Skeletal Robusticity

Skeletal robusticity refers to the strength of a skeletal element relative to some mechanically relevant measure of body size, and is generally considered to reflect the magnitude of the mechanical loads that are habitually incurred by that element as the organism interacts with its environment. Through both genetically-mediated adaptations for functionally competent skeletal systems and through epigenetic bone modeling processes that alter the amount and distribution of bone tissue in response to mechanical stimuli, levels of skeletal robusticity serve as an important indicator of the magnitude and nature of the forces normally operating on the skeleton. Robusticity is variable across the skeleton within a single individual, between individuals in a population, between populations, and between species of hominoids, all of which reflects great variation in the magnitudes of biomechanical loads and the behaviors that produce them – variation which operates at all of these levels. Because most apes are quadrupedal in both climbing and terrestrial locomotion, and because the acceleration and deceleration of body mass during locomotion is generally a significant source of mechanical loads on the bones of the limbs involved, the bones of the fore and hind limb tend to exhibit comparable levels of robusticity (usually quantified by geometric analysis of the amount and distribution of cortical bone in the shafts of the long bones of the arm, forearm, thigh and leg). Humans depart from this pattern in having hind limb bones that are substantially more robust than those of the fore limb. This reflects the differential loading of the two limbs during bipedal locomotion, and the lack of a weight bearing role in the upper limb. The earliest members of our genus (Homo habilis sensu stricto) appear to exhibit the ape-like pattern (subequal levels of robusticity in fore and hind limbs), with the modern human pattern (more robust lower limb relative to the upper limb) emerging by about 1.6 million years ago with early African Homo erectus (H. ergaster).