Origin Of Bipedalism
Upright walking (bipedalism) is the adaptation that defines hominins, and preceded the origin of tool use and enlarged brains by at least 2 million years. Many hypotheses have been put forward to explain the adaptive path that led to this mode of locomotion, including improved energetics and efficiency of posture for harvesting food resources.
Although Homo sapiens is not the only primate to walk on two feet—for instance, chimpanzees, a small species of orangutan, and gibbons often use this form of posture in certain environmental circumstances—no other primate does so habitually or with a striding gait. The rarity of habitual bipedalism among primates—and among mammals as a whole—has given rise to the assumption that it is inefficient and therefore unlikely to evolve. As a result, anthropologists have often sought "special"—that is, essentially human—explanations for the origin of bipedalism. Strictly biological explanations are, however, more likely to be correct.
Human evolution is often cast in terms of four major novelties: upright walking, reduction of anterior teeth and enlargement of cheek teeth, elaboration of material culture, and a significant increase in brain size. As the current fossil and archeological records indicate, however, these novelties arose at separate intervals throughout hominin evolution. In other words, hominins show a pattern of mosaic evolution.
Stone-tool making appears to have originated at roughly the same time as significant brain expansion, approximately 2.5 million years ago (see unit 23). The earliest hominin fossils discovered so far—from Ethiopia and Kenya—are dated 2 million years earlier (see unit 19); they show significant adaptation to bipedalism in combination with a hominin dental pattern that has distinct apelike overtones. It is therefore possible that the first hominin might have been apelike in all respects, apart from an adaptation to upright walking. If true, then bipedalism would represent the primary hominin adaptation.
In this unit we will examine some of the mechanics of bipedalism, the ecological context in which it might have arisen, and the development of hypotheses that purport to account for its evolution.
Biomechanics of bipedausm
The striding gait of human bipedalism involves the fluid flow of a series of actions—collectively, the swing phase and the stance phase—in which one leg alternates with the other. The leg in the swing phase pushes off using the power of the great toe, swings under the body in a slightly flexed position, and finally becomes extended as the foot again makes contact with the ground, first with the heel (the heel-strike). Once the heel-strike has occurred, the leg remains extended and provides support for the body—the stance phase—while the other leg goes through the swing phase, with the body continuing to move forward. (See figure 17.1.)
Three key features differentiate human and chimpanzee bipedalism. First, chimpanzees are unable to extend their knee joints—to produce a straight leg—in the stance phase. Thus, muscular power must be exerted in order to support the body. Try standing with your knees slightly bent, and you'll get the idea. The human knee can be "locked" into the extended position during the stance phase, thereby minimizing the amount of muscular power needed to support the body. The constantly flexed position of the chimpanzee leg also means that no toe-off and heel-strike occur in the swing phase.
Second, during each swing phase the center of gravity of the body must be shifted toward the supporting leg (otherwise one would fall over sideways). The tendency for the body to collapse toward the unsupported side is countered by contraction of the muscles (gluteal abductors) on the side of the hip that has entered the stance phase. In humans, because of the inward-sloping angle of the thigh to the knee (the valgus angle), the two feet at rest are normally placed very close to the midline of the body. Therefore, the body's
Stance phase
Swing phase
Stance phase ft
Swing phase
- Stride length -
Push-off -Walking cycle -
Heel-strike
Heel-strike
Figure 17.1 Phases of bipedalism:
Upright walking in humans requires a fluid alternation between stance phase and swing phase activity for each leg. Key features are the push-off, using the great toe, at the beginning of the swing phase, and the heel-strike, at the beginning of the stance phase.
center of gravity need not be shifted very far laterally back and forth during each phase of walking.
Third, the transverse and longitudinal arches of the human foot make it a propulsion-contributing lever, as compared with the grasping function of the chimpanzee foot.
Modern human anatomy is a fully terrestrial adaptation, although the earliest hominins also demonstrated some arboreal adaptation. As we shall see later, these differences have implications for energetic efficiency.
The suite of anatomical adaptations that underlie human bipedalism includes the following characters:
- A curved lower spine;
- A shorter, broader pelvis and an angled femur, which are served by reorganized musculature;
- Lengthened lower limbs and enlarged joint surface areas;
- An extensible knee joint;
- A platform foot in which the enlarged great toe is brought in line with the other toes; and
- A movement of the foramen magnum (through which the spinal cord enters the cranium) toward the center of the basicranium. (See figures 17.2-17.6.)
EcoLogicaL contExt of thE origin of bipEdausm
The nature of the evolution of bipedalism in hominins depended, of course, on the nature of the locomotor adaptation of the immediate ancestor. The ancestor might have been a knuckle-walker, like the chimpanzee, or a species much more arboreally adapted. In any case, the quadrupedal to bipedal transformation is not as dramatic a shift as it might at first appear, because primates are not true quadrupeds (like a horse), and body posture is often relatively upright, such as in tree-climbing.
