The killer ape theory or killer ape hypothesis is the theory that war and interpersonal aggression was the driving force behind human evolution. It was originated by Raymond Dart in the 1950s; it was developed further in African Genesis by Robert Ardrey in 1961.
According to the theory, the ancestors of humans were distinguished from other primate species by their greater aggressiveness, and this aggression remains within humanity, which retains many murderous instincts.
The theory gained notoriety for suggesting that the urge to do violence was a fundamental part of human psychology. The hunting hypothesis is often associated with the theory, because of similarities and because Robert Ardrey developed both.
The expression killer ape does not mean an outstanding aggressive kind of ape; in fact the term refers to anthropological analysis of human aggression. Accordingly, the killer ape is a notably belligerent species on which our instincts might be rooted, because this very ancestor could establish itself due to its special aggression.
There are three intuitive ways to define the evolutionary step from ape to human: the increase of brain size, the acquisition of speech, or upright posture. Australian anatomist Grafton Elliot Smith excluded this last option; otherwise the gibbon, which walks erect, would have been a possible ancestor of humans. For him, the most relevant differentiator was the bigger brain. It would have made bipedalism possible, and bipedalism would have freed up the animal's hands for tool use, further accelerating the enlargement of the brain.
This approximately 2.5-million-year-old cranial bone, also known as “Taung Child”, was a first proof of bipedal apes. Robert Broom (1866–1951), primarily a Scottish physician, who spent his life as an archeologist in Australia since 1892, agreed to this statement, too. Five years later he decided to spend the rest of his life in South Africa. His excavations from 1946 pointed into the same direction, when he also discovered bones from the Australopithecus africanus.
However, further examinations showed that, in these cases, the size of the brain was not to be equated with the evolution’s level.[clarification needed] In fact, it is much more popular to connect the accomplishment of more and more complex movements directly with an evolutionary response, which caused the brain to grow.
Both Dart and Broom, as well as Charles Darwin (1809–1882), agreed that this new type of locomotion brought a remarkable advantage in comparison to other co-specifics, to rival animals or to the quarry.[clarification needed]
Osseous findings at a limestone cave located in Makapan, South Africa, led to the question to what extent this advantage, in combination with improved tool-using skills, affected the behavior of the apes.
The fossil bones showed explicit cracks and fractures, which seemed likely to have been done on purpose. Additionally, there were clubs, bludgeons, and spears made from long limb bones or the horns of antelopes. This new special weapon leaves small punctured, round, and triangular holes in skulls, depending on how it was formed.[clarification needed]
This new development in building weapons is taken[by whom?] to indicate an overall increase in aggression.
Dart claims that this new type of "carnivorous and killing" ape ("proto-men" in his own words) was equipped with weapons. Furthermore, he describes them as organized in a tribe, so they were able to hunt bigger animals. The ability of making fire and remarkable social skills prompt Dart to bring them more in line with humans.
Observations from Sgt. H. B. Potter (Zululand, South Africa) show that this kind of development is still up to date[clarification needed] as it is mentioned in The predatory transition from ape to man by Raymond Dart. He describes a pride of baboons that intentionally hunts antelopes when food is scarce.
Concerning the eating habits from then until now, Dart argues that there has always been an ambition[clarification needed] to eating meat: grubs and insects, bigger mammals and even human flesh (i.e. cannibalism).
The comment[clarification needed] written by Dart's editor Dr. Alan H. Kelso shows how few scientists accepted Dart and Ardrey's new ideas. Not only did Dart require a long time to publish his work, but also the epilogue contains notices[by whom?] such as: “Professor Dart's thesis that the South African apemen, at the stage they were found, were omnivorous, must be considered as proven. Of course, they were only the ancestors of the modern Bushmen and Negroes, and of nobody else.”
Dart's thesis was rejected by a scientific convention at Livingstone, Zambia, which led Ardrey to write his book African Genesis. He felt himself forced to defend the opinion of his mentor.
The ethologist Konrad Lorenz showed interest in similar ideas in his book On Aggression (1963). In his introduction he describes how rival butterfly fish defend their territory, leading him to raise the question of whether humans, too, tend to intraspecific conflict.
A 2008 article in Nature by Dan Jones stated that "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." Evolutionary psychologists generally argue that violence is not done for its own sake but is a by-product of goals such as higher status or reproductive success. Some evolutionary psychologists argue that humans have specific mechanisms for specific forms of violence such as against stepchildren (the Cinderella effect). Chimpanzees have violence between groups which are similar to raids and violence between human groups in non-state societies, and produce similar death rates. On the other hand, intra-group violence is lower among humans living in small group societies than among chimpanzees. Humans may have a strong tendency to differ between ingroup and outgroup, which affects altruistic and aggressive behavior. There is also evidence that both intra-group and inter-group violence were much more prevalent in the recent past and in tribal societies. This suggests that tendencies to use violence in order to achieve goals are affected by social mores. Reduced inequalities, more available resources, and reduced blood feuds due to better-functioning justice systems may have contributed to declining intra-group violence.
The idea that man is naturally warlike has been challenged; in, for example, the book War, Peace, and Human Nature (2013), edited by Douglas P. Fry. The Seville Statement on Violence, released under UNESCO auspices in 1986, specifically rejects any genetic basis to violence or warfare.
The association of intra-species and inter-species violence with a quantum leap in human evolution can be seen in the opening sequence of 2001: A Space Odyssey.
The television show Sliders made extensive use of the killer ape theory in storyline arcs involving the Kromaggs.