Classification: Efficiency
Mnemonic: Locomotor Decoupling
Specific Model: Locomotor Decoupling Hypothesis
Original Proponent(s): Adam Sylvester (2006)    
Basic Summary: The selective pressure that produced bipedalism was the need for effective suspensory and terrestrial movement. This testable hypothesis, termed the Decoupling Hypothesis, posits that bipedalism is an adaptation that enables the shoulder to maintain a high degree of mobility, a feature important to suspensory behaviors, in the face of significant demands for a high degree of stability, a feature important for highly effective terrestrial quadrupedism.     
Assessment: The Locomotor Efficiency Category was ranked 3rd out of 9 with 58% of texts surveyed referring to them. None, however, specifically referred to this hypothesis.
Simple: #30 (=4) / 42 (48%)
Detailed: #30 / 42 (52%)
Discussion: An sophisticated updated form of one of the original ideas on bipedal origins, the Hylobatian (gibbon-like) theory of Keith (1923), was published in 2006 by Adam Sylvester. Although it overlaps signifiicnatly with the hylobatian model, I have classified it here under the energy efficiency category here because Sylvester's thesis is based on human biomechanics much more than comparative anatomy with gibbons.

The paper starts by reviewing various ideas on bipedal origins and he picks out two questions that he regards as pivotal:

1) What might have been part of the daily locomotor repertoire of early hominins and their relative capacities for different locomotor adaptations, and
2) What were the selective pressure that caused hominin bipedalism to evolve?

For the later, Sylvester lists out some of the ideas under review here: Provisioning. vigilance, terrestrial efficiency, transporting food, transporting infants, transporting tools, effective tool use, behavioral displays, seed eating, increased foraging efficiency, feeding posture, hylobatian model and thermoregulation.

Sylvester spells out his objective clearly:
"The purpose of this paper is to explore a possible increase in fitness resulting from the relaxation of the selective pressure for shoulder stability, which would have accompanied the adoption of bipedalism."
Sylester 2006:582.

He defines the hypothesis thus:
"The DH (Decoupling Hypothesis) posits that hominin bipedalism is an adaptation that reduces the selective pressure for shoulder stability so that hominins could simultaneously achieve highly effective terrestrial and suspensory behaviors, a combination not accessible to quadrupeds because of a trade-off in shoulder stability and mobility. Negative affects on fitness related to changes in hind limb morphology are not included in the models, but are discussed."
Sylvester 2006:582.

Locomotor decoupling is where a complex locomotor repertoire becomes split into two or more components. Sylvester provides a few very good examples in evolution where it has occurred in the past such as the evolution of avian bird flight and argues that the evolution of human bipedalism is a such example, where a previously complex locomotor repertoire has decoupled into a dependence on bipedalism with a mobile shoulder.

Basically, Sylvester sets out a theoretical case that primate shoulder traits include conflicting selection pressures for mobility and stability and that bipedalism would reduce the selection on traits requiring stability. The study was purely theoretical, using a computer simulation, but he proposed that a baboon-like model where open terrestrial locomotion interspersed with regular arboreality might provide the right kind of mix to select for shoulders with human levels of mobility and reduced stability due to increased bipedality. However, whereas baboons tend to move in trees above branch, apes (due to their relatively large size) tend to move below branch.

Sylvester tests this through computer simulations of habitats and provides some falsifiable predictions about primate locomotion and human evolution.
 Strengths: The strengths of Sylvester's idea is that it is not teleological, that it provides a plausible precursor to human walking and knuckle walking and that it attempts to provide some falsifiable predictions.    
Weaknesses: It's weaknesses are that it is purely theoretical and does not provide any concrete idea which helps hominin survival value, improved food acquirement etc.    
1.1 Survival Value 3 (Poor) The "decoupling hypothesis" does not provide any improvement for survival value other than some rebalancing of biomechanical stresses for support from the shoulder down to the lower limbs.    
1.2 Sexual Selection 5 (Fair) This model was judged neutral by this criterion.    
1.3 Not Teleological 7 (Good) The decoupling hypothesis is a plausible Darwinian model for a shift in hominin locomotor  repertoire that does not require any anthropocentric assumptions.    
2.1 Improved Food Acquisition 3 (Poor) The "decoupling hypothesis" does not provide any improvement for food acquisition.    
2.2 Accounts for Predation 5 (Fair) This model was judged neutral by this criterion.    
2.3 Why Apes are not Bipedal 3 (Poor) Sylvester doesn't provide a strong ultimate explanation for the divergence of hominin locomotion, only a proximal one.    
2.4 Extant Analogues 5 (Fair) This model was judged neutral by this criterion.    
2.5 Applies to Both Sexes 9 (Good) This model applies equally to both sexes.    
3.1 Hominid Anomalies 2 (Poor) Sylvester does not attempt to explain australopithecine postcranial anatomical anomalies.    
3.2 Fits Paleoecological Record 5 (Fair) This model was judged neutral by this criterion.    
3.3 Precursor to Strider and knuckle Walker 7 (Good) Sylvester's ideas provide a plausible locomotor repertoire which predates both human waling and knuckle walking.    
4.1 Extended Explanatory Power 3 (Poor) The Locomotor decoupling hypothesis doesn't attempt to provide an explanation for anything more than bipedal origins.    
4.2 Complimentary 5 (Fair) This model was judged compatible with almost all models and complementary to those involving a degree of arboreality.    
4.3 Falsifiable or Testable 7 (Good) Sylvester does provide a few falsifiable predictions and his model is framed in a rigorous scientific way.    
References Sylvester, A. Locomotor decoupling and the origin of hominid bipedalism. Journal of Theoretical Biology 242:581-590, (2006).
Sylvester, A., Kramer, P. Brief Communication: Stand and Shuffle: When Does it Make Energetic Sense?. American Journal of Physical Anthropology 135:484-488, (2008).