The molar size relationship is one of the peculiar characteristics of hominins species, and various theories have been proposed to account for this, as well as the differences in shape between the types of teeth (incisors, canines, premolars and molars). The latest theory, called the inhibitory cascade model, arose out of experiments with mice embryos, and in 2016, it was applied theoretically to fossil hominins with satisfactory results.
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Source: Phys.org