164 CHAPTER 7 | The First Bipeds
consumption. The biped does not have to remain out in
the open, exposed and vulnerable, to do all of its eating.
Besides making it possible to carry food, bipedalism
could have facilitated the food quest in other ways.
With their hands free and body upright, the animals
could reach otherwise unobtainable food on thorny trees
too flimsy and too spiny to climb. Furthermore, with both
hands free, they could gather other small foods more
quickly using both hands. And in times of scarcity, being
able to see farther, with the head in an upright position,
would have helped them locate food and water sources.
Food may not have been the only thing transported by
early bipeds. As we saw in Chapters 3 and 4, primate infants
must be able to cling to their mothers in order to be carried;
because the mother is using her forelimbs in locomotion,
to either walk or swing, she cannot hold her infant as well.
Chimpanzee infants, for example, must cling by themselves
to their mother, and even up to 4 years of age, they make
long journeys on their mother’s back. Injuries caused by
falling from the mother are a significant cause of infant
mortality among apes. Thus the ability to carry infants
would have made a significant contribution to the survi-
vorship of offspring, and the ancestors of Australopithecus
would have been capable of doing just this.
Another suggestion—that bipedal locomotion arose as
an adaptation for nonterritorial scavenging of meat^15 —is
unlikely. Although it is true that a biped is able to travel
long distances without tiring, and that a daily supply of
dead animal carcasses would have been available to early
bipeds only if they were capable of ranging over vast areas,
no evidence exists to indicate that they did much in the
way of scavenging prior to about 2.5 mya. Furthermore,
the heavy wear seen on australopithecine teeth is indica-
tive of a diet high in tough, fibrous plant foods. Thus
scavenging was likely an unforeseen byproduct of bipedal
locomotion, rather than a cause of it.
Although bipedalism appeared before our ancestors
lived in the savannah, it is still possible that bipedalism
served as a means to cope with heat stress out in the open
as the forested environments disappeared. In addition to
bipedalism, one of the most obvious differences between
humans and other living hominoids is our relative naked-
ness. Body hair in humans is generally limited to a fine
sparse layer over most of the body with a very dense cover
of hair limited primarily to the head. Peter Wheeler, a
British physiologist, has suggested that bipedalism and the
human pattern of body hair growth are both adaptations
to the heat stress of the savannah environment.^16 Building
upon the earlier “radiator” theory of paleoanthropologistemphasize that bipedalism does not result in particularly
fast running; quadrupedal chimpanzees and baboons, for
example, are 30 to 34 percent faster than we bipeds. For
100-meter distances, our best athletes today may attain
speeds of 34 to 37 kilometers per hour, while the larger
African carnivores from which bipeds might need to run
can attain speeds up to 60 to 70 kilometers per hour. The
consequences of a leg or foot injury are more serious for a
biped while a quadruped can do amazingly well on three
legs. A biped with only one functional leg is seriously
hindered—an easy meal for some carnivore.
Because each of these drawbacks would have placed
our early ancestors at risk from predators, paleoanthro-
pologists have asked what made bipedal locomotion
worth paying such a high price. It is hard to imagine bi-
pedalism becoming a viable adaptation in the absence of
strong selective pressure in its favor; therefore, a number
of theories have been proposed to account for the adaptive
advantages of bipedalism.
One once-popular suggestion is that bipedal locomo-
tion allowed males to gather food on the savannah and
transport it back to females, who were restricted from
doing so by the dependence of their offspring.^14 This
explanation is unlikely, however, because female apes, not
to mention women among food-foraging peoples, rou-
tinely combine infant care with foraging for food. Indeed,
among most food foragers, it is the women who com-
monly supply the bulk of the food eaten by both sexes.
Moreover, the pair bonding (one male attached to
one female) presumed by this model is not character-
istic of terrestrial primates, nor of those displaying the
degree of sexual dimorphism that was characteristic of
Australopithecus. Nor is it really characteristic of Homo
sapiens. In a substantial majority of recent human societ-
ies, including those in which people forage for their food,
some form of polygamy—marriage to two or more indi-
viduals at the same time—is not only permitted but pre-
ferred. And even in the supposedly monogamous United
States, it is relatively common for an individual to marry
(and hence mate with) two or more others (the only re-
quirement is that he or she not be married to more than
one mate at the same time).
Although we may reject as culture-bound the idea of
male breadwinners provisioning stay-at-home moms, it
is true that bipedal locomotion does make transport of
bulky foods possible. (See the Biocultural Connection for
another example of the influence of socially defined roles
and theories about the evolution of human childbirth.)
Nevertheless, a fully erect biped on the ground—whether
male or female—has the ability to gather such foods
for transport back to a tree or other place of safety for
(^14) Lovejoy, C. O. (1981). The origin of man. Science 211, 341–350.
(^15) Lewin, R. (1987). Four legs good, two legs bad. Science 235, 969–971.
(^16) Quoted in Folger, T. (1993). The naked and bipedal. Discover 14 (11),
34–35. Reprinted with permission.