The earliest hominins appear to have evolved under ecological circumstances (that is, heavily wooded) similar to those typical for living and extinct apes (see unit 19), not in relatively open savannah, as has long been assumed.
Pelvis tilts
Gluteus medius -and minimus
Pelvis tilts
Gluteus medius -and minimus
Figure 17.2 The pelvic tilt: Gluteus medius and minimus muscles link the femur (thigh bone) with the pelvis. They contract on the side in the stance phase, preventing a collapse toward the side of the unsupported limb. Nevertheless, the pelvis tilts during walking. (Courtesy of David Pilbeam.)
DEvElopmENt of idEas on thE origin of bipEdansm
As we saw in unit 3, Darwin essentially equated hominin origins with human origins, proposing an evolutionary package that included upright walking, material culture, modified dentition, and expanded intelligence. In the 1960s, this incipient "Man the Hunter" scenario found an added advant-
FICURE 17.3 Anatomical adaptations to bipedalism: The principal adaptations involve a lumbar curve of the spine; a short, broad pelvis; and long hindlimbs. These characters bring the knees closer to the center of the body (adduction) to form the valgus angle of the femur, and bring the great toe in line with the other toes (adduction).
FICURE 17.3 Anatomical adaptations to bipedalism: The principal adaptations involve a lumbar curve of the spine; a short, broad pelvis; and long hindlimbs. These characters bring the knees closer to the center of the body (adduction) to form the valgus angle of the femur, and bring the great toe in line with the other toes (adduction).
age in bipedalism: although humans are slower and less energy-efficient than quadrupeds when running at top speed, at a slow pace bipedalism allows for great stamina such as might be effective in tracking and killing a prey animal. Recently, with the replacement of the "Man the Hunter" image by "Man the Scavenger" (see unit 26), it has been suggested that the endurance locomotion provided by bipedalism enabled the earliest hominins to follow in the wake of migrating herds, opportunistically scavenging the carcasses of the inexperienced young and the infirm old. (See figure 17.7.)
One problem arises with both these explanations: not only do stone tools that are required for cutting meat from carcasses apparently postdate hominin origins by as much as 3.5 million years, but also no indication of regular meat-eating has been found in the dentition of the earliest known hominins. In fact, evidence from microwear patterns on the surface of teeth (see unit 18) shows that hominin diets remained predominantly vegetarian until approximately 1.8 million years ago—that is, until the origin of Homo erectus.
Other explanations offered for the origin of bipedalism have included the following:
- Improved predator avoidance, as the biped would be able to see further across the "open plain" than the quadruped;
- Display or warning;
- A shift in diet, such as seed-eating; and
- Carrying things.
The last explanation has been featured in two hypotheses in recent years: the "Woman the Gatherer" hypothesis, and the "Man the Provisioner" model.
The "Woman the Gatherer" hypothesis, advanced initially in the early 1970s, shifted putative evolutionary novelty from hunting meat (a male activity) to gathering plant foods (a female activity), which might have required technological innovations such as digging sticks and means of carrying many small items. As often happens in modern chimpanzees, females are envisaged as having foraged together and with their offspring, with whom they shared food. Males were socially peripheral (see unit 13). The "Woman the Gatherer" hypothesis is more conservative than the "Man the Hunter" model, in that the first hominins are viewed as being basically apelike rather than already essentially human. Nevertheless, it focuses on the need to carry things: specifically, food for sharing with infants.
Another hypothesis that focuses on the need to carry things is "Man the Provisioner," in which males gathered food and returned it to some kind of home base; there, the food was shared with females and offspring, specifically "his" female and offspring. Proposed in 1981 by Owen Lovejoy of Kent State University, this model envisages pair bonding between male/female couples, with the male providing an important part of the dietary resources. Such a provisioning pattern would enable females to reproduce at shorter intervals, thus giving them a selective advantage over other large hominoids, which, says Lovejoy, were reproducing at a dangerously slow rate. The system would work only if a male could be reasonably certain that the infants he was helping to raise were his—hence the need for pair bonding and sexual fidelity. Although it received widespread attention, Lovejoy's hypothesis has been widely criticized, not least because the
- FICURE 17.4 Pelvic anatomy: In apes the pelvis is long and narrow; in humans it is short and broad.
FICURE 17.5 The valgus angles in humans, apes, and an early hominin:
The angle subtended by the femur at the knee, the valgus angle, is critical to bipedal locomotion. With the femur angled as in humans, the foot can be placed underneath the center of gravity while striding. An ape's femur is not angled in this way, causing the animal to "waddle" during bipedal locomotion. The valgus angle of Australopithecus afarensis, a 3 million-year-old (or older) hominin, is humanlike, indicating its commitment to bipedality. Also note the humanlike shape of the A. afarensis pelvis. (Courtesy of Luba Gudz.)
very large degree of sexual dimorphism in body size seen in these creatures is very difficult to explain, given the putative monogamous social structure proposed (see units 12 and 13).
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