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Read the proposed theories for the origins of bipedalism
presented below. Select one theory that you find compelling and
present two points of data in support of this theory. Select a
second theory that you find improbable and present at least one
point of data to counter this theory. Any information that you pull
from a website you must cite (by including the web address next to
the fact). Present your argument in essay format, using complete
sentences. Do not simply copy and paste from other sources. Your
total response should be no longer than 300 words in length.ORIGINS OF BIPEDALISMWhy do we walk on two legs? If you asked a roomful of
anthropologists, you’d likely not get the same answer from any two
of them. Specialists cite everything from changing landscapes to
needing to keep cool to heightening sexual attraction as probable
causes of our upright stance, generally agreeing only on one point:
everyone else’s hypothesis is wrong. Think about the anatomical
changes that accompanied bipedalism in light of the following
theories. Do the theories hold up to inspection?01 Hauling FoodAs the African landscape shifted gradually from dense forests
toward large patches of savannah, early hominids found their food
supplies waning, leading them to descend from the trees and become
ground-dwellers. Because these early human ancestors could no
longer feed where they lived, they were forced to begin carrying
large amounts of sustenance over long distances back to their home
bases—a tricky task had they remained quadrupeds. While some
anthropologists contend that early hominids gathered fruits and
nuts, a few argue that they were scavengers, stealing predators’
kills. An upright stance would have enabled our ancestors to lug
carcasses to safer areas for consumption, while also allowing them
to see other food sources or potential danger at greater
distances.02 A New WorldMany anthropologists hypothesize that our ancestors developed an
upright posture in order to carry food over long distances, but
others believe they stood up merely to find it. As early hominids
left the comfort of the forest to explore the savannah, they no
longer needed a body structure suitable for climbing. Those who
could walk upon two feet were better able to survive because they
expended less energy and could travel longer distances than
knuckle-walkers; they were also better able to see potential
dangers lurking in the distance. Other anthropologists have
suggested further environmental factors that might have helped urge
our ancestors to stand upright, such as the cold and wet ground
conditions that today lead chimpanzees to become temporary bipeds
until they reach dry land.03 Attracting MatesAnthropologist C. Owen Lovejoy stirred controversy in 1981 when
he attributed sex— specifically males’ desire to get more of it—as
a direct reason for why we walk upright. According to Lovejoy’s
behavioral model, males who could walk bipedally freed their arms
to carry more food than their quadruped counterparts could hold,
thus making the knucklewalkers seem far less appealing to females.
In this model, the upright males were simply better breadwinners.
Their ability to ration more food for females (who remained at the
home base to care for the offspring) ensured that they were able to
reproduce, thus leading to future generations of adept bipeds who
in turn were able to pass on their own genes.04 Grabbing a BiteSome anthropologists argue that early hominids could not have
become ground-dwellers and bipeds in a single evolutionary step, as
many hypotheses imply. Instead, they contend, the ability to walk
upright was in part a serendipitous by-product of new feeding
habits. As our ancestors descended from the trees to forage on the
ground for low-hanging fruits and berries, they began to feed from
a squatting position. Over time, physiological changes occurred in
their upper bodies, backbones, and pelvic areas, causing their
weight and centers of balance to shift to a lower point in the
body. This gave the hominids a steadier stance as well as the
ability to stand upright with greater ease than their quadruped
cousins. When our ancestors developed the need to reach higher and
stand, these new physical traits came in handy—just as evolving a
long neck proved favorable for the giraffe.05 Keeping CoolWalking on two feet did more than help early hominids conserve
energy, as some hypotheses suggest—it also protected them from
overheating. According to evolutionary biologist Peter Wheeler,
early bipeds were generally exposed to less direct sunlight on the
savannah than quadrupeds of the same size. In fact, when the sun
shone directly overhead, the heat load upon a hominid on two feet
would have been 60 percent less than that upon a knuckle-walker.
Additionally, bipedalism raised hominids’ bodies above the ground,
enabling their skin to come in better contact with cooler and
faster-moving breezes. This allowed for further heat dissipation
through convection, and, says Wheeler, it meant that biped hominids
needed to consume only about three pints of water per day, whereas
quadrupeds needed five.06 Aquatic ApesAlthough most paleoanthropologists, despite their many
differences, tend to agree that our ancestors became bipeds on dry
land, a few suggest an alternate possibility. Aquatic Ape Theory,
posed by marine biologist Alister Hardy in the 1930s, postulates
that several human traits, from relatively minimal body hair to the
ability to sweat moisture and salt, can be explained only through
the idea that early hominids once lived in semi-aquatic
environments. The hypothesis claims that our ancestors had to wade
regularly through shallow lake- or riverside waters in order to
reach shellfish, aquatic plants, and other potential food sources.
With their heavy upper bodies, quadrupeds would have had a more
difficult time adjusting to walking upright on the savannah than in
buoyant water.07 Weapons and ToolsSome of the oldest and most popular suggestions for why we
developed into bipeds state that our upright posture relates
directly to our need to use weapons and tools. While some
researchers hypothesize that it was bipedalism that brought forth
our ability to use these primitive devices, others believe the
reverse—that the advent of tool and weapon use encouraged us to
become bipedal. Charles Darwin, for one, felt that early hominids
would have been “better able to defend themselves with stones or
clubs, to attack their prey, or to otherwise obtain food” if they
stood, walked, and ran erect, whereas quadrupeds of the same size
would not have been able to exert the same force from a sitting or
squatting position.

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