Evidence that habitual upright bipedality is a synapomorphy that defines a hominiform clade of hominoids including humans and all extant apes.
77th Annual Meeting of the American Association of Physical Anthropology, April, 2008
Department of Neurosurgery, Institute for Spinal Disorders, Cedars Sinai Medical Center, Los Angeles, CA.
Developmental anatomy of young juveniles in all extant hominoids reveals a homological discontinuity producing a transposition of the dorso-ventral body septum and the neuraxis when compared with other catarrhines. This study evaluates this among 250 mammalian species.
Osteological evidence shows the character in Morotopithecus, but not in proconsulid hominoids such as Proconsul and Nacholapithecus. The functional implications have a major bearing on the biomechanics of postural spinal architecture and reflect the onset of habitual orthogrady in this clade as synapomorphic. This study also suggests that heelstrike bipedalism, revealed in juvenile Siamangs in this study, is also synapomorphic for a hominiform clade.
Since the boxlike cross-section of the lumbar transverse process in Morotopithecus and Homo is unusual and adaptive to habitual bipedal as opposed to suspensory orthogrady, it constitutes a fourth synapomorphy for hominiforms. These four traits place habitual orthograde bipedalism as a synapomorphy at the stem of an Early Miocene hominiform clade. Since this proposed ancestral, clade-founding species is a hominoid with habitual upright posture and heelstrike bipedalism, this constitutes a “Humanian” model as opposed to the “Troglodytian” model in which bipedalism arises from a knuckle walking ancestor or the “Hylobatian” model in which it arises from a brachiating ancestor.
This Humanian model proposes a continuous series of primarily upright bipedal hominoids from the Early Miocene to the time of the chimp/human split 6 million years ago (e.g. Oreopithecus, Pierolapithecus, Sahelanthropus). Hominoids with other specialized forms of locomotion emerge to enter different niches at various intervals.