Wednesday, October 29, 2014

Our Chimp Ancestors Murdered—and Played Pacifist. Should We Expect Differently?

About 90% of chimp males participated in a killing at some point in their lives. ENLARGE
About 90% of chimp males participated in a killing at some point in their lives. GETTY IMAGES
Rousseau versus Hobbes: one of the most famous philosophical smackdowns ever. Are humans noble savages by nature, or are our lives innately nasty, brutish and short? Today, the best insight into this classic question comes from asking how our close primate relatives fit on the Rousseau/Hobbes spectrum.
A key observation concerns chimpanzees, with whom we share as much as 99% of our genes. As researchers have documented repeatedly over the last 35 years, chimps are no strangers to murder. Males cooperate to kill males from neighboring groups; males kill rivals for alpha positions in their own groups; and females kill the children of other females as a form of genetic competition. These killings, moreover, are not spontaneous outbursts; they are filled with intent and strategy.
The fact that chimps murder could mean two things. The first would be that chimps have always murdered; it’s basic to their behavior. If so, then a look at our evolutionary tree would suggest that the shared propensity for murder among chimps and humans stretches back to our last common ancestor, some eight million years ago. In other words, killing one’s kind is a basic, long-standing part of human behavior.
An alternative view would hold that chimp murder is unnatural and solely reflects the disruptive effects of human intrusion—higher chimp population densities due to habitat destruction; more competition for food because of humans hunting out stocks of game; and the battle for highly desirable foods provided by researchers in order to habituate chimps to the presence of humans (supplying a finite stock of bananas is a great way to get chimps at each other’s throats).
A recent paper in Nature addressed this debate. The authors, an all-star team of primatologists, had a collective 492 years of experience observing chimps. Pooling their data, they examined whether rates of “lethal aggression” across populations were best predicted by intrinsic features of the social lives of the chimps or by extrinsic factors reflecting human impact (for example, proximity to humans, or whether the chimps lived in a protected game park).
Vast amounts of data were crunched and back came the answer: Patterns of human disruption didn’t predict rates of chimp murders. For example, some of the highest rates of killing occurred in the most pristine populations. Instead, rates most reflected standard features of chimp social competition.
Remarkably, the 152 killings worked out to about 3.5 murders for every decade of observation. Males made up 92% of the killers and 73% of the victims. Killing occurred in 83% of these populations across the African continent. In most killings, groups of males ambushed someone from a neighboring troop, with an average of eight males ganging up on the victim. And about 90% of males participated in a killing at some point in their lives.
Whoa. This would seem to be Hobbes out the wazoo—in apes who share almost all their DNA with us.
Naturally, this paper hasn’t settled the debate. Proponents of the “it’s due to human disturbance” explanation question whether the right measures of human impact were used. Critiques and rebuttals are flying online and in the media, because this is a big deal. If this sort of violence is fundamental to chimps, if it’s “in their genes,” then it’s overwhelmingly likely to be in ours as well.
But that wouldn’t be the right conclusion to reach. Because the chimp research was only half the paper. The authors also examined bonobos, the “other” species of chimp, famed for their social affiliation and female dominance. What is the bonobo rap sheet after 92 years of behavioral observation? One suspected killing, a mere 3% of the rate in chimps.
Critically, we share as much as 99% of our DNA with bonobos as well (and chimps and bonobos share about the same percentage of genes with each other).
We’re not chimps. Sadly, we’re not bonobos either. We’re their cousins, the species that invented both Quaker pacifism and the atrocities of Islamic State. What a cross-species analysis like this teaches us, in short, is the evolutionary roots of our potential, not the inevitabilities of our behavior.

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