Chimpanzees and gorillas are quadrupeds which have a form of walking on the ground generally called knuckle-walking. When they walk, they are on all fours, supporting their weight on the knuckles of their hands. When they do walk on two legs, chimpanzees waddle: they walk with their legs far apart and their bodies sway from side to side. Their swaying walk reminds some observers of the gait of an inebriated human. Sober humans, on the other hand, have a striding gait in which there is little sway. Instead of having to expend energy on trying to stabilize the upper body, humans use their energy for forward motion.
Unlike other primates, humans are bipedal, meaning that we walk on two legs. Bipedalism is generally considered to be the central feature of being human and has consequences for much of our anatomy. In discussing the anatomical adaptation to bipedal walking and running, Alan Walker and Pat Shipman, in their book The Wisdom of the Bones: In Search of Human Origins, write:
“While it is obvious that the major bones of the leg and foot must be adapted to bipedalism, the evolution of this new mode of locomotion also produced subtle changes in other parts of the body in order to maintain balance and equilibrium during bipedal walking or running. Put plainly, bipedalism is a precarious and difficult way to move around the world.”
In his entry on human nature in The Oxford Handbook of Archaeology, Jonathan Marks writes:
“The adoption of bipedalism is uniquely human only insofar as it occurred on a primate frame, with considerable independence of movements of the pelvic and shoulder girdles.”
With regard to evolution, McNeil Alexander, in his book Human Bones: A Scientific and Pictorial Investigation, reminds us:
“We have not retained five toes, and two lower leg bones because those are necessarily the best numbers, but because they are the numbers our ancestors had. Evolution often leaves things as they are when there is no strong reason for change.”
Briefly described below are some of the anatomical modifications which accommodate bipedalism in the feet, legs, and buttocks.
Feet
When we look at the foot of a chimpanzee, we see that it resembles the chimpanzee hand in that the big toe splays out from the other digits. In comparing the human foot with the chimpanzee foot, Daniel Fairbanks, in his book Evolving: The Human Effect and Why it Matters, writes:
“The human foot, by contrast, is shorter and narrower, and it has highly derived adaptations for bipedalism.”
Daniel Fairbanks also writes:
“The parallel, forward-pointing conformation of our toes also enhances our bipedalism. Among all living primates, we are the only ones whose large toe is parallel with the other toe, and our toes are relatively short.”
Compared to ape toes, human toes are also relatively short, a feature which stabilizes the foot for walking.
In addition, the chimpanzee foot does not have an arch: when apes stand, the whole length of the foot is on the ground. The foot of the chimpanzee is designed for climbing, not walking. Jonathan Marks writes:
“We see many similarities between the chimpanzee’s foot and the human foot, for they are built of roughly the same parts, in roughly the same relations. Yet the chimp foot is a grasping structure and the human foot is a weight-bearing structure.”
In comparing the feet of humans with those of chimpanzees and other apes, Robert Winston and Don Wilson, in their book Human, report:
“Our feet are arched so that the heel and the ball of the foot carry our weight as we move. In contrast, apes stand with the whole length of the foot on the ground.”
The arch in the human foot functions as a kind of spring to absorb shocks. In his book The Story of the Human Body: Evolution, Health, and Disease, Daniel Lieberman explains:
“Since running uses the legs like springs, some of the most important adaptations for running are literally springs. One key spring is the dome-shaped arch of the foot, which develops from the way ligaments and muscles bind together the foot’s bones as children start to walk and run.”
With regard to how the human foot works when walking, Daniel Lieberman reports:
“Walking humans usually land first on the heel and then, as the rest of the foot makes contact with the ground, we stiffen the foot’s arch, enabling us to push the body upward and forward at the end of the stance, mostly with the big toe.”
Daniel Lieberman also reports:
“The feet of chimps and other apes lack an arch, preventing them from pushing off against a stiffened foot, and their toes are unable to extend as much as humans.”
Legs
While chimpanzees and other apes can stand somewhat upright on two legs, this is not their normal posture and their upright stance looks rather awkward. Daniel Fairbanks writes:
“When in an upright stance, chimpanzees bend their legs and lean forward to balance themselves, a stance that is consistent with the curvature of their vertebral column, but it is awkward for them. They typically do not maintain upright stances for long periods of time.”
In looking at humans standing upright, one of the first things that becomes apparent is that human legs are not only longer than those of the chimpanzees, but human legs are straight rather than being bent like those of a chimpanzee.
One of the key anatomical features that allows humans to have straight legs is the knee joint. The knee, like the elbow, is a hinge joint, but, unlike the elbow, the knee is not a close-fitting joint. McNeill Alexander explains:
“Curved surfaces on the end of the femur rest on the almost flat top surface of the tibia. The gaps between the two bones are filled in by wedges of cartilage called menisci, which are held in place by ligaments.”
McNeill Alexander also writes:
“As an engineering design, the knee seems clever but rather unsatisfactory. Of all the joints in the body, it is the one most often damaged.”
It was, by the way, the discovery of a fossil knee that provided the first evidence that Australopithecus afarensis (i.e., “Lucy”) was bipedal and, thus, a possible human ancestor. When viewed from the front, human legs are knock-kneed, that is, they slant inward from the hips to the knees, while ape legs go straight down. In their book Ancestors: In Search of Human Origins, Donald Johanson, Lenora Johanson, and Blake Edgar explain it this way:
“As bipeds, we have widely spaced hips and narrowly spaced knees to keep us balanced as we walk.”
With regard to the knee, this means that a chimpanzee knee joins the tibia and the femur (the leg bones) in a straight line, while a biped’s knee joins the tibia and the femur at an angle. When Donald Johanson discovered Lucy’s knee, the bones fit perfectly at an angle, meaning that Lucy was a biped.
Buttocks
One of the unusual features of human anatomy is the rounded, muscular buttocks. While this feature is considered sexually attractive in many cultures and may have been evolutionarily enhanced by sexual selection, anatomically human buttocks are a result of bipedalism. The large muscle—the gluteus maximus, the largest muscle in the human body—is needed to support and balance the weight which is carried above the pelvis because of bipedalism. In his book Evolving: The Human Effect and Why it Matters, Daniel Fairbanks explains:
“The most highly derived are the gluteal muscles, which give humans our distinctively rounded buttocks. The enlargement and positioning of these muscles during the evolutionary transition to upright posture assisted our ancestors in maintaining themselves upright while walking and running.”
The large muscles allow humans to have a distinctive walk: humans can walk and run upright continually facing forward. While the great apes do walk upright occasionally, their gait is very different from the human walk. Daniel Fairbanks writes:
“A chimpanzee, gorilla, or orangutan typically twists side-to-side when walking on two legs in part because its gluteal muscles lack the size, strength, and positioning to fully stabilize it while walking.”
In other words, the large muscles—the rounded buttocks—make the more efficient human walk possible.
More Human Origins
Human Origins: Bipedalism
Human Origins: The human face
Human Origins: Eyes
Human Origins: Teeth
Human Origins: Humans as naked apes
Human Origins: The Human Hand
Human Origins: Transitional Humans
Human Origins: Fossil Evidence