He who rejects with scorn the belief that the shape of his own canines, and their occasional great development in other men, are due to our early progenitors having been provided with these formidable weapons, will probably reveal by sneering the line of his descent. For though he no longer intends, nor has the power, to use these teeth as weapons, he will unconsciously retract his “snarling muscles”… so as to expose them ready for action, like a dog prepared to fight.
Charles Darwin's analysis of the size and shape of human canine teeth convinced him that at some point, human male ancestors possessed large and projecting canines, similar to those of male gorillas or orangutans. In Darwin's view, male canine teeth, like other male armaments such as horns in mammals or spurs in birds, were the result of sexual selection: the advantage that members of the same sex have over one another in “exclusive relation to reproduction.”
Darwin conceived of sexual selection happening in two ways: first, through competition among males for mating opportunities with females, and second through differences among males in their ability to attract females. These two sides of the sexual selection coin are called intrasexual and intersexual selection, respectively. In the first case competition occurs among males and in the second, females select males who they find most attractive (or on the basis of other traits that would presumably enhance the survival of their offspring). Thus ensues the evolution of armaments, like the large curved horns of male bighorn sheep, and ornaments, like the peacock's tail. Slashing canine teeth, in Darwin's view, were a male armament which evolved through time because males with larger and sharper teeth were able to win more contests for females and thus pass these traits to their offspring.
Darwin unfortunately did not have a satisfactory answer for why there is a general pattern in nature of male-male competition for mates and female choice of them, but later work (116, 117) offered an explanation in terms of sex differences in investment in offspring. In mammals, it is usually females that make an enormous investment in offspring through gestation and lactation. Males, who by comparison invest much less, compete with one another for access to females.