Walking Tall: Understanding the Biological Adaptations that Led to Bipedalism

If we look at our hominid (human) ancestors we will see several defining characteristics that differentiate hominids from other primate species.  While early scholars believed it was our large brains that defined humans as humans it was actually the ability to walk bipedally, or the ability to walk on two legs.  This is a feature that is unique to hominids and humans as we are the only species that are able to walk bipedally.  But how exactly did our hominid ancestors adapt the ability to walk on two legs?  Today’s blog post will discuss the morphological (the form and structure of the body) changes that led to bipedal adaptations in hominids and, ultimately, humans.

Figure 1: Pelvic Girdle & Upper Leg Differences Among Three Primate Species

There are several anatomical features that lend themselves to providing organisms the ability to walk on two legs: pelvic, upper leg, spinal column, and cranial changes.  In order for an organism to have the ability to be a biped the pelvis (hips) must change to allow for a wide range of motion in the legs.  If you look at hominids and humans and their ability to move they have a ball and socket joint in both of their hips, which allow for unrestricted motion (as long as the individual has the flexibility to do so).  Quadrupedal primates have limited ranges of motion based on the placement and angle of the hip bones (Figure 1), which leads to some but not full bipedal abilities.  Typically, if quadrupedal primates (or animals, in general) attempt to walk bipedally it is for short periods of time and can also be quite painful.  

Figure 2: Lumbar Curve

In addition, the placement of the pelvic bones allows for the angling of the upper leg bones.  This allows for an upright posture, which is necessary for bipedal locomotion.  The angling of the legs, however, is not the only feature required for upright posture.  A lumbar curve, the curvature of the lower vertebrae/part of the spine, also enable hominids and humans to stand upright (Figure 2).  This is why individuals with degenerative spinal disorders have difficulties walking because the lumbar vertebrae are most often affected, causing a hunching forward that inhibits the upright posture and effective bipedal locomotion.

Figure 3: Foramen Magnum Placement Among Primate Species

Last but not least, cranial changes are also necessary for effective bipedal locomotion.  The foramen magnum is a large hole in the skull where the spinal cord means the skulls, and the placement of the foramen magnum is a determining factor in the locomotion pattern of each species.  A posterior placement causes quadrupedal locomotion, whereas an inferior placement allows for bipedal locomotion because it allows for the upright posture (Figure 3).

These anatomical features first appeared in our hominid ancestors approximately 4 million years ago.  It is unclear as to why our hominid ancestors developed bipedal locomotion as it has its disadvantages (e.g. reduction in speed as compared to quadrupedal species), but it may have also had several advantages.  But this is a topic for a later blog post (stay tuned!)    

Bibliography

Jurmain, Robert, Kilgore, Lynn, Trevathan, Wenda, Ciochon, Russell L. Introduction to Physical

Anthropology. Belmont, California: Wadsworth, 2014. Print.

Stein, P.L. & Rowe, B.M. (2014). Introduction to Human Evolution and Prehistory.  United

 States of America: McGraw Hill Education.