Shadows of Forgotten Ancestors

The delight shown by the members of a patrol after having successfully returned from dangerous—perhaps enemy-held—territory is little different from what happens when chimps unexpectedly encounter a substantial cache of food. They screech, kiss, hug, hold hands, pat one another on the shoulders and the rump, and jump up and down. Their camaraderie is reminiscent of teammates in mutual embrace just after winning the national title At the start of a heavy rain, male chimps often perform a spectacular dance. On coming upon a stream or waterfall they display, seize vines, swing from one tree to another, and cavort high above the water in a breathtaking acrobatic performance that may last for ten minutes or more. Perhaps they are awed by the natural beauty or entranced by the white noise. Their evident joy sheds a revealing light on the eighteenth-century doctrine³² that humans are right to enslave other animals because we are unmatched in our capacity to be happy.

The prescription offered by Sewall Wright for a successful evolutionary response to a changing environment closely matches many aspects of chimp society. The species is divided into free-ranging groups, generally comprising between ten and one hundred individuals. They have different territorial ranges, so that if the environment alters the impact will be at least slightly different from group to group. A staple food at one end of a vast tropical forest may be a rare delicacy at the other. A blight or infestation that might result in serious malnutrition or famine for chimps in one part of the forest might have negligible consequences in another. Each territorial group is enough inbred that the gene frequencies differ systematically, group to group. And yet the pattern of inbreeding is relieved by exogamy (outbreeding). There are key sexual encounters with chimps from adjacent territories, initiated either when a patrol penetrates into alien territory or when a foreign female wanders over. These unions provide genetic communication, group to group, so that if in an adaptive crisis one group were more fit than the others, the adaptation would rapidly spread to the entire chimpanzee population through a sequence of sexual contacts—perhaps hundreds of copulations in a chain linking the remotest groups of a vast tropical forest. If there’s a modest environmental crisis, the chimpanzees are ready.

If this is indeed the explanation, at least in part, of the territoriality, ethnocentrism, xenophobia, and occasional exogamy that characterize chimpanzee society, we do not imagine that individual chimps understand the reasons for their behavior. They simply can’t stand the sight of strangers, find them hateful and deserving of attack—except, of course, for the chimps of the opposite sex, who are unaccountably exciting. The females occasionally run away with strange males, no matter what crimes they may earlier have committed against their land and kin. Perhaps they feel something of what Euripides makes Helen of Troy feel:What was there in my heart, that I forgotMy home and land and all I loved, to run awayWith a strange man?…

    Ah, husband still, how shall thy hand be bentTo slay me? Nay, if Right be come at last,What shalt thou bring but comfort for pains past,And harbour for a woman storm-driven:A woman borne away by violent men …³³

Mothers know who their sons are and so can preferentially resist their (very rare) sexual advances. But fathers are not sure who their daughters are, and vice versa. Thus, when a female comes of age in a small group, there’s a significant chance of an incestuous union, further inbreeding, more infant mortality, and fewer of their genetic sequences passed on to future generations. So around the time of first ovulation, a female often feels an inexplicable urge to visit the neighboring territory. This can be a dangerous undertaking, as she probably understands full well. The compulsion, then, must be strong, which in turn underscores the evolutionary importance of her mission. Combine this not uncommon itch to wander at first ovulation with the rarity of brother-sister and, especially, mother-son unions and it’s clear that a high-priority, well-functioning incest taboo is operating among the chimps.