Science Literature

When I first heard of treadmill studies involving chimps and humans, I wondered if the researchers would be open to design paradigms as well as Darwinian adaptation. Would they get beyond the narrow conceptual window exhibited by another group studying assisted bipedalism in orangutans? The answer to that question would appear to be no.
"The researches collected metabolic, kinematic and kinetic data from five chimpanzees and four adult humans walking on a treadmill. The chimpanzees were trained to walk quadrupedally and bipedally on the treadmill. Humans walking on two legs only used one-quarter of the energy that chimpanzees who knuckle-walked on four legs did. On average, the chimpanzees used the same amount of energy using two legs as they did when they used four legs." This is certainly an interesting result. Those who have declared human upright movement to be associated with numerous bad design features will be given food for thought by this research. Those who already recognise design in the human body will regard the new data as strengthening this understanding.
The researchers, however, pursue an adaptationist agenda. The new study is said to provide support "for the hypothesis that walking on two legs, or bipedalism, evolved because it used less energy than quadrupedal knucklewalking." Also, "it has been hypothesized that the reduced energy cost of walking upright would have provided evolutionary advantages by decreasing the cost of foraging." OK, this is an adaptationist hypothesis, but it needs to be tested. Demonstrating an energy differential does not prove a hypothesis: in this case it just stimulates a hypothesis.
This is a situation where the Adaptive Landscape concept may be helpful. When considered holistically, bipedalism involves a whole raft of characters, all of which are needed to form a workable organism. Bipedalism involves arched feet, strong big toes, long legs, upright knee joints, angled femur bones, upright hip joints, straight back, upright skull, flat face and a very fine sense of balance. Taking all these characters into account means that the adaptive landscape looks like to very sharp peaks separated by a wide plain. We could call the peaks Mount Ape Improbable and Mount Human Improbable. The claim that there is an incremental route for an apelike animal to move from one peak to the other has so far eluded Darwinians. The authors comment: "why our unique two-legged gait evolved remains unknown." We have no evidence that apes have ever climbed a Mount Improbable, and there is a long way to go before an adaptive story based on the "evolutionary advantages [of decreased] cost of foraging" can begin to be convincing. Studying evidences of a "more extended hip and a longer hindlimb" do not get close to a holistic appreciation of the issues.
In 2002, a film was produced with the name Most Vertical Primate. In it, a chimpanzee becomes a competent skateboarder. A trailer can be found here. The task of training the chimpanzee to skateboard was impossible. One foot had to be tied to the board and the other positioned carefully so that the animal could get a grip with its toes. Even then, the chimp could not steer or balance. The film of skateboarding was pierced together from tiny clips of a few seconds each. Stuart Burgess, a design engineer, concluded: "Despite the title of the film, the film actually demonstrated that apes are not designed to be vertical!"

Chimpanzee locomotor energetics and the origin of human bipedalism
Michael D. Sockol, David A. Raichlen, and Herman Pontzer
Proceedings of the National Academy of Sciences US, published July 16, 2007, 10.1073/pnas.0703267104
http://www.pnas.org/cgi/content/abstract/0703267104v1?

Bipedal walking is evident in the earliest hominins [Zollikofer CPE, Ponce de Leon MS, Lieberman DE, Guy F, Pilbeam D, et al. (2005) Nature 434:755-759], but why our unique two-legged gait evolved remains unknown. Here, we analyze walking energetics and biomechanics for adult chimpanzees and humans to investigate the long-standing hypothesis that bipedalism reduced the energy cost of walking compared with our ape-like ancestors [Rodman PS, McHenry HM (1980) Am J Phys Anthropol 52:103-106]. Consistent with previous work on juvenile chimpanzees [Taylor CR, Rowntree VJ (1973) Science 179:186-187], we find that bipedal and quadrupedal walking costs are not significantly different in our sample of adult chimpanzees. However, a more detailed analysis reveals significant differences in bipedal and quadrupedal cost in most individuals, which are masked when subjects are examined as a group. Furthermore, human walking is ~75% less costly than both quadrupedal and bipedal walking in chimpanzees. Variation in cost between bipedal and quadrupedal walking, as well as between chimpanzees and humans, is well explained by biomechanical differences in anatomy and gait, with the decreased cost of human walking attributable to our more extended hip and a longer hindlimb. Analyses of these features in early fossil hominins, coupled with analyses of bipedal walking in chimpanzees, indicate that bipedalism in early, ape-like hominins could indeed have been less costly than quadrupedal knucklewalking.

See also:

Study Identifies Energy Efficiency As Reason For Evolution Of Upright Walking, Science Daily July 17, 2007

Burgess, S. The Origin of Man, Day One Publications, 2004.