Quadrupedal To Bipedal

Many of the changes that occur along the hominin lineage involve adapting to bipedalism from a quadrupedal ape's body (Figure 3.6). Built for climbing, swinging, and brachiating through the trees, ape bodies feature short, flat trunks, broad hips, long collarbones (clavicles), shoulder blades (scapulas) on the back instead of on the side of the body (like a monkey or a dog), a round head of the humerus which makes for a flexible rotator cuff at the shoulder, semierect posture, long arms, short legs, elongated forelimbs, long curved fingers, and no tail.

Humans still possess many ancestral ape-like traits that were the springboard for not only the evolution of bipedalism but also the evolution of overarm throwing and tool making which depends, in part, on our retention of some of this brachiating anatomy. Major changes in the limb proportions occurred due to bipedalism, since it is better to have long legs for a striding gait, humans developed much longer legs for the body size than apes have. We also have much shorter arms than apes relative to our body size and a few of the reasons are probably to do with greater control during tool use and throwing and also the optimal arm length for swinging arms by the side during walking and running.

The earliest hominins had a similar body plan to living gorillas, chimpanzees, and bonobos and it is debated whether or not they walked on their knuckles like them too. The knuckle-walking wrist must be strong and rigid to support the weight of the animal, so the morphology of the radius shows a locking mechanism to prevent collapse at the wrist. There may be anatomical remnants of knuckle-walking adaptations in early australopiths but there are few arm and wrist bones and the traits appear to be intermediate. Evidence from earlier hominins, with less bipedal adaptations (and presumably a higher chance of retaining such adaptations if they were there in the first place) has not been discovered yet.

Parsimony supports the LCA as a knuckle-walker. If the common ancestor of gorillas and chimpanzees was a knuckle-walker, then it makes sense that the LCA of humans and chimpanzees was one too, and that humans lost knuckle-walking adaptations as they evolved bipedalism. Nature does not always obey our rule of parsimony, so it is also possible that knuckle-walking arose independently on both the gorilla and chimpanzee lineages, meaning that it is possible the LCA did not yet have knuckle-walking behavior. Genetic analyses have shown that chimpanzees and humans are more closely related than chimpanzees are to gorillas so independent acquisition of knuckle-walking in the great apes cannot be ruled out yet. More fossils of ape and human ancestors from the crucial period during the late Miocene and early Pliocene will eventually sort out the hominin knuckle-walking question.

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