Chimpanzees are naturally violent, according to an international study involving Fife experts.
St Andrews University researchers have dismissed previous claims chimps adapt aggressive behaviour because of human interference.
Instead, the study suggests killing among chimpanzees, while rare, is a result of natural competition.
The team said the findings could shed new light on our understanding of human violence and warfare.
St Andrews researchers Dr Catherine Hobaiter and Professor Klaus Zuberbühler contributed findings from their chimpanzee study site in Budongo, Uganda, to the project.
Their data was then analysed alongside an Africa-wide study of a further 22 sites.
Overall, the study compiled 50 years' of information from 18 chimpanzee communities and four bonobo communities.
Dr Hobaiter said: "Chimpanzees and bonobos are our closest relatives. By looking at the similarities and differences between their behaviour and ours we can start to trace the evolutionary origins of behaviours such as language, tool use or even violence and warfare."
The study re-examines pioneering primatologist Jane Goodall's reports of chimpanzee violence, in the 1970s.
Since then, scientists have argued over whether chimps become violent through intergroup aggression, or because of human activities such as destruction of habitat.
However, the new research, led by Minnesota University and involving 29 co-authors, suggests that human impact, or even contact, is not the root cause of lethal aggression in chimps.
From the data spanning 50 years, only one suspected killing was observed in the bonobos, whereas 152 killings were reported in 15 chimpanzee communities. Variation in killing rates was unrelated to measures of human impact on those communities.
Killings increased in larger populations and groