Classification: Efficiency
Mnemonic: Long Distance Walking
Specific Model: Slow, long-distant walking efficiency
Original Proponent(s): Rodman & McHenry (1980)    
Basic Summary: Bipedalism evolved as an adaptation for locomotor efficiency primarily long distances at slow speeds.    
Assessment: Popularity: The general efficient model classification was ranked 3rd most popular (out of 9) of the texts reviewed. 47% referred to this idea specifically making it the 3rd (equal with Wheeler's thermoregulatory hypothesis) most popular model.
Simple: #15 (=4) / 42 (56%)
Detailed: #20 (=5) / 42 (55%)
Discussion: Bioenergetics and the Origin of Hominid Bipedalism. American Journal of Physical Anthropology Vol:52 Pages:103-106 Rodman, PS & McHenry, H. (1980).

The remarkably straightforward idea that hominid bipedality was adopted simply because it is more energetically efficient than the quadrupedalism of their immediate ancestors, proposed by Rodman & McHenry (1980), is very compelling in that offers a plausibly strong motive for chimp-like apes to move from a quadrupedal to a bipedal mode of locomotion. It is backed up by several strands of evidence.

Most significantly, human bipedality does appear, in some situations, to be a slightly more efficient form of locomotion than general quadrupedalism. Taylor et al (1970) compared the energy cost of movement in several different animal species, of different weights, and at different velocities and found that there was a predictable relationship between those factors. Although at running speeds humans were found to be much less (about twice as) efficient that a general quadruped, at slower walking, speeds they turn out to be slightly more efficient. Furthermore Taylor & Rowntree (1973) found that common chimpanzees and capuchin monkeys used more oxygen (148% and 132%, respectively) than would be predicted by this relationship and that there was no significant energetic difference, in those species, between the cost of moving bipedally and quadrupedally. Rodman & McHenry argued from this that human bipedalism is a clearly a big energetic advantage compared to hominoid quadrupedalism and that “there was no energetic rubicon for an early hominid to cross.”

This model is based on two assumptions:

a. that the earliest bipedal apes were rather chimp-like and certainly quadrupedal on the ground, and
b. that they started moving bipedally, thereby gaining in energy efficiency immediately, without any significant rubicon to cross.

The ‘quadrupedal last common ancestor’ assumption has long been held due to arguments based more upon parsimony (three out of four of the Hominoidae families are quadrupedal) than fossil evidence, which is all but non-existent. It is an assumption that appears even less certain today than it did in 1980 after recent new fossil finds and data from molecular evidence (more on this later). The ‘no energetic rubicon’ assumption also has a major difficulty: If it really involves no extra effort for extant African apes to move bipedally on the ground rather than quadrupedally, as the Taylor & Rowntree (1973) data suggests, then one would predict that they’d do so around 50% of the time, or at least that it would be a significant part of their locomotor repertoire. However studies have consistently found that chimpanzees and gorillas are very reluctant to move bipedally. For example, Hunt (1994) found that chimpanzees are usually less than 3% (measured by time) bipedal when moving on land. This suggests that other factors may outweigh energy efficiency in determining the chosen mode of locomotion.

Also there is another problem: If the last common ancestor was a terrestrial quadruped, and if one (hominid) lineage since that ancestor became bipedal because it was more energetically efficient to do so, why not the other lineages too? One suggested solution to that problem was offered by Isbell & Young (1996), who suggested that the different evolutionary strategies resulted because hominids had larger group sizes than Pan & Gorilla.

In summary, although the energetic efficiency model does seem irresistible as a means for modern human bipedalism to have become perfected it is far from clear that it answers the question as to how hominid bipedality began in the first place. The evidence which its proponents cite is rather slight at best and only applies at slow, walking, speeds and under perfect (treadmill) conditions. It is not at all clear that such a slight energetic advantage would favour bipedalism in less than optimal sort of walking conditions that early hominids might have encountered. For example Zamparo et al 1992 found that the cost of walking on dry sand was 2.5 times as high as on concrete and Pandolf et al 1996 found that the cost of walking in deep snow was up to 5 times as high as that on a treadmill. It is interesting to speculate as to whether slow human bipedality still provides an energetic advantage in such situations, and indeed if there are any conditions in which it has a much clearer advantage over quadrupedalism. Overall, the energy efficiency model appears strongest when used to explain the later specialisations of human locomotion and weakest when used to explain how the evolutionary trajectory towards bipedalism may have begun in the first place.

