or How we traded half our legs for all of our arms.
We
Homo sapiens are the only living members of the
primate order who are
bipedal, that is, we have a funny
habit of
walking around on two legs. Neither our early
predecessors, whom we share with the
apes, nor the
apes themselves do this. How was the
transition made? At first glance, it might even seem a
disadvantage. We're so slow compared to four-legged animals that it
boggles the mind and we have a relatively high
center of gravity positioned over a very small
surface area. Understanding how and why our ancestors became
bipedal is an important step in
understanding our
evolution, as it is one of the few
major traits that set us apart from the
apes and our common
ancestors.
In accordance with the currently favored model of
evolution, for a trait to be carried on, it must occur as a
mutation and then be
favored by the
environment in which the
organism lives. So we must assume that the practice of
ambulating on two limbs is no
exception. In order for
bipedalism to
proliferate, it must have granted some
advantages to the
primates in which it appeared.
Most of the
fossilized remains of our earliest
predecessors have been
discovered in
Africa, east of the
Rift Valley. This area is now mostly dry
savannah, but it's believed that
bipedal primates evolved either during or before the
tectonic shifting that caused the area to dry up and allowed its
forests to give way to
grasslands. So we have a forested
environment as the stage on which these
strange, two-legged
creatures make their
debut. By the time they started walking bipedally, they must have already been spending some time on the
ground, since the
bipedal adaptation suggests walking on the
surface and not
swinging through the
trees. Since they were most likely
scavengers, they often had to walk long distances from one area to another in order to
secure a fresh supply of
food. Then, since they faced
predators even in the forests, they would probably have to carry the food to a
safe place before
consumption. Bipedalism, though making them much slower at a
running pace, is much more
efficient for walking
long distances. It also allowed them to have two limbs free to carry food, or, in the case of the
mothers, their
offspring.
Some other
aspects of
bipedalism are easier to
understand if we examine the
bonobo. The
bonobo is thought to be the closest living
relative to the
common ancestor of
apes and
humans. The
bonobo spends much more time walking on two
legs than any other
ape, and it displays
sexual consciousness, using
sex not only
unconsciously, as a
reproductive method, but also as a
means of
bonding and ensuring
unity in a
group. Our
enjoyment of this particular
behavior is thought to have
appeared at the same time as
bipedalism, with the disappearance of
estrus and resulting constant
sexual receptivity, which made reproduction much more
convenient, and freed more time for food gathering; the male felt more secure leaving his
companion during
gathering because he didn't have to worry about her going into
heat and
fucking anything that moved.
Although they occurred after the
initial development of
bipedalism, certain things are thought to have
reinforced it later, on the
savannah, such as the ability to see
farther due to a
higher viewpoint. Also thought to be a later
advantage was the
cooling effect of exposing less area to
solar radiation and keeping the
torso and
head in the area with
cooler air currents, both of which could have
fostered the
development of our
large,
complex brains; the brains are liable to suffer a
terminal meltdown when
exposed to an
excessive level of
heat.
It may be
slow and hard to get the hang of it, at first, but
walking around on two legs does seem to have its
advantages, not to mention the key role it played in our
subsequent world domination, which will probably result in the
destruction of most
species on the
planet, if not the very
ability of the
planet to
sustain life. Well, maybe we'll
evolve again,
sooner rather than
later.
This was originally an essay for my biological anthropology course. I followed the advice of sensei and noded my homework.
For more information about bipedalism on Everything2, check out Kaytay's excellent wu Major Features of Hominid Bipedalism.