Civilization is at war with nature. That is true at least with regard to facial hair. In the heat of this centuries-long battle, on which billions of dollars are spent each year, few have paused to consider how the war began in the first place. Why did nature give men—and some women—beards? How did they end up with a band of hair on their cheeks and chins that society requires they scrape off every day? If one hopes to discover the meaning of beards, it makes sense to start with these basic questions. And that will require us to peer into the mists of the evolutionary past.

It is tempting to think that beards are a holdover from our much hairier progenitors, that for whatever reason, this trait survived as we developed into the naked ape. Yet bonobos, our closest relative in the animal kingdom, lack hair around their mouths—precisely where the human beard grows. It would seem that, if anything, human beings have added hair to their faces, even as they lost it most other places. Even if our ape ancestors had had hairy faces, a question would remain: Why did women lose this hair while men retained it? As it is, a hairy chin and upper lip are virtually unique to the human male.

Beginning with Charles Darwin himself, evolutionary theorists have pondered the origins of the beard. In The Descent of Man (1871), Darwin described a process of sexual selection that operates in tandem with natural selection in shaping the course of human development. Natural selection changes a species by favoring individuals with traits that enhance their chances of survival and procreation. When it comes to procreation, however, there is another level of selection as individuals within a species compete with one another for the favor of sexual partners. Darwin reckoned that, for the purposes of this competition, animals evolved many secondary sexual characteristics that functioned either as weapons to defeat sexual rivals, such as horns or tusks, or as ornaments to attract potential mates, such as colored hair and feathers. Individuals with the more appealing ornaments or stronger weapons would succeed in reproducing themselves and propagating their distinctive traits. Darwin assigned the human beard to the category of ornament, and imagined that it had the power to attract women. Over the millennia, the theory goes, bearded men were more successful in procreation than their smoother competitors, and the human beard evolved into its present form. In short, men now have beards because our prehistoric female ancestors liked them.

But Darwin saw a problem with this idea. Anthropologists of his day reported that human populations varied widely in the fullness of the male beard. It was believed that Native Americans, for example, were nearly incapable of growing them. Darwin surmised that some ancestral women in some particular places must not have liked the beard and because of that prejudice continually selected against it. That is, the beard functioned as an ornament only among peoples who in fact considered it to be an ornament. To help resolve this conundrum, Darwin invoked still another evolutionary process: the inheritance of acquired characteristics. Before Darwin, Jean Baptiste Lamarck had argued that species change over time by passing along newly acquired traits to their offspring. If a giraffe, for example, spends a lifetime stretching its neck to reach food in the treetops, its progeny will be born with longer necks.

Though many a schoolteacher or professor might dismiss the inheritance of acquired characteristics as un-Darwinian, Darwin repeatedly invoked this this principle in The Descent of Man, and did so again on the matter of beards. Noting anthropological observations of peoples who were relentless in plucking unwanted facial hair, and referring to (very dubious) experiments that appeared to show that surgical alterations in animals could be passed on to the next generation, he concluded that “it is also possible that the long-continued habit of eradicating the hair may have produced an inherited effect.” In other words, men who cut or pulled their facial hair would beget boys who grew less facial hair as adults. The inheritance of acquired characteristics thereby completed a process begun by sexual selection, leaving some groups of men with great thick beards and others with almost none. This analysis assigned women a great deal of influence over beard evolution: they chose more or less bearded men according to their tastes, and men plucked their hairs to accommodate them, which in turn led to permanent physiological changes.