Two main theories have been proposed to explain the evolution of molars.
1) CONCRESCENCE THEORY OF ROSE
2) TRLTUBERCULAR THEORY OF COPE & OSBORN.
According to the concrescence theory, the molar tooth has evolved by the fusion of several simple cone-like teeth. This theory Is now abandoned in favor of the second with is now accepted with certain modifications. -
Cope & Osborn theory is based on the paleontological evidence. As such the mammalian molar has been derived from a simple reptilian cone by the development of additional cusps. The Mesozoic mammal, the crown of the molar tooth has three cusps arranged more or less in the form of a triangle. This pattern of tooth has been termed tritubercular. It is assumed that the molars of the modem mammals have originated from the primitive tooth by the development of lateral cusps.
In the upper molars the three cusps are known as paracone, protocone, and metacone, The protocone lies on the apex at the inner side, the paracone lies external to it and the metacone to the back of the paracone. In the lower molars the cusps are named protoconid, paraconid and metaconid. The protoconid and metaconid are anterior and posterior to it in position. These cusps are joined by the ridges and later bell-like extensions or talons are produced from the ridges and additional cusps form on them. In this way, the various types of mammalian molars have evolved.