Bipeds have adapted a number of interdependent morphological characteristics that solve challenges posed by habitual bipedalism. These anatomical adaptations evolved over millions of years and differences exist between earlier and later hominin species (i.e., Australopithecus, Paranthropus, and Homo). Australopith and paranthropine evolution represents a notable step in the evolution of humans because these species are among the earliest hominins known to have evolved the adaptation of bipedalism.
Major morphological features diagnostic (i.e., informative) of bipedalism include: the presence of a bicondylar angle, or valgus knee; a more inferiorly placed foramen magnum; the presence of a reduced or nonopposable big toe; a higher arch on the foot; a more posterior orientation of the anterior portion of the iliac blade; a relatively larger femoral head diameter; an increased femoral neck length; and a slightly larger and anteroposteriorly elongated condyles of the femur. Each of these features is a specific adaptation to address problems associated with bipedalism.
All of the anatomical adaptations necessary for habitual bipedalism can be found in the fossil record. By reconciling the fossils evidence with the geologic time scale, it is possible to hypothesize about the evolutionary origins of bipedalism. The following is a detailed discussion of each morphological adaptation for habitual bipedalism.
