Bipedalism in Humans and Chimpanzees

Humans exhibit habitual bipedalism. Chimps, on the other hand, exhibit facultative bipedalism. This simply means that a chimp can walk bipedaly for short distances, if needed. Humans are obligate bipeds. Due to our upright form of locomotion, this creates anatomical differences between both humans and chimpanzees.

For humans, some of these include: an angled femur (moves the center of mass toward the middle of the body, promoting stability), non-divergent toes (big toe acts like a spring and aids in bipedal gait), arched feet (provides shock absorption), longer legs (allows mass to be located in the lower body), wider pelvis (to assist in upright muscle attachment), weaker neck muscles (results from a centrally located foramen magnum), a curvy spine (allows the backbone to act like a spring), narrower rib cage (for arm swinging), altered inner ear bones (aids in balance), and a centrally located foramen magnum (balances the head). When a human walks, or runs, our center of gravity is located directly in the pelvis. For chimpanzees, their center of gravity is located below the pelvis in the space between the rib cage and in front of the legs. This means that chimps "expend more energy to keep from falling forward" (Park, 2008). Nevertheless, due to anatomical differences, it is much more energy efficient for chimpanzees to be knuckle-walkers. This, of course, creates stronger wrists and "tough, hairless skin on the dorsum of the middle fingers" (France, 2007). They are also extremely well-adapted to arboreal life. For humans, walking upright is more efficient for slower, non-running locomotion - in other words, walking across great distances:

The average [human] male mammalian biped moving between 2.4 and 6 kilometers an hour saves 100 and 500 KJ by moving bipedaly. The average female saves between 100 and 600 KJ. Compared to the energy used in [nonhuman] primates for locomotion, bipedal walking is almost half as expensive. (Leonard & Robertson, 1997).

There are numerous reasons as to why bipedalism may have evolved, and it may well depend on whether one accepts that it evolved in a forested or a savannah-like environment. (It must be mentioned that it has not been clearly shown that Orrorin, Ardipithecus, or Sahelanthropus were bipedal. Because of this, their affiliated environments do not necessarily indicate that bipedalism arose in a particular, ecological niche.) In one regard, "bipedalism may have evolved more as a terrestrial feeding posture than as a walking adaptation" since chimpanzees are often observed adopting "bipedal postures much more commonly when feeding on the small fruits of low, open-forest tress" (Conroy, 2005). It should be noted this hypothesis "only addresses upright posture, not necessarily upright locomotion" (Park, 2008). There is also the possibility that bipedalism arose from arboreal walking. In other words, species walking upright throughout the canopy of tress ultimately resulted in "on the ground" bipedalism. Another hypothesis concerning the evolution of bipedalism for humans involves heat dissipation. With new fossil discoveries, it seems the savannah hypothesis is losing ground in the scientific community but here the "vertical orientation helps cool the body by presenting a smaller target to the intense equatorial rays of the sun" (Park, 2008).

References
Conroy, G. (2005). Reconstructing Human Origins. New York: Norton.
France, D. (2007). Lab Manual and Workbook for Physical Anthropology. Belmont, CA: Thompson Wadsworth
Leonard, William., & Robertson, M. (1997). Rethinking the Energetics of Bepedality. Current Anthropology 38, 304-309.
Park, M. (2008). Biological Anthropology. New York: McGraw-Hill.