Photo of the Day #44: Adult Chimpanzee

As a follow-up to yesterday's photo of a juvenile Chimpanzee (Pan troglodytes), here's a photo of the adult. The differences between the adult and juvenile are quite striking; the adult skull has a more pronounced brow ridge, smaller incisors compared to the rest of the skull, more prominent canines, and is a bit more prognathus (the jaw sticks out more). Indeed, the infants and juveniles of these apes are not just small clones of the adults but change in important ways as they grow and mature, although as I noted in the comments yesterday juvenile chimpanzees seem to be strikingly familiar, as is the famous Taung Skull of a juvenile Australopithecus. The infant and juvenile features in these organisms seem to share many traits with humans, and one of the most interesting hypotheses is that our own species is paedomorphic, more specifically exhibiting neoteny in which juvenile characters seen in our closest relatives (such as flattened face, large head, projected brain growth rate, and hair being less dense over the body) are retained in adults by the development of some traits being slowed. Many of the features that we often point to as those that make us human and distinguish us from living apes may have more to do with changes in development than natural selection working on individual variations in adults over a much longer period of time, so when we contemplate increase is brain size (for example) we cannot simply say "Larger brains were good and those with larger brains had more intelligence lived longer and had more offspring," and consider the case closed.

[Hat-tip to Chris for schooling me on developmental terms.]