A model for morphological change in the hominid vestibular system in association with the rise of bipedalism
This study re-examines the morphological data and conclusions of Spoor, Wood, and Zonneveld concerning the morphology of the vestibular apparatus in relation to locomotor behavior in hominids (1994). The pedal and labyrinthine morphology of early hominid taxa are functionally analyzed for classification as either obligate bipeds or habitual bipeds with primarily arboreal locomotion. The bony labyrinth is investigated since the anatomy of the semicircular canals of the vestibular auditory system can be determined in fossil crania through computed tomographical analysis. It is thought that a relationship exists between semicircular canal size and locomotor behavior. Functionally modern pedal morphology precedes modern vestibular morphology in the fossil record. Complete modern pedal morphology, however, appears concurrently with modern vestibular morphology first at Homo erectus. A comparison of the genes involved in the development of both pedal and labyrinthine morphology was undertaken. It was found that only fibroblast growth factor 8 (FgfS) and sonic hedgehog (Shh) are shared between these systems in the determination of positional information. It is found that the function of Fgf8 in otic induction and in limb bud formation is very different. It is also found that the function of Shh in vestibular and pedal morphogenesis is different. Therefore, it is unlikely for alteration in the function or in the expression of either gene to result in the observed differences in pedal and vestibular morphology between early hominid taxa: Australopithecus afarensis, Australopithecus africanus, Homo habilis; and Homo erectus. My examination of the data on the timing of changes in pedal morphology rejects Spoor, Wood, and Zonneveld's conclusion. Moreover I find no gene mutation which could account for simultaneous change in the shape of the semicircular canals and the proportions of the metatarsals and pedal phalanges. Instead, it is postulated that the change to modern vestibular morphology at Homo erectus is in response to a concurrent enlargement in cranial capacity. It is also postulated that persistence of panid vestibular morphology in the semicircular canals of hominid taxa: Australopithecus afarensis, Australopithecus africanus, and Homo habilis is a functionally neutral trait in regard to bipedal locomotor capability.