This is one of the best books on human nature that I have read. It combines hard-headed reasoning with attention to detail, willingness to propose bold hypotheses, careful analysis of arguments, and honesty about points where the evidence is weak.
The primatologist Richard Wrangham posits that our ancestors became Homo sapiens by a process of simultaneous reduction of reactive aggression (response to attack and threat) and increase of proactive aggression (premeditated attack against others). The reduction of reactive aggression occurred in a process of self-domestication, while proactive aggression got its boost from a combination of language, the coalition of males against tyrannical alpha males, and inter-group conflict.
Wrangham explores the evolutionary basis for these changes compared to our latest common ancestors with great apes and also, as far as the evidence allows us to say anything, Neanderthals. His controversial explanation is what he calls the execution hypothesis. At some point maybe 300,000 years ago, our ancestors acquired the ability to form coalitions. The males in a group started collaborating to control and, in the ultimate case, execute any tyrannical alpha male. Over the generations, this reduced the propensity for reactive aggression. The new-found power of the male collective also led to the development of morals, shame, and guilt, as regulators of behavior as mechanisms for avoiding the potentially lethal disapproval of the group.
Wrangham is that kind of scientist and thinker who does not shirk from the idea that terrible mechanisms can yield good results. He displays palpable disdain for the Rousseauian idea that humans are naturally good, while civilisation has made us twisted and evil. But neither is he a fully committed Hobbesian; he thinks morality, altruism and cooperation are part of our nature, and not something an authority has to force us into.
In the final chapter of the book Wrangham writes: "... evolution has left us with biases that affect our behavior in predictable and sometimes disturbing ways, and we would do well to acknowledge those biases." It is my belief that modern political philosophers need to question their various assumptions about human nature, morality and society in the light of the evolutionary insights that Wrangham and many others have provided during the last 30 or so years.