"The anthropologist Sarah Hrdy noted that to pack hundreds of chimpanzees into close quarters on an airplane would be to invite violent chaos, whereas most human passengers behave sedately even when they are crowded. As Dale Peterson observed, however, intense screening is needed to ensure that a secret enemy will not carry a bomb on board."

This captures the phenomenon (or "paradox") the book is trying to explain: humans have very low reactive aggression, that is uncontrollably attacking someone in the anger of the moment. We can walk down the street, see strangers, they can even bump into us, and we'll virtually never hit them and usually won't even send a dirty look their way. Virtually no other wild animal is like that. But humans also have extraordinary levels of proactive aggression, we can plot wars, genocides, and much more that kill millions of humans in the process--something that no other animal could come remotely close to doing.

Richard Wrangham advances a bold thesis for how this came about: evolutionary changes driven by hundreds of thousands of years of capital punishment that effectively "self domesticated" humans, much like the process that turned reactively aggressive wolves into tame dogs and many other examples of domestication. Basically, language (a bit of a deus ex machina in this account, and given that it doesn't fossilize it may always be in every account) enabled humans to coordinate to kill overly aggressive people using gossip, plotting, and the like. Over hundreds of thousands of years--or about 12,000 generations--this led to genetic changes that separated from earlier humans not to mention chimpanzees.

The evidence Wrangham puts forward for this hypothesis is draws on evolutionary biology, animal behavior, genetics, neuroscience, anthropology and more. A lot of it draws on studies of the way the domestication changes animals, many of them in the direction of paedomorphic changes in which animals retain more juvenile features as adults, including reduced differences between males and females, certain aspects like bone density, and interesting things that have simultaneously evolved multiple times like floppy ears and white tufts. Wrangham looks at the fossil record and modern humans and sees many of these features diverging from our ancestors--and also differing from very recent relatives like neanderthals.

Wrangham contrasts his hypothesis to other explanations. One explanation he debunks (following a long-standing tradition of arguing against it) is group selection, because this generally cannot explain why individuals will not benefit from defecting--something that fear of execution can explain. He also criticizes cultural explanations for human aggressive behavior because of the strong evidence about how deeply rooted it is, observable in babies, in children even when given contrary instructions, etc.

He also applies this idea to a variety of areas. For example, he explains several moral puzzles about people's behavior (e.g., trolley-problem like issues around people not wanting to touch or directly engage in certain behavior that they would do indirectly) as humans evolving to be risk averse, trying to avoid being (unfairly) blamed when they were trying to help. He also analyzes war which is an example of "coalitionary proactive aggression," contrasting the primitive version which relied on voluntary consensus without leaders leading to raids with a very high probability of success with the modern version which entails leaders getting their followers to do things that are highly non-adaptive from an evolutionary perspective--requiring intense drilling, rules, and created camaraderie to make it work.

The above does not do justice to what is a very rich, dense, but highly readable book that draws on a lot of cutting-edge, peer-reviewed research. Although I am not 100 percent convinced of the execution hypothesis there is a rich set of evidence for it, not just an ex post just so story. Also, even if you do not agree with the hypothesis there is a lot to get out of the book, including a better understanding of aggression, some history of science (and particularly, some appalling politicalization that led scientists to resist admitting things like chimpanzee infanticide or hunter-gatherer warfare because they were afraid it would legitimize it in humans), and much much more.

Ultimately Wrangham is at pains to distance himself from the naturalistic fallacy that just because something is natural or evolved it is legitimate. He points out the ways that culture has changed over time to reduce violence--and that even nature itself builds in responses to incentives (e.g., the frequency of chimpanzee infanticide depends on factors that change the evolutionary rewards for engaging in it). Ultimately he agreed with Katherine Hepburn's character from The African Queen that "Nature... is what we are put in this world to rise above."

Very, very highly recommended.

P.S. Another image, like the opening quote, I cannot get out of my head is how humans have never really been led by alpha males, whether in hunter gathers or sophisticated societies. Our leaders are not obeyed because they could win a wrestling match with any other challenger but because they can organize a coalition to engage in violence to enforce the law. This means that humans have obedience in way that no wild animal does.