Several theories suggest the primary reason humans evolved bipedalism was to conserve energy while running, and to free the use of upper limbs.
[citation needed] The killer ape theory posits that violence was a driving factor in evolving bipedalism, freeing the upper limbs to wield weapons.
[citation needed] In Creatures of Cain: The Hunt for Human Nature in Cold War America by Erika Lorraine Milam (2018), she states that, "in the 1970s, the theory unraveled altogether when primatologists discovered that chimpanzees also kill members of their own species."
[3] In his introduction, he describes how rival butterfly fish defend their territories, leading him to raise the question of whether humans, too, tend to intraspecific conflict.
[4] A 2008 article in Nature by Dan Jones stated, "A growing number of psychologists, neuroscientists, and anthropologists have accumulated evidence that understanding many aspects of antisocial behaviour, including violence and murder, requires the study of brains, genes, and evolution, as well as the societies those factors have wrought."