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C O U R S E 
Human Evolution: Past and Future
John Hawks, University of Wisconsin
C O U R S E   L E C T U R E 
Hominid Bipedality
Notes taken on July 26, 2016 by Edward Tanguay
humans walk upright
walking up a hill, you realize one of the consequences of that
we're able to walk relatively efficiently
this has compromises for other forms of locomotion
we're not especially good at climbing
we've descended from a mammal that are especially good at:
good at walking quadrupedally
as hominids, we took a different pathway
which had consequences for our skeleton
changes in our skeleton due to adaptation to bipedality
big toes in line with other toes
that's a specialization so that we can toe off with each step directing our energy forward
the major element of our propulsion during our walking
longitudinal arch
allows us to capture forward motion energy and spring it back into the next step
an specialized ankle
much better for moving back and forth than making our feet close side to side to e.g. grasp around branches
specialized knees
instead of our femur sitting on top of our tibia as a column
our femus is angled
each time we put our weight on one leg, we need to put our center of gravity over the foot that supports us
hip joint including femur and pelvis
our pelvis has blades which wrap around and create a basin which supports our abdomen
in our relatives, these are relatively flat
humans have a very large muscle in the back side called the gluteus maximus
this is actually a minor muscle in other kinds of primates
in hominids, this muscle allows us to powerfully extend our legs
if you are climbing a set of stairs of climbing, you'll feel those muscles working and getting tired
in humans the change in orientation of our pelvis makes us use that muscle
in primates, who have a very differently shaped pelvis, they use the hamstring muscles more than their gluteus maximus
the hamstring muscles are the muscles that a sprinter uses to start in the starting blocks
primates use it all the time because their bodies are in that stance habitually
in hominids, we carry our weight upright
as a consequence, the hamstring muscles are not especially effective for us unless our bodies are bending over
but when we walk and especially when we walk upwards, we are exercising our gluteus maximus
in order reorient ourselves to a vertical posture, we have a curavture of the lower spine
Lumbar lordosis
each of Lumbar vertebrae are angled somewhat to the next
to allow the architecture of our spine to carry our body upright
we haven't changed the angle of our pelvis relative to the ground
what we've changed is the anatomy of that pelvis in such a way that a primate-like pelvic carriage is operating in a bipedal stance
the head
in the base of the head, the foramen magnum
the hole in the base of the skull through which the spinal cord passes
is relatively anterior, toward the front, because we have a vertical carriage of our head
in other kinds of primates, that have a more inclined or even horizontal spine posture during locomotion, the foramen magnum is more toward the back of the skull
bipedalism is similar and different in humans and other Hominidae
we think of bipedality as part of human adaptation today
if you think of chimpanzees, orangutans, and other apes, they are all able to walk bipedally
they use bipedal locomotion as part of their repertoire when they are on the ground
all of them can carry objects in both hands and move around this way
but they do it a little differently than we do
the architecture of their pelvis and the musculature of their legs are not well adapted to having their weight supported as a straight up-and-down column
yet this is very efficient for humans
we are able to stand for long periods of time with relatively little muscle action
if they are going to be upright, they are going to have to have a bent knee
they have to maintain their femura at an angle relative to their pelvis so their muscles have leverage
they have to walk with a bent knee gait
this gets tiring after awhile
humans, however, are able to maintain an upward gait
with great energy efficiency
what led our ancestors to abandon the ability to move fast on all fours and to climb fast into through trees
advantage of efficiency
we are able to stand for long periods of time
walk at a normal walking pace for long periods of time
with great efficiency
it doesn't take us very much muscle action in order to maintain a normal walking pace
with each step when we push off with our toe, our leg swings forward
that swinging is a pendulum
gravity is doing most of the work to move our leg forward
all I have to do for each step is to fire my leg forward a little bit to start the swing forward
and transfer the weight of my body
allow my body to fall forward, and then push off with the toe of the other foot
this forward movement is not costing me much energy at all
advantage for having head high
having our eyes and ears higher up above the ground was an advantage
one advantage in predation is how big you are, how tall you stand, and how big you appear
a small bit of bluff might make a difference