Since Darwin’s Descent of Man, the ongoing debate on human nature has taken a new form. Rather than debate human nature itself, we instead debate which primate is our closest relative in terms of our evolution. The standard scientific consensus is based on a single piece of evidence: over 99% of the base pair sequences of the human genome are identical to chimpanzees. The debate does not end there, though, because there are two subspecies of chimpanzees: Pan troglodytes, the “common” or “robust” chimpanzee (hereafter referred to simply as “chimpanzee,” or “chimp” for short), and Pan paniscus, the “pygmy,” “dwarf,” or “gracile” chimpanzee, more commonly known as the “bonobo.” The two types of chimpanzee are more closely related to each other than either one is to humans, having diverged from one another after the last common ancestor of chimpanzees, bonobos and humans. On a genetic basis, there is no evidence to prefer one chimpanzee over the other as our “closest relative.” We are forced, instead, to choose based on behavior–and that means rehashing the debate on what we consider to be our essential human nature, but in veiled terms.

Chimpanzees were once seen as gentle herbivores, but the research of Jane Goodall turned that perception upside down. Goodall studied chimps in the wild, rather than in captivity, and she found that in the wild, chimps hunt and even wage war. In his article, “The Brutal Ape vs. the Sexy Ape?” for the American Scientist (March-April 2000), Craig Stanford sums up the shock of Goodall’s research:

That chimpanzees are not vegetarian pacifists came as a surprise in anthropological circles when Goodall first reported the chimps’ omnivorous appetites. Some scholars even alleged that the lethal aggression seen during encounters between neighboring social groups was aberrant behavior, occurring only in animals disturbed by human contact. But as the field data accumulated it became clear that the brutal side of chimpanzees is quite real. Males strive to ascend a rigid dominance hierarchy and on reaching high rank wield their political power in brutal ways. Sexual coercion and beating of females who do not submit to male desires are routine. Males patrol the perimeter of their territory, attacking and sometimes murdering their unwary neighbors. Chimpanzees at two study sites in Tanzania (Gombe National Park and Mahale National Park) were observed to fission into two separate communities, after which the larger community in each case systematically exterminated the smaller community. Such “warfare” has been seen in only two primate species, humans and chimpanzees.

For those who believe that humans are inherently evil and violent, Goodall’s research provided the evidence they needed. In Demonic Males, Dale Peterson and Richard Wrangham cite chimpanzee behavior to make an argument that human males are, by their very nature, violent, and that human evolution is a five million story of warfare, violence, rape and aggression.

Of course, the general lack of archaeological evidence for warfare among humans prior to the Neolithic stands in sharp denial of such a claim. Why can’t we find any Mesolithic skeletons with arrowheads in their rib cages, or the kind of skull wound you’d get from a heavy stone hammer?

The primatologist’s answer to a case like Peterson’s and Wrangham’s is typically the bonobo. In the abstract for a paper published in the 2000 Annals of the New York Academy of Sciences titled, “The Other ‘Closest Living Relative’: How Bonobos (Pan paniscus) Challenge Traditional Assumptions about Females, Dominance, Intra- and Intersexual Interactions, and Hominid Evolution,” Amy Parish, Frans de Waal and David Haig sum up the bonobo counter-argument:

Chimpanzee (Pan troglodytes) societies are typically characterized as physically aggressive, male-bonded and male-dominated. Their close relatives, the bonobos (Pan paniscus), differ in startling and significant ways. For instance, female bonobos bond with one another, form coalitions, and dominate males. A pattern of reluctance to consider, let alone acknowledge, female dominance in bonobos exists, however. Because both species are equally “man’s” closest relative, the bonobo social system complicates models of human evolution that have historically been based upon referents that are male and chimpanzee-like. The bonobo evidence suggests that models of human evolution must be reformulated such that they also accommodate: real and meaningful female bonds; the possibility of systematic female dominance over males; female mating strategies which encompass extra-group paternities; hunting and meat distribution by females; the importance of the sharing of plant foods; affinitive inter-community interactions; males that do not stalk and attack and are not territorial; and flexible social relationships in which philopatry does not necessarily predict bonding pattern.

Bonobos are the only animals other than humans that engage in sex recreationally. For bonobos, such sexual activity replaces aggression as a means of resolving conflict–leading to the characterization of bonobos as the “hippies of the forest” that “make love, not war.” If the bonobo is more closely related to humans than the chimp, then human nature is essentially good, loving, open and affectionate. The dominant role of females in bonobo society is a point many feminists have honed in on. Unfortunately, de Waal has suffered from the very same trap as early chimp researhcers: the bonobos he researched were in captivity.

In the same article cited above, Craig Stanford outlines how de Waal’s reliance on captive bonobos has skewed our perception.

