1Anthropology, New Mexico State University, 2Anthropology, University of Michigan
Thursday Morning, Ballroom C
Plantigrade foot posture during terrestrial locomotion, where the heel strikes the ground first, is an adaptation that is uniquely shared by African apes and humans. All other primates utilize a semi-plantigrade foot position with a heel-elevated posture or have adapted their feet for suspension without heel contact with the substrate.
The foot of Kenyapithecus africanus from Maboko Island displays features for plantigrady in common with African apes. The most notable feature of Kenyapithecus is enlargement and plantad orientation of the navicular tuberosity (KNM-MB 31335), where the tendon for tibialis posterior inserts. The navicular tuberosity extends distally beyond the cuneiform facets, as in gorillas. During the mid-stance phase of terrestrial walking, the plantar portions of gorilla and chimpanzee tarsals contact the substrate. Since African apes lack longitudinal arches, weight-bearing tubercles are produced on the plantar sides of the tarsals. This orientation and development of the navicular tuberosity is not present among other Miocene hominoids. Another African ape-like feature is the enlargement, plantad orientation and laterally extensive nature of the peroneal tubercle of the fifth metatarsal (KNM-MB 28397), which is oriented plantarly.
The evolution of terrestrial plantigrady among African apes and humans is not well understood. Dating from 15 MA, Kenyapithecus africanus is the first Miocene hominoid to display terrestrial plantigrade features and provide context for understanding the origin of African ape and human locomotion. The foot of Kenyapithecus indicates that the last common ancestor of African apes and humans was ape-like and not monkey-like as claimed from Ardipithecus foot remains.