Complimentary 4 (Fair) The mosel was judged complimentary to other arboreal models and contradictory tocarying models and those based in open habitats.    
4.3 Falsifiable or Testable 2 (Poor) Prost's (1980) paper lacks falsifiable predictions for his hypothesis.    
References Keith, A. (1923) Man's posture: it's evolution and disorders.
Prost, J H (1980). Origin of Bipedalism. American Journal of Physical Anthropology Vol:52 Pages:175-189