Classification: Efficiency
Mnemonic: Biomechanical inevitability
Specific Model: Biomechanical Inevitability
Original Proponent(s): Reynolds (1985)    
Basic Summary: Shuffling bipedalism whilst moving from tree to tree was the precursor to human bipedality.    
Assessment: Popularity: The general efficient model classification was ranked 3rd most popular (out of 9) of the texts reviewed. 11% referred to this idea specifically.
Simple: #36 / 42 (39%)
Detailed: #35 / 42 (47%)
Discussion: “Stresses on the Limbs of Quadrupedal Primates” Reynolds (1985)  
Along similar lines to the energy efficiency model of Rodman & McHenry is the view that bipedalism is really just the logical conclusion of a trend towards increasingly orthograde posture in the primates. Reynolds’ paper in the American Journal of Physical Anthropology gave detailed evidence showing that primates, even when quadrupedal, bare a larger percentage of their body weight through their hind limbs than is the norm amongst mammals.

The basic argument is that where animals use their upper limbs for other actions than quadrupedal weight bearing locomotion, they are likely to take more of their body weight on the lower limbs and that this trend would ultimately favour the evolution of bipedalism. Reynolds argued that whereas non-hominoid primates typically possess narrow thoraces and laterally positioned scapulae, which have glenoid fossae that face largely ventrally, hominoids have relatively broad flat thoraces and dorsally placed scapulae with glenoid fossae that face more cranially and laterally. Consequently, vertically directed forces on the forelimbs of hominoids during quadrupedalism have a large component that shears across the glenohumeral joint. Reynolds (1985).

His study captured force plate data for the ground reaction forces during quadrupedal locomotion of eight primates, from five species, to estimate the distribution of the support of the body weight in order to test the hypothesis that those primates with more dorsally orientated scapulae should decrease the forces on their forelimbs during quadrupedal locomotion. The data presented (Reynolds 1985:357) did not support this hypothesis but it was shown that primates generally do support most of their body weight on their hind limbs than do typical cursorial mammals. Of the species tested, Ateles showed the greatest hind limb support at around 70-72% with Pan at around 54%-56%, both increasing with higher speeds of locomotion. It was found that there was significant variation in the weight supported by the forelimbs and proposed that this variation correlated with the locomotor adaptations of the primate, be it brachiation or vertical climbing.

Reynolds' conclusion was that
“the occurrence of bipedalism in primates represents the extreme expression of the tendency in primates to reduce the compressive forces on their forelimbs”
Reynolds (1985:351.)

This was characterised by Rose (1991:41) as ‘biomechanical inevitability’ although Reynolds never used that term himself.

One of the strongest points argued was that “t[T]he stability of a joint is maintained by congruence of the articulating surfaces, strength of the joint capsule, and muscles spanning the joint. Mobility is obtained via laxity of the capsule and lack of congruence of the articulating surfaces. Thus mobility is generally obtained at the expense of strength and stability. Consequently, the forelimb joints of primates are likely to be less stable than those of most other mammals.” Reynolds (1985:359.)

As primates, particularly large ones, have evolved the ability for various degrees of vertical climbing and brachiation, it is clear that this evolutionary trend must have reduced the stability of the forearms when used in ‘standard’ cursorial quadrupedalism. Trends do seem to be identifiable, also, within those primates which are quadramanous climbers. “The rank order of these stresses suggests a source of variation. Hylobatids are bipedal; spider monkeys support 70% of their weight on their hind limbs when quadrupedal; common chimpanzees support roughly 55% of their weight on their hind limbs and also decrease the stresses on their forelimbs by other means at higher speeds, and the limited data on orang-utans indicate that they support approximately 50% of their weight on their hind limbs.” Reynolds (1985:360) However, the main hypothesis of the study, that primates with more dorsally orientated scapulae should show this trend more, was not actually borne out by his data.

If one is postulating the evolution of hominin bipedalism as ‘biomechanical inevitability’ (Rose 1991), then this view is not supported by the findings here either, because the taxon closest to humans, Pan, was found to be significantly less bipedal than Hylobatids and Ateles. Data from Pongo and Gorilla, not part of this study, would contradict that proposed trend even more as Pongo (more distant) are more bipedal than Pan, whereas Gorilla (less distant) is less bipedal.

Reynolds’ paper strongly supports the assertion that the high mobility of the upper arm, which must have evolved in both old world and new world monkeys, is a strong factor in predisposing some primates to move with more of their body weight on their hind limbs. Although this strongly implies that arboreality was a necessary prerequisite to hominin bipedalism, it seems insufficient to explain it alone, as only Hylobatidae and Homo amongst the primates could be said to exhibit bipedalism whereas our closest relatives, Pan and Gorilla, rarely do.
Strengths: Reynolds' paper does provide good evidence in favour of arboreal models of evolution, in that the most handlimb dominated forms of locomotion are found in species tht are most committed bipeds.  
Weaknesses: Anomalies in these data contradict Rose's label of Reynolds' work as "biomechanical inevitability". There are other weakesses in the idea too, notably a lack of proposed selection pressure and a distinct teleological slant to the argument.  
1.1 Survival Value 3 (Poor) No factors are included in thie idea that might actually lead to improved survivability in more bipedal hominins.  
1.2 Sexual Selection 5 (Fair) This model was judged neutral in this respect.  
1.3 Not Teleological 0 (Poor) This model is one of the most anthropocentric in that it assumes that humans are the end of some natural trend towards increasing bipedality.  
2.1 Improved Food Acquisition 5 (Fair) This model was judged neutral in this respect.  
2.2 Accounts for Predation 5 (Fair) This model was judged neutral in this respect.  
2.3 Why Apes are not Bipedal 3 (Poor) This model was udged poorly by this criterion as it's teleological message is not very helpful in answering this question.  
2.4 Extant Analogues 5 (Fair) This model was judged neutral in this respect.  
2.5 Applies to Both Sexes 9 (Good) This model applies equally to both sexes.  
3.1 Hominid Anomalies 4 (Fair) This model was judged slightly worse than neutral by this criterion as it did not refer to any eraly hominin attributes.  
3.2 Fits Paleoecological Record 5 (Fair) This model was judged neutral in this respect.  
3.3 Precursor to Strider and knuckle Walker 5 (Fair) This model was judged neutral on this criterion.  
4.1 Extended Explanatory Power 0 (Poor) The idea does not attempt to explain anything else about human evolution.  
4.2 Complimentary 5 (Fair) This model was judged compatible (no more, no less) with most other models.  
4.3 Falsifiable or Testable 3 (Poor) Although Reynolds' paper did provide a lot of empirical data, most of it contradicted the assertion that there wwas some kind of phylogenetic trend towards increasing bipedalism in the human lineage.  
References Reynolds, T R (1985). Stresses on the Limbs of Quadrupedal Primates. American Journal of Physical Anthropology 67 :351-362.
Rose, M. (1991). The Process of Bipedalization in Hominids. In: Senut, B., Coppens Y; (Eds) Origine(s) de la bipedie chez les hominides. In: Coppens, Y., Senut, B. (eds.), (1991). Origine(s) de la bipedalie chez les hominides. CNRS (Paris)