<P> 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 chin's evolution . </P> <P> The terms cleft chin, chin cleft, dimple chin, or chin dimple, refer to a dimple on the chin . It is a Y - shaped fissure on the chin with an underlying bony peculiarity . Specifically, the chin fissure follows the fissure in the lower jaw bone that resulted from the incomplete fusion of the left and right halves of the jaw bone, or muscle, during the embryonal and fetal development . For other individuals, it can develop over time, often because one half of the jaw is longer than the other, leading to facial asymmetry . </P> <P> This is an inherited trait in humans, where the recessive gene causes the cleft chin, while the dominant genotype presents without a cleft . However, it is also a classic example for variable penetrance with environmental factors or a modifier gene possibly affecting the phenotypical expression of the actual genotype . Cleft chins are common among people originating from Europe and the Middle East . </P> <P> In Persian literature, the chin dimple is considered a factor of beauty, and is metaphorically referred to as "the chin pit" or "the chin well": a well in which the poor lover is fallen and trapped . </P>

Where does the dimple in the chin come from