On this blog, in my book The End of War and elsewhere (see Further Reading and Viewing), I have knocked the deep roots theory of war, which holds that war stems from an instinct deeply embedded in the genes of our male ancestors.

Inter-community killings are rare among chimpanzees and non-existent among bonobos, according to a new report in Nature, undercutting the theory that the roots of war extend back to the common ancestor of humans and chimps.

Proponents of this theory—notably primatologist Richard Wrangham—claim it is supported by observations of inter-community killings by chimpanzees, Pan troglodytes, our closest genetic relatives.

Skeptics, including anthropologists Robert Sussman and Brian Ferguson, have pointed out that chimpanzee violence might be not an adaptation but a response to environmental circumstances, such as human encroachment.

This "human impacts" hypothesis is rejected in a new report in Nature by a coalition of 30 primatologists, including Wrangham and lead author Michael Wilson. In “Lethal aggression in Pan is better explained by adaptive strategies than human impacts,” Wilson et al. analyze 152 killings in 18 chimpanzee communities and find "little correlation with human impacts."

Given that the primary interest in chimp violence is its alleged support of the deep-roots theory, it might seem odd, at first, that Wilson et al. do not mention human warfare. Actually, this omission is wise, because the Nature report undermines the deep-roots theory of war, and establishes that the "human impact" issue is a red herring.

Of the 152 killings analyzed by Wilson et al., 58 were directly observed by researchers and 41 were "inferred" (a dead chimp is found with bite marks or other signs of violence). Another 53 chimps were "suspected" of being killed, usually because they disappeared; the researchers exclude these data from most of their analyses "to be conservative," so I will too.

I will also exclude infanticides and "intracommunity" (within-group) killings, because chimp killings that provide the closest analog to human warfare involve a group of adults from one community killing one or more adults or adolescents (Wilson et al. call them "weaned victims") from another community.

After studying 18 communities for a total of 426 years (or 23 years per community, on average), Wilson et al. directly observed only 15 intercommunity killings of weaned victims. That comes to one killing every 28 years in a typical community--or one every 15 years if 14 "inferred" killings of weaned chimps are included.

Wrangham himself, in an exchange with Sussman, conceded that lethal intercommunity killings are "certainly rare." Why then does he insist that they are adaptive?

Also, Wilson et al. record only one "suspected" killing among 4 communities of Pan paniscus, or bonobos, a less common chimp species clustered in the Congo, that scientists have observed for a total of 92 years. Pan paniscus is just as genetically related to Homo sapiens as Pan troglodytes is. Deep-rooters have never offered compelling reasons why Pan troglodytes is more illustrative of our ancestry than Pan paniscus.

In spite of its flimsy support, the deep-roots theory of war has become popular not only among scientists—notably Wrangham, Edward O. Wilson and Steven Pinker--but also some U.S. leaders. Barack Obama was alluding to the theory when he said in 2009, while accepting the Nobel Peace Prize, “War, in one form or another, appeared with the first man.” [Journalists also love the deep roots theory, as coverage of the Nature paper demonstrates. See Addendum below.]

Former Secretary of Defense Donald Rumsfeld, who oversaw the U.S. invasions of Afghanistan and Iraq, says in the 2013 Errol Morris documentary The Unknown Known, "Human nature being what it is, I'm afraid we'll have to continue to ask young men and women to come serve our country."

The popularity of the deep-roots theory stems less from its merits than from the militarism of our culture. Ending war will be far easier if we take responsibility for our warlike ways and stop blaming them on evolution, genes and "human nature."

Further reading (and viewing):

See my next post, in which Brian Ferguson provides a detailed critique of Wilson et al. http://blogs.scientificamerican.com/cross-check/2014/09/18/anthropologist-brian-ferguson-challenges-claim-that-chimp-violence-is-adaptive/. See also the response of Wilson et al. to criticism of Ferguson and me: http://blogs.scientificamerican.com/cross-check/2014/10/01/chimp-violence-researchers-respond-to-criticism-on-cross-check/.

I talk to George Johnson about the deep-roots theory (among other topics) in a recent chat on Bloggingheads.tv. http://bloggingheads.tv/videos/30767

I talked to Richard Wrangham about the deep-roots theory (among other topics) in 2009 on Bloggingheads.tv. http://bloggingheads.tv/videos/2106

I talked to Steven Pinker about war and other forms of violence in 2011 on Bloggingheads.tv. http://bloggingheads.tv/videos/3164

“Quitting the hominid fight club: The evidence is flimsy for innate chimpanzee–let alone human–warfare.” http://blogs.scientificamerican.com/cross-check/2010/06/29/quitting-the-hominid-fight-club-the-evidence-is-flimsy-for-innate-chimpanzee-let-alone-human-warfare/

“New Study of Foragers Undermines Claim That War Has Deep Evolutionary Roots.” http://blogs.scientificamerican.com/cross-check/2013/07/18/new-study-of-foragers-undermines-claim-that-war-has-deep-evolutionary-roots/

“New Study of Prehistoric Skeletons Undermines Claim that War Has Deep Evolutionary Roots.” http://blogs.scientificamerican.com/cross-check/2013/07/24/new-study-of-prehistoric-skeletons-undermines-claim-that-war-has-deep-evolutionary-roots/

“Survey of Earliest Human Settlements Undermines Claim That War Has Deep Evolutionary Roots.” http://blogs.scientificamerican.com/cross-check/2013/08/02/survey-of-earliest-human-settlements-undermines-claim-that-war-has-deep-evolutionary-roots/

Addendum: Here is a sampling of coverage of the Nature paper. James Gorman, writing in The New York Times, is cautious, quoting skeptics such as Ferguson and Sussman. Other reporters proclaim that the paper corroborates the deep roots theory, even claiming that chimps and humans are both "natural born killers." USA Today: "The results bolster the theory that we humans have a tendency toward violence rooted deep in history." The Independent: "Chimpanzees and humans have one key trait in common--both are natural born killers, scientists have shown." Daily Mail: "The study suggests that violence has deep evolutionary roots, and is not a modern man-made phenomenon."

USA Today: http://www.usatoday.com/story/news/nation/2014/09/17/chimps-study-nature-violence/15775355/

Daily Mail: http://www.dailymail.co.uk/sciencetech/article-2759619/Natural-born-killers-Chimpanzees-inherently-violent-wage-war-like-human-cousins-study-claims.html

Independent: http://www.independent.co.uk/news/science/chimps-and-humans-are-both-natural-born-killers-9738988.html

Daily News: http://www.dailynewsen.com/science/natural-born-killers-chimpanzee-violence-is-an-evolutionary-strategy-h2648852.html

New York Times: http://www.nytimes.com/2014/09/18/science/lethal-violence-in-chimps-occurs-naturally-study-suggests.html?hpw&rref=science&action=click&pgtype=Homepage&version=HpHedThumbWell&module=well-region&region=bottom-well&WT.nav=bottom-well&_r=5

Photo: Wikimedia Commons, http://commons.wikimedia.org/wiki/File:Knoxville_zoo_-_chimpanzee_teeth.jpg.