Anthropologists once thought that the ancestors
of modern humans began to walk upright because
it freed their hands to use stone tools, which they
had begun to make as the species evolved a brain of
increased size and mental capacity. But discoveries
of the three-million-year-old fossilized remains of
our hominid ancestor Australopithecus have yielded
substantial anatomical evidence that upright walking
appeared prior to the dramatic enlargement of the
brain and the development of stone tools.
  Walking on two legs in an upright posture (bipedal
locomotion) is a less efficient proposition than walking
on all fours (quadrupedal locomotion) because several
muscle groups that the quadruped uses for propulsion
must instead be adapted to provide the biped with
stability and control. The shape and configuration
of various bones must likewise be modified to allow
the muscles to perform these functions in upright
walking. Reconstruction of the pelvis (hipbones) and
femur (thighbone) of “Lucy,” a three-million-year-old
skeleton that is the most complete fossilized skeleton
from the Australopithecine era, has shown that they
are much more like the corresponding bones of the
modern human than like those of the most closely
related living primate, the quadrupedal chimpanzee.
Lucy's wide, shallow pelvis is actually better suited to
bipedal walking than is the rounder, bowl-like pelvis of
the modern human, which evolved to form the larger
birth canal needed to accommodate the head of a
large-brained human infant. By contrast, the head of
Lucy's baby could have been no larger than that of a
baby chimpanzee.
  If the small-brained australopithecines were not
toolmakers, what evolutionary advantage did they
gain by walking upright? One theory is that bipedality
evolved in conjunction with the nuclear family:
monogamous parents cooperating to care for their
offspring. Walking upright permitted the father to
use his hands to gather food and carry it to his mate
from a distance, allowing the mother to devote more
time and energy to nurturing and protecting their
children. According to this view, the transition to
bipedal walking may have occurred as long as ten
million years ago, at the time of the earliest hominids,
making it a crucial initiating event in human evolution.