Human and chimpanzee babies share a similar skull shape. As chimpanzees develop certain facial bones grow rapidly compared to others resulting in the characteristic pronounced brows and elongated jaw of an adult chimpanzee. These facial bones grow in humans too, but at a much slower rate. A consequence of this is that our adult skulls look more like the juvenile skulls of both species, but a mature chimpanzee's skull looks pretty different!
Growing different parts of the body at different rates is called allometric growth and it plays a central role in proportioning the adult form. Because a small genetic change can alter relative growth rates significantly, mutations that affect allometric growth can result in very different looking adult forms in closely related species.
An image of Pinkie, a chimpanzee who lives in a sanctuary for orphaned primates in Tacugama. Her unusual coat of white fur makes this infant picture of her quite striking. Click it to see a larger version.
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