Human Origins - The Origins of Bipedalism.

        The morphology of the foot has also changed to accommodate bipedal walking.  The foot of an ape has a large gap between the last toe and the other four and is relatively flat on the bottom.  Humans have gained a reinforced heel and an arch to absorb shock.  The toes have become aligned to point forward, allowing for a smoother ending to the “propulsive phase” of the stride.

        Other adaptations for bipedalism pertain to the location and function of muscles.  The gluteus maximus in apes is located at the side of the hip, in order to pull the thigh outward.  In humans, it is at the back of the illium, pulling the thigh back during the “propulsive phase” while walking and running.

        Skeletal changes that have occurred due to bipedalism include the flaring out of the illium, the s-shaped curvature of the vertebral column, and the repositioning of the foramen magnum.  The iliac blades are shorter and wider, forming a basin that cups and protects the organs.  Additionally, they can better support the direct pressure of upright posture.  The s-shaped spine centers the weight of the upper body over the pelvis.  This is in place of a quadruped’s straighter spine.  The foramen magnum is repositioned from the back of the skull to the base of the skull.  This change allows humans to look forward while upright.

        All of these changes occurred over the course of many millions of years.  We understand how they occurred, but there are many different theories that attempt to explain why.  Some of the most compelling come from Peter Wheeler, C. Owen Lovejoy, and Kevin Hunt.

        Wheeler’s thermoregulatory theory proposes that bipedalism originated because an upright stance allows the body to cool more effectively than that of a quadruped.  The theory is based on the idea that at noon, in an open area such as the seashore or the savannah grasslands of Africa, an upright individual would be exposed to more wind and less sun than a quadruped.  The wind would facilitate cooling through sweat evaporation, allowing the individual to be much more efficient.  This would allow upright individuals to exploit the “midday niche” while facing little competition from quadrupeds, who are more susceptible to overheating. 

        Lovejoy’s behavioral model for bipedalism centers on the theme of a family.  Instead of both parents taking care of their offspring and getting food for the family, bipedalism would allow one parent to travel long distances while carrying food.  The other parent would be left to take care of the offspring, increasing the chance of survival.  The larger area that can be exploited by a bipedal individual allows for more available food for that individual’s family.  This also increases the survival rate of their offspring, essentially selecting for bipedalism.

        Lovejoy backs his theory up by connecting it to human sexual behavior.  He claims that the monogamous mating structure of this model accounts for the elimination of any sort of mating season in humans.  Mating seasons exist so that mating can coincide with a time when energy and resources are not needed for offspring.  Since bipedalism allows families access to more resources, copulation can occur at any time.

        Hunt’s postural feeding hypothesis suggests that bipedalism originated in quadruped feeding activities.  Essentially, the terrestrial quadrupeds that could stand up were able to get the most food.  His hypothesis is backed up by studies of chimpanzee behavior.  According to Hunt, “Eighty per cent of chimpanzee bipedalism was during feeding.”

        Hunt claims that retained morphological aspects of arm hanging helped the early quadrupeds reach for food.  He explains evidence that these traits, including long arms and fingers, were present in australopithecines.

        Although these theories all seem to be solid and well based, they each contain at least one implausible aspect.  Wheeler’s thermoregulatory theory does not apply to enough geographical situations.  This limitedness only becomes worse when the areas he proposes are considered rationally.  For example, even the African savannah had tall vegetation.  Bipedal hominids that walked by some dense shrubs or even tall grass would be blocked from the wind.  Additionally, the bipedal advantage only lasts for a few hours in the middle of the day.  The benefits do not last long enough to really affect the efficiency of an individual.  It is therefore unlikely that the thermoregulatory theory was entirely responsible for bipedalism.

        Lovejoy’s theory is the most probable of the three.  His claims make a lot of sense because of how he ties in other aspects of human development to bipedalism.   The only problem that isn’t addressed in his article is how women developed bipedalism.  According to his theory, males went out to provision for the family while females stayed behind and took care of the babies.

        Hunt’s theory makes interesting points, but it conflicts with more sound evidence that early hominids and apes evolved from a knuckle-walking quadruped.  It is also much more vague than the other theories.

        Many other theories exist that attempt to explain the shift from quadrupedal locomotion to bipedalism.  Each offers compelling evidence to support its claims.  Additionally, each is flawed in one way or another.  Without new evidence, we may never know the true origin of our most unique characteristic.