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Common Ground
Natural History, Dec, 2000 by Barbara Smuts
Studies of the social and emotional lives of forest apes reveal the evolutionary roots of human nature.
What sort of species are we, and how did we come to be this way? When scientists attempt to answer these questions. they tend to emphasize our upright stance, large brains, manual dexterity, complex use of tools, hunting ability, and capacity for spoken language and abstract reasoning--all ways in which we differ from other animals. When scientists focus on human social and emotional tendencies, however, they uncover striking resemblances between ourselves and the great apes. Any full definition of what it means to be human must include the traits we share with our closest living primate relatives.
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Recognition of our profound kinship with great apes began 141 years ago, when Charles Darwin startled the world with the implication that humans had evolved from an apelike ancestor. Yet serious study of apes in the wild did not begin until a century after the publication of the Origin of Species, and only in the past decade have scientists reached a consensus about the evolutionary relationships between humans and great apes. DNA evidence indicates that we last shared a common ancestor with the orangutan roughly 15 million years ago, with the gorilla roughly 8 million years ago, and with chimpanzees and bonobos (the latter sometimes known as pygmy chimps) about 5 million to 7 million years ago. These molecular data not only confirm that chimps and bonobos are our evolutionary kin but also show that they are more closely related to us than they are to gorillas. (Because the DNA evidence also implies that humans are a kind of great ape, I refer to the others as forest apes.)
In several early issues of Natural History, in articles written before scientific behaviorism made the study of animal emotion unfashionable, authors described captive apes raised in association with humans, usually as pets or as circus performers. A good example is Henry Sheak's "Anthropoid Apes I Have Known," published in 1923. Sheak, a lecturer at the Museum, gave a detailed account of how affectionate some chimps could be. He told of their suffering when their favorite ape or human companions went away, of the enthusiasm with which they greeted returning friends, and how they shared food. Most remarkable to Sheak was that the chimpanzee "understands how to express affection and gratitude by hugging and kissing without being taught. This can only mean that these modes of expression are very, very old in the primate group."
Sheak's conclusion anticipated discoveries made decades later, when field studies revealed that many aspects of human emotional expression, including hugging, kissing, begging, and bowing, are homologous with those of chimpanzees. To understand these and other emotional similarities between forest apes and humans, we need to look at how natural selection has shaped the apes' (and ultimately our own) social tendencies.
Orangutans, which live in Southeast Asia, are largely solitary, but the other forest apes, of Africa, live in complex societies that revolve around long-term bonds between particular individuals. African apes are strongly motivated to form such relationships. Among the chimpanzees of Gombe Stream National Park in Tanzania studied by primatologist Jane Goodall, the strength of the mother-infant bond can determine whether or not the infant survives. Similarly, the degree of trust between two allied males can influence each one's ability to obtain status in the group and to father young. Among bonobos, a male's position in the community and probably also his mating success depend in part on his maintaining a close bond, even as an adult, with his mother. A mountain gorilla female can raise an infant only if she has a close association with a particular silverback male, and a male gorilla's success in siring offspring reflects his ability to woo females and keep their loyalty over the course of many years. Bonobo females rely on strong friendships with other females to gain access to the best foods and to prevent males from bullying them and their young.
Forest ape relationships are often long term, based on a history of interaction, and intensely personal. Because each ape's social success depends on what other group members are up to, natural selection has favored the capacity for social maneuvers and counterploys, as illustrated by primatologist Frans de Waal's accounts of Machiavellian power struggles among male chimpanzees at Arnheim Zoo in the Netherlands. Selection has also promoted the ability to influence the relationships of others. Witness Mama, the most influential female in the colony that de Waal studied. If two males failed to reconcile after a tense encounter, Mama would sometimes make contact with one of them and then go sit next to the other, pulling the first male by the arm if he failed to follow her. Both males would then groom her, and when she melted away, the males would find themselves placidly grooming each other. Chimps and bonobos appear to place value on maintaining good relations, renewing friendly contact after a fight. Bonobos reconcile by having sex; chimps literally kiss and make up.
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