Re: Did evolution welded pre-molar into molar teeth?

Are human molars the equivalent of two premolars?

During the course of evolutionary history one of the innovations of early vertebrate animals was to form different types of teeth (the heterodonts). These teeth became specialized in their functions, so that when mammals evolved we had a basil pattern of: 2-1-4-3 / 2 -1-4 -3 (the slash means that these numbers should be written one on top of the other. The numbers indicate the numbers of each type of tooth in each half (left right halves) of the upper and lower jaws. So, the basil mammalian pattern reads 2 incisors, 1 canine, 4 premolars and 3 molars for the one half of the upper jaw and 2 incisors, 1 canine, 4 premolars and 3 molars for one half of the lower jaw. Each of these tooth types has a different basic function, but for the most part premolars are used for slicing and molars are used for grinding.

When Old World primates evolved (the catarhines) the tooth pattern had shifted slightly. The first two premolars were lost, so that the tooth pattern now reads: 2-1-2-3 / 2-1-2-3. This is the same pattern we see in apes and humans.

Early human ancestors, that differentiated from the other apes, are noted for the increased use of the grinding tooth function. Human ancestors have much bigger molars and thicker tooth enamel than is seen in other apes. As the evolutionary process progressed the grinding function became more emphasized, and so the human premolar lost some of its “slicing” function and adopted more of a “grinding” function. This means that the human premolars began to look more like molar teeth because they were functioning more like molar teeth. In the robust australopithecines (one of the dead end side branches of human evolution) the premolars and even the canines look very much like molar teeth.

So, your observation that molar teeth look like two premolars put together is not too far off. But, what actually happened is that the premolars evolved to look something like “half” a molar tooth.