The study of craniofacial form and variation has always been one of the most important areas for those interested in shaping primate evolution. Skulls were the most frequently collected specimens in museums. Skull parts, especially teeth, are most frequently found in the fossil record. Skulls and teeth are easily examined in the living. The bones of the face allow some estimation of how their owners appeared. Appearance and change in appearance as produced by medical and dental technologies have profound effects upon individual well-being. All these are good reasons why this is one of the most critical of anatomical regions.

At the same time, however, skulls, faces, jaws, and teeth are the most complex region of the body. More, perhaps, than in any other region, do a number of completely different functions have to be integrated in its structure. The genetics underlying cranium, face, jaw, and teeth are even now not well known and clearly far more complicated than the postcranium. The development and growth of the head depends upon complex mechanisms and processes, many of which have only been elucidated in the last two decades. In evolutionary terms, the “head problem” in chordates, reflecting at the same time both very ancient and very recent elements, has always been more difficult to understand than, say, the equivalent trunk problem or limb problem (which problems do not even rate quotation marks).