Balance of the head in hominid evolution

In primates it is useful to distinguish three basic types of bipedal posture: (1) agonial, with extended hips and knees as in modern humans, (2) monogonial, with flexed hips but extended knees. and (3) digonial, with flexed hips and knees as in pongids. Early hominids retained an ancestral, forwardly inclined posture of the neck and head. Therefore the body posture of australopithednes must have differed from that in modem man, in which the centre of gravity of the head can be aligned with that of the body, other major centra of gravity, and important axes of rotation in a single frontal plane. It is suggested that in australopithednes the gravitational tilt of the head was counterbalanced by bent hips in association with hyperextended knees (monogonial posture). In australopithecines the increase in brain weight would have counteracted an improvement in the balance of the head. After the neck had assumed a more vertical posture as a consequence of shortening of the face, selection for an improved balance system in the bipedal posture favoured an increase in the weight of the postcondylar portion of the head, accentuated by selection for a posterior shift of the superior nuchal line in order to minimise the force of the nuchal muscles. At this stage the evolutionary increase in brain weight may have been largely a by-product of the process towards perfecting the bipedal posture. When the centre of gravity of the head had first become aligned with that of the body, the conditions of balance of the head had become favourable for a dramatic increase of brain size, as a result of selection for greater learning and storage capacity of the brain.