The area of the chin is known anatomically as the mental region. It tends to be smaller and more rounded in human females, while bigger and more square in human males. It is formed by the lower front of the mandible. In humans there is a wide variety of chin structures, e.g. cleft chin. The chin developed as a point of muscular attachment facilitating minute movements of the lips associated with speech. In human evolution, the chin is a cladistic apomorphy, partially defining anatomically modern humans as distinct from archaic forms. Non-human anthropoid apes have a simian shelf for example. Elephants are the only other animals considered to display such a feature, although this leads to debate over the use of the term. The chin emerged during the Middle and Late Pleistocene, but its origin and biomechanical significance are the subjects of controversy. Prominent hypotheses include buttressing the jaw against stresses resulting from speech or chewing as well as simple sexual selection through mate choice. With the advent of more advanced computational facilities, finite element analyses have been used to support hypotheses involving mechanical stress. On the other hand increased availability of data regarding sexual dimorphism in chins has also lent support to the sexual selection hypothesis as sexual dimorphism is more difficult to explain under other regimes. It is possible that multiple causal factors have played a role in the evolution of this bony protuberance.