Chimpanzee locomotor energetics and the origin of human bipedalism. PNAS (2007) Sockol et al
This model was bolstered recently by a paper by Sockol et al. (2007) which studied energy efficiency in chimpanzees who analysed walking energetics and biomechanics for adult chimpanzees and humans. They found that bipedal and quadrupedal walking costs were not significantly different in their sample of adult chimpanzees. However, a more detailed analysis revealed significant differences in bipedal and quadrupedal cost in most individuals, which are masked when subjects are examined as a group. They found that individuals that were more efficient adopted more upright, human-like, gaits which led them to propose that an efficient human like gait could have evolved merely though selection on simple variation in populations.
 Strengths: The strength of this model is that it is very plausible in explaining modern human postcranial anatomy.    
Weaknesses: The model is weakest in explaining how bipedalism began in the first place and places early hominins in situations which would make them very vulnerable to predation.    
1.1 Survival Value 8 (Good) As all organisms are faced with clear-cut, net energy considerations, any adaptation to reduce energy expenditure, especially in species postulated to have done more movement than normal, is likely to provide for strong selection.    
1.2 Sexual Selection 5 (Fair) This model was judged neutral by this criterion.    
1.3 Not Teleological 3 (Poor) This model is weakest in explaining the earliest stages of bipedalism and strongest in explaining how human anatomy became specialised for optimal walking once it had already begun.    
2.1 Improved Food Acquisition 5 (Fair) This model was judged neutral by this criterion.    
2.2 Accounts for Predation 1 (Poor) One weakness of this model is that, by definition as it proposes slow open terrestrial locomotion over relatively long distances, it exposes putative hominids to distinctly greater risk of predation than would be the case otherwise.    
2.3 Why Apes are not Bipedal 7 (Good) One weakness of the Rodman & McHenry model, especially as it specifically suggests that there was no energetic rubicon to cross for early hominoid ancestors, is the question of why bipedalism did not evolve in other apes, considering it is more energetically efficient to do so. Sockol et al's paper (2007) provides support for this deficiency however.    
2.4 Extant Analogues 2 (Poor) As chimpanzees have been found to be approximately 45% less efficient than humans at locomotion (bipedal and quadrupedal) it might not be expected that they should exhibit this kind of locomotion in any case. However, as Rodman & McHenry make such use of the Taylor & Rowntree (1973) evidence that chimpanzees are equally efficient on two legs as four, one might expect to see greater evidence for bipedalism in extant apes. That we do not implies that there is some kind of behavioural rubicon to cross, whether or not it be anything specific to do energy efficiency    
2.5 Applies to Both Sexes 9 (Good) Although there are known anatomical compromises between the sexes in humans due to competing and slightly contradictory demands of child birth and efficient bipedalism, both sexes are similarly efficient at slow locomotion.    
3.1 Hominid Anomalies 3 (Poor) The model, as articulated by Rodman and McHenry, has nothing to say about australopithecine or any early hominid anatomy.    
3.2 Fits Paleoecological Record 5 (Fair) Once again there is some contradiction here in the fossil record. On the ‘macro-geographic’ level there is no doubt that East African habitats become more open and arid, as the energy efficiency model would predict, but locally it is equally clear from the faunal and floral fossil assemblages associated with hominids that there remained significant gallery forest refugia where ‘exotic’ species continued to live. It is not clear which of these two habitats were inhabited by our ancestors, or indeed, if they inhabited both of them.    
3.3 Precursor to Strider and knuckle Walker 4 (Poor) The proponents of this model assume that the precursive form of locomotion that preceded human bipedalism was quadrupedalism, a form with very little overlap with human bipedalism.    
4.1 Extended Explanatory Power 5 (Fair) The model does not set out to explain any other aspect of ape-human divergence but could be used to reinforce those that suggest it happened in more open habitats    
4.2 Complimentary 5 (Fair)  This model was jidged complimentary to those models set in open habitats but contradictory to arboreal ones.    
4.3 Falsifiable or Testable 5 (Fair) As the model is laid out, it does not suggest any new testable hypotheses but it is largely based upon the hypothesis that as human locomotion, at normal speeds, is slightly more efficient than typical mammalian quadrupedalism. It could be argued that this was the prediction that the hypothesis made and that it met with that prediction.    
References Rodman, P S; McHenry, H (1980). Bioenergetics and the Origin of Hominid Bipedalism. American Journal of Physical Anthropology 52:103-106
Sockol, Michael D; Raichlan, David A; Pontzer, Herman (2007). Chimpanzee locomotor energetics and the origin of human bipedalism. Proceedings of the National Academy of Sciences of USA 104(30) :12265-12269