Classification: Habitat Compulsion
Mnemonic: "Upwardly Mobile"
Specific Model: "Upwardly Mobile" Hypothesis
Original Proponent(s): Tuttle 1975, 1981    
Basic Summary: An updated version of the Hylobatian hypothesis - that humans evolved from gibbon-like briachiators - with the addition of a vertical climbing component.    
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. 14% of reviewed texts referred to this idea specifically.
Simple: #7 (=2) / 42 (63%)
Detailed #6 (=3) /42 (66%)
Discussion: Evolution of Hominid Bipedalism and prehensile capabilities Tuttle (1981)
Tuttle's paper is perhaps the clearest exposition of this idea. He hypothesises our 'gibbon-like' ancestors to have been relatively small creatures that were especially adapted for the vertical climbing on tree trunks and vines and for bipedalism on horizontal boughs but they were, according to Tuttle not brachiators like gibbons are today and no more disposed towards suspensory behaviour than are modern chimpanzees and bonobos and argued that their ore modern specialisations like richotal arm-swinging were not practiced at all.

According to his scenario, bipedalism preceded the emergence of the Hominidae. These were bipeds that, according to Tuttle, stood with full extension of the knee joints and walked with greater extension of the lower limbs than is common in non-human primates. Tuttle is confident that the fossil record supported the idea.

Tuttle hypothesised these "hylobatians" moving like this:
"Vertically climbing on tree trubks and vines and bipedalism on horizontal boughs were conspicuous components of their locomotor repertoire. They commonly stood bipedally while foraging in trees and employed bipedalism during intraspecific displays. Short bursts of bipedal running and hindlimb propelled leaps may have been important for the manual capture of insects and small vertebrates with which they supplemented their vegetable fare."

Tuttle (1981:90).

This model overlaps greatly with Keith's original 'hylobatian' ideas as well as Thorpe et al's "Hand Assisted" arboreal bipedalism model.
 Strengths: Arboreal models are at their strongest in their evidence in extant apes and in their accordance with the growing consensus on the paleocological habitat for early hominins.    
Weaknesses: The model is weakest in explaining Pan-Homo-Gorilla divergence.    
1.1 Survival Value 3 (Poor) Vertical climbing is not judged as a valuable extra component of behaviour that increases survival value.    
1.2 Sexual Selection 5 (Fair) This model was judged neutral by this criterion.    
1.3 Not Teleological 9 (Good) This model is based on behaviour of extant primates and not humans.    
2.1 Improved Food Acquisition 6 (Fair) This model was judged better than neutral by this criterion because it places hominins in relatively food-rich wooded habitats.    
2.2 Accounts for Predation 9 (Good) The "hylobation" model is among the best with regard to predation.    
2.3 Why Apes are not Bipedal 2 (Poor) As with other arboreal models, this one is poor in explaining Pan-Homo_Gorilla divergence.    
2.4 Extant Analogues 9 (Good) This model is largely based on extant ape behaviour.    
2.5 Applies to Both Sexes 9 (Good) This model applies to both sexes.    
3.1 Hominid Anomalies 8 (Good) Tuttle argues that australipithecines' curved pedal phalanges, robust hallux, broad sacrum, and shortness and lateral orientation of the iliac blades are evience for the model.    
3.2 Fits Paleoecological Record 8 (Good) This model fits the currently accepted paradigm for early hominin evolution.    
3.3 Precursor to Strider and knuckle Walker 4 (Fair) Although vertical climbing would place hominins in an upright posture, it is not judged as a close precursor to human walking.    
4.1 Extended Explanatory Power 2 (Poor) This model doesn't explain much else other than bipedal origins.    
4.2 Complimentary 4 (Fair) This model was judged complimentary to othr arboreal models and contradictory to most carrying models and those set in open habitats.    
4.3 Falsifiable or Testable 5 (Fair) Tuttle makes no falsifiable prediction for his model other than arguing that hominins in the fossil record support it.    
References Tuttle, R H (1975). Primate morphology and evolution. Mouton Publishers (The Hague)
Tuttle, R H. (1981) Evolution of Hominid Bipedalism and prehensile capabilities. Philosopical Transactions of the Royal Society London 292 (1057):89-94.