Evolution And History

The Anatomy of Bipedalism 147

The shape of the pelvis also differs considerably be-
tween bipeds and other apes. Rather than an elongated
shape following the arch of the spine as seen in chimps, the
biped pelvis is wider and foreshortened so that it can pro-
vide structural support for the upright body. With a wide
bipedal pelvis, the lower limbs would be oriented away from
the body’s center of gravity if the thigh bones (femora) did
not angle in toward each other from the hip to the knee,
a phenomenon described as “kneeing-in.” (Notice how
your own knees and feet can touch when standing while
your hip joints remain widely spaced.) This angling does
not continue past the knee to the shin bones (tibia), which
are oriented vertically. The resulting knee joint is not sym-
metrical, allowing the thigh and shin bones to meet despite
their different orientations (Figure 7.3).
Another characteristic of bipeds is their stable arched
feet and the absent opposable big toe. The ape big toe is
in an abducted position (sticking out away from the mid-
line) while the human big toe is pulled in toward the mid-
line (adducted). In general, humans and their ancestors
possess shorter toes than the other apes.
These anatomical features allow paleoanthropolo-
gists to “diagnose” bipedal locomotion even in fragmen-
tary remains such as the top of the shin bone or the base
of a skull. In addition, bipedal locomotion can also be
established through fossilized footprints, preserving not
so much the shape of foot bones but the characteris-
tic stride used by humans and their ancestors. In fact,
bipedal locomotion is a process of shifting the body’s
weight from one foot to the other as the nonsupporting
foot swings forward. While the body is supported in a
one-legged stance, a biped takes a stride by swinging the
other leg forward. The heel of the foot is the first part
of the swinging leg to hit the ground. Then as the biped

abduction Movement away from the midline of the body or

from the center of the hand or foot.

adduction Movement toward the midline of the body or to

the center of the hand or foot.

Figure 7.3 Examination of the upper hip bones and lower

limbs of (from left) Homo sapiens, Australopithecus, and

an ape can be used to determine means of locomotion. The

similarities between the human and australopithecine bones

are striking and are indicative of bipedal locomotion.

Homo sapiens Australopithecus Ape

continues to move forward, he or she rolls from the heel

toward the toe, pushing or “toeing off ” into the next

swing phase of the stride (Figure 7.4). While one leg is

moving from heel strike to toe off of the stance phase,

Fossilized footprints were preserved in volcanic ash at the
3.6-million-year-old Tanzanian site of Laetoli. As shown here, the
foot of a living human fits right inside this ancient footprint, which
shows the characteristic pattern of bipedal walking. The actual trail
of footprints is 24 meters (80 feet) long.

© Andrew Hill/ Anthro-Photo