The field data show that in two important respects, female bonobos are not more sexual than their chimpanzee counterparts. First, there is no difference in frequency of copulations when wild bonobos from Wamba are compared with wild chimps at either Tanzanian site, Gombe or Mahale. Second, the idea that bonobo females are released from estrus results from data on the duration of sexual swelling taken mainly from animals at Yerkes, where they maintain their sexual swelling for 23 days, nearly half of their 49-day cycle (in captivity). This dwarfs the receptive period of wild female chimpanzees from Gombe, who swell for about 13 days of their 36-day cycle. The equation changes if we consider wild bonobos rather than captive specimens, whose excellent nutrition may produce earlier menarche and ratcheted-up reproductive cycling. Bonobos from Wamba in the Congo are swollen for only 13 days of a 33-day cycle, numbers that are much closer to those of wild chimpanzees. A recent report about bonobos in the Antwerp Zoo shows that even in captivity, bonobos do not necessarily have longer swelling durations than chimpanzees. The supposed release from estrus that is said to characterize bonobos has been overstated because the data are based on captive animals.

Other aspects of bonobo behavior bear a second look as well. Female bonobos, it is true, are often dominant to males, but this domination occurs in only two settings, when either food or sex is involved. A male often gets sex by acceding to a female’s desire to feed and so might be thought of as strategically submissive in select situations. Cleverness through subordination is certainly not unknown in other primate societies.

And are bonobos entirely peace-loving? About half of all the intercommunity encounters seen by Kano’s team involved aggression of some sort. The difference between chimpanzee aggression and bonobo aggression is that bonobo attacks and injuries are often directed by females at males, rather than the reverse as in chimpanzees. There are even reports from zoos of female bonobos brutalizing a male so badly that his penis was severed.

Meat eating, while certainly less common among bonobos than among chimpanzees, may be under reported because bonobos are so little studied. Barbara Fruth and Gottfried Hohmann of the Max Planck Institute for Evolutionary Anthropology in Leipzig have observed extensive meat eating and meat sharing by bonobos at Lomako, but most “chimp-ologists” still refer to the bonobo as the “vegetarian” great ape.

Jospeh Carpenter’s “Political Apes?” accuses de Waal of a political bias in his research, but fails to see the same bias on the other side of the equation, involving those who see chimpanzees as evidence that war, violence, rape and patriarchy are humanity’s natural state. Both sides of the debate are biased by their politics; neither one is arguing about evolution, genetics, morphology or ethology nearly so much as they are arguing the question of human nature. Neither species can be argued to be “closer” to humans than the other except on political grounds; the only evidence for either one is genetic, and it applies equally to both. Whether we prefer chimps or bonobos says more about us and our ideas about our own nature, than it does about genetics or phylogeny.

As we discussed in thesis #5, humans are neither good nor evil. As I wrote there:

We are not good, we are not evil, and we are not torn between the two. There are characteristics of human nature, but none of those characteristics can truly be called “good” or “evil.” We are what we are, and nothing more. We live more easily, and more fully, when we work with that rather than against it. That nature, though, is neither “good” nor “evil”–it simply is.

Of course, if we are honestly interested in which surviving primate is our closest relative, then we also need to take a look at the less popular theory advanced by an old professor of mine, the “orangutan theory.” While the evidence for chimpanzees is based solely on genetic evidence, Schwartz dares to ask just how reliable such genetic relationships really are? After all, we may share 99% of our genes with chimpanzees, but we also share 60% of our genes with a banana. Meanwhile, as Schwartz points out, orangutan teeth are routinely mistaken for human, even by dental experts. In fact, in The Red Ape, Schwartz highlights at least 35 known characters that appear to be either exclusive to humans and orangutans, or largely absent in outgroups. Like humans, orangutans have culture–even complex culture, going so far as developing gloves and napkins, apparent sports and games, building shelters, and even regional variation between distinct cultures.

Of course, most scientists reject the orangutan theory out of hand, based solely on the genetic evidence, even though such human-like culture is far more fundamental to “human nature” than either bonobo “good” or chimp “evil.” In an article for the Pittsburgh Post-Gazette, “New exhibit renews argument over orangutan theory of human origins,” the reporter writes:

Grehan said he became intrigued by the orangutan theory when he read Schwartz’ book several years ago. His interest increased when he tried to get critics of the theory to explain why orangs and humans could have so many physical similarities, while the genetically similar chimp could be so different from humans.

“I got nothing,” he said, noting most anthropologists refuse to even discuss it.

Which primate is most closely related to us, then? The debate will likely continue without resolution, but in the end, it is a philosophical and academic debate, because in the end, regardless of which primate is most closely related to us, we remain the only “us” there is. No other species is our species, but us, and the question of how we define our nature as “good” or “evil” is entirely our own.

See also

• Bonobo Sex and Society: The behavior of a close relative challenges assumptions about male supremacy in human evolution
• The Hunting Apes: Meat-eating, meat-sharing, and the roots of human social cognition
• Study reveals complex orangutan culture