Walking the Walk: The Bipedal Adaptation
All primates show adaptations for erect body posture, and some species are occasionally bipedal. Of all living primates, however, efficient bipedalism as the primary form of locomotion is seen only in hominins. Functionally, the human mode of locomotion is most clearly shown in our striding gait(walking), where weight is alternately placed on a single fully extended hind limb. This specialized form of locomotion has developed to a point where energy levels are used to near peak efficiency. This isn’t true of nonhuman primates, who move bipedally with hips and knees bent in a much less efficient manner. From a survey of our close primate relatives, it’s apparent that while still in the trees, our ancestors were adapted to a fair amount of upper-body erectness. Lemurs, lorises, tarsiers, monkeys, and apes all spend considerable time sitting erect while feeding, grooming, or sleeping. Presumably, our early ancestors displayed similar behavior. What caused them to come to the ground and embark on the unique way of life that would eventually lead to humans is still a mystery. Perhaps natural selection favored some Miocene hominoids coming occasionally to the ground to forage for food on the forest floor and forest fringe. In any case, once they were on the ground and away from the immediate safety offered by trees, bipedal locomotion could become a tremendous advantage.
First of all, bipedal locomotion freed the hands for carrying objects and for making and using tools. Hominins were bipedal for at least 2 million years prior to the first archaeological evidence of tool use. Early cultural developments such as habitual tool use had an even more positive effect on speeding the development of yet more efficient bipedalism—once again emphasizing the dual role of biocultural evolution. In addition, in a bipedal stance, animals have a wider view of the surrounding countryside, and in open (or semi-open) terrain, early spotting of predators (particularly large cats, such as lions, leopards, and saber-tooths) would be of critical importance. We know that modern ground-living primates, including savanna baboons and vervets, occasionally adopt this posture to “look around” when out in open country. Moreover, bipedal walking is an efficient means of covering long distances, and when large game hunting came into play (several million years after the initial adaptation to ground living), further refinements increasing the efficiency of bipedalism may have been favored. It’s hard to say exactly what initiated the process, but all these factors probably played a role in the adaptation of hominins to their special niche through a special form of locomotion.
The Mechanics of Walking on Two Legs
The problem is to maintain balance on the “stance” leg while the “swing” leg is off the ground. In fact, during normal walking, both feet are simultaneously on the ground only about 25 percent of the time, and as speed of locomotion increases, this percentage becomes even smaller. Maintaining a stable center of balance calls for many drastic structural/anatomical alterations in the basic primate quadrupedal pattern. The most dramatic changes are seen in the pelvis. The pelvis is composed of three elements: two hip bones, or ossa coxae , joined at the back to the sacrum . In a quadruped, the ossa coxae are vertically elongated bones positioned along each side of the lower portion of the spine and oriented more or less parallel to it. In hominins, the pelvis is comparatively much shorter and broader and extends around to the side. This configuration helps to stabilize the line of weight transmission in a bipedal posture from the lower back to the hip joint .
Moreover, the foot must act as a stable support instead of a grasping limb. When we walk, our foot is used like a prop, landing on the heel and pushing off on the toes, particularly the big toe. In addition, our legs became elongated to increase the length of the stride. An efficient bipedal adaptation required further remodeling of the lower limb to allow full extension of the knee and to keep the legs close together during walking, in this way maintaining the center of support directly under the body . We say that hominin bipedalism is both habitual and obligate. By habitual bipedalism, we mean that hominins, unlike any other primate, move bipedally as their standard and most efficient mode of locomotion. By obligate bipedalism, we mean that hominins are committed to bipedalism and cannot locomote efficiently in any other way. For example, the loss of grasping ability in the foot makes climbing much more difficult for humans. The central task, then, in trying to understand the earliest members of the hominin lineage is to identify anatomical features that indicate bipedalism and to interpret to what degree these individuals were committed to this form of locomotion (that is, was it habitual and obligate?).
What structural patterns are observable in early hominins, and what do they imply regarding locomotor function?
By 4.4 mya, we have good evidence that hominins had adaptations in their pelvis and feet that allowed for fairly efficient bipedal locomotion while on the ground. They were, however, still surprisingly primitive in many other respects and spent considerable time in the trees (where they could also move about very efficiently).
Only after around 4 mya do we see all the major structural changes required for bipedalism. In particular, the pelvis, as clearly documented by several excellently preserved specimens, was remodeled further to more efficiently support weight in a bipedal stance.
Other structural changes shown after 4 mya further confirm the pattern seen in the pelvis. For example, the vertebral column (as known from specimens in East and South Africa) shows the same curves as in modern hominins. The lower limbs are also elongated, and they seem to be proportionately about as long as in modern humans (although the arms are longer in these early hominins). Further, the carrying angle of weight support from the hip to the knee is very similar to that seen in ourselves.
Fossil evidence of early hominin foot structure has come from several sites in South and East Africa. Some of this evidence, especially well-preserved fossils (as well as footprints) from East Africa, show a well-adapted form of bipedalism. However, some earlier (and recently analyzed) finds from East Africa, as well as some from South Africa, indicate that the large toe was divergent like that seen in great apes. Such a configuration is an ancestral trait among hominoids, important in allowing the foot to grasp. In turn, this grasping ability (as in other primates) would have enabled early hominins to more effectively exploit arboreal habitats.
Finally, since anatomical remodeling is always constrained by a set of complex functional compromises, a foot highly capable of grasping and climbing is less capable as a stable platform during bipedal locomotion.
From this evidence, some researchers have recently concluded that many forms of early hominins spent considerable time in the trees. What’s more, the earliest hominins were likely habitual bipeds when on the ground, but not necessarily obligate bipeds. Only after about 4 mya did further adaptations lead to the fully committed form of bipedalism that we see in all later hominins, including ourselves.