of mathematical expression. There are also thousands of ways of being violent, and equally, ways of counteracting such violence. But none of this takes away from the importance of biology, especially its role in constraining the form that these expressions take in different environmental settings. To think otherwise is just wrong. The debate about version 1.5 of lethal aggression gains interest if we restrict the conversation to the similarities and differences between chimpanzee and human killing. Similarities speak to our shared evolutionary history, including the mechanisms we inherited and the pressures that favored this form of violence. Differences speak to both changes in our biology and the environments we confronted and created. Those who argue that the comparison between human and chimpanzee killing lacks any analytical value come from two different camps. On the one hand are anthropologists such as Robert Sussman and Brian Ferguson who suggest that chimpanzee killing is infrequent, has little benefit in terms of resources or competition, and is restricted to populations that are either artificially provisioned by humans or crowded in by us. They also suggest that the archaeological evidence for human warfare doesn’t really begin until about 12,000 years ago. As Ferguson notes “To argue that war is a result of some sort of innate predisposition to wage it requires that war be practiced throughout our prehistoric past.” This date, so Ferguson continues, is too recent to invoke natural selection as a cause, and leaves unexplained why there is no earlier evidence of massive killing if our last common ancestors had this capacity. These criticisms either fly in the face of contradictory evidence or have little to do with the original ideas. Concerning chimpanzee killing, the evidence comes from multiple sites in East and West Africa, including sites with no provisioning and no crowding from humans. Further, analyses by Wrangham and his colleagues show that h