Anthropology - Bipedalism Bipedalism: Evolutionary

Anthropology - Bipedalism

BIPEDALISM: EVOLUTIONARY SIGNIFICANCE and CONSEQUENCES Introduction:

Traditional anthropological theory emphasized the relative size of the human brain, as measured by cranial volume and capacity, as the primary evolutionary adaptation that accounts for the profound difference between homo sapiens and other hominid species. Recent studies contradict this conclusion and suggest that bipedalism was the most significant evolutionary adaptation rather than cranial capacity, and that the former may actually have played at least an equal role in the evolution of human intelligence.

As a student of anthropology, I was fascinated by this revelation in three different levels: first, it represents a dramatic shift in our understanding of an area of human evolution that is described in contemporary textbooks as established fact; second, it suggests that many other conclusions in the field may be less certain than I had previously considered them to be by virtue of their inclusion in academic texts; and finally, it highlights the fascinating trade-off, in principle, between long-term evolutionary benefits of particular adaptations and the negative short-term consequences in terms of the lives of the individual.

Behavior-Driven Adaptation:

Bipedalism in humans is one example of an anatomical evolutionary transition resulting as a consequence of behavior, rather then the reverse. It is believed that early hominids began standing upright and walking on two legs instead of four first, and that this behavior triggered a very gradual evolutionary adaptation of bipedalism to take advantage of the profound benefits of the behavior. Specifically, bipedalism is a much more mechanically efficient mode of locomotion compared to quadruped locomotion. It conserves significant amounts of energy, primarily through recapturing as much as 65% of the kinetic energy required to swing the legs by virtue of the upside-down pendulum motion that accurately describes the act of walking in engineering terms.

Since bipedal locomotion requires a more efficient cooling system to maintain a physiologically optimal internal temperature, this single adaptation is thought to have triggered many others, including the evolution of sweat glands, the disappearance of significant body hair, and numerous anatomical structures that distinguish modern human anatomy from all of our simian relatives. For example, only humans have flexible tendons, complex shock-absorbent heel bones, and comparatively straight legs, all of which are considered to have been directly related to the natural transition from bipedal walking to running. In fact, running on two legs is much faster and more efficient than the fastest sustained quadruped locomotion; it is believed to have played a significant role in the evolution of larger brains by virtue of the increased ability to pursue prey species for their rich nutrients and for triggering the simultaneous adaptation of larger brains necessary to communicate and coordinate hunts in early hominid species.

Short-Term Consequences of Long-Term Evolutionary Benefits:

Over the long-term, bipedal locomotion provides such a profound evolutionary advantage that it outweighs even significant negative consequences to the individual. In fact, many of the most common medical complaints of modern humans relate directly to the physiological realities of the transition of anatomical systems originally designed for quadruped locomotion to the later evolution of bipedalism. Chronic lower back pain, for example, afflicts approximately 80% of the human population. According to physiologists, this is a direct function of the changed role of a spinal column originally designed as an arched support connecting two sets of load-bearing limbs to a vertically- loaded column supporting the compression load required by the shift to bipedalism.