Origins of Bipedality and Canine Reduction

Chimpanzees and gorillas are our closest cousins. Most presume that the behaviors of the earliest hominins were like those seen living apes. One effect of chimpanzee-centric models of human evolution has been a tendency to view Australopithecus as transitional between an ape-like ancestor and early Homo.

Ardipithecus fossils show that the anatomy of living African apes is not totally primitive but has itself evolved in its own direction. It is incorrect to assume that the anatomy and behavior of early hominins was exactly like that of chimps. The earliest ancestors, all placed in the Ardipithecus group for this class (although some are referred to with different genus names) were already starting to be bipedal. It provokes many questions about the nature of the common ancestor. Could it have been more bipedal than chimps or gorillas are today? Additionally, the Ardiptithecus is not fully bipedal.

Ardipithecus comes before Australopithecus, and it has smaller molars compared to Australopithecus. It also has canines that are much smaller than a chimp's. Comparisons of the Ardipithecus dentition with those of all anthropoids suggests than male-to-male aggression is less of a factor in early hominins. Some have suggested than male parenting and provisioning, related to bipedality, is part of this.

Human (left), Ardipithecus (center), and chimp (right). Note canine size.

There are many other hypotheses also. Possible reasons for the evolution of human bipedalism include freeing the hands for tool use and carrying, sexual dimorphism in food gathering, changes in climate and habitat (from jungle to savanna) that favored a more elevated eye-position, and to reduce the amount of skin exposed to the tropical sun.

One thing that is for sure- big brains are still millions of years off. We will not start to push our brian size beyond chimps for millions and millions of years after the split with chimps. Bipedality comes first.

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