Classification: Habitat Compulsion
Mnemonic: "Thin branch tree wobbling"
Specific Model: "Orang-utan-like" hand assisted bipedalism
Original Proponent(s): Thorpe et al 2007    
Basic Summary: Orang-utan like upright posture in thin branches was the precursor to human bipedalism.    
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. No text referred to this model specifically, as it is still very new.
Simple: #12 / 42 (59%)
Detailed: #6 / 42 (66%)
Discussion: Origin of human bipedalism as an adaptation for locomotion on flexible branches. Thorpe et al 2007
One of the most recent published ideas on bipedal origins made the front page of Science in 2007. The authors hypothesis was based on a long running study of wild orang-utan locomotor behaviour and their observations of "hand assisted" bipedalism whilst clinging ofr balance in relatively thin branches of trees. Perhaps most controversially, the idea promotes the idea that this form of bipedalism was a part of the locomotor repertoire of the ancestor of all the great apes, a very different view to the largely held mainstream view that the LCA was a quadruped.
One difficulty with this is explaining why gorillas and chimpanzees evolved away from this form of bipedalism and became knuckle-walkers on land whereas humans diverged from that so greatly. Thorpe et al do not offer a convincing adaptive explanation and one is left to conclude that they believe it was purely down to genetic drift.
Strengths:   The strengths of this model are its plausibility in an arboreal early paleocological setting, it's strong evidence in extant apes and in accounting for predation.
Weaknesses: .It's major weakness is that it does not provide an adaptive explantion for the evolution of human-like bipedalism or Pan-Homo-Gorilla divergence.    
1.1 Survival Value 3 (Poor) This model was judged porr by this criterion as little adaptive value was proposed for the evolution of human bipedalism.    
1.2 Sexual Selection 5 (Fair) This model was judged neutral by this criterion.    
1.3 Not Teleological 9 (Good) The model proposes a form of bipedalism that is very much ape-like.    
2.1 Improved Food Acquisition 6 (Fair) The model was judged slightly better than neutral by this criterion because its arboreal setting would provide a relatively rich setting for food.    
2.2 Accounts for Predation 9 (Good) The arboreal setting of this model gives an excellent response to questions on predation.    
2.3 Why Apes are not Bipedal 1 (Poor) The model is weak here in that it does not provide an adaptive explanation for Pan-Homo-Gorilla divergence,    
2.4 Extant Analogues 9 (Good) The study was based on a very comprehensive study of extant orang-utan behaviour.    
2.5 Applies to both sexes 9 (Good) This model applies equally to both sexes.  
3.1 Hominid Anomalies 7 (Good) Thorpe et al do try to explain some postcranial anatomical features of australopithecines and some earlier hominids.    
3.2 Fits Paleoecological Record 9 (Good) The currently growing concensus of a wooded habitat for early hominins fits this model very well.    
3.3 Precursor to Strider and knuckle Walker 5 (Fair) This model was judged neutral by this criterion.  
4.1 Extended Explanatory Power 1 (Poor) This model does not offer any other explanations of human traits.    
4.2 Compatibility 4 (Fair) This model was judged incompatible with most carrying models and complimentary to most arboreal ones.    
4.3 Falsifiable or Testable 3 (Poor) The predictions made by Thorpe et al are only those related to orang-utan behaviour itself.    
References Thorpe, S., Holder, R., Crompton, R. Origin of Human Bipedalism As an Adaptation for Locomotion on flexible Branches. Science 316:1328-1332, (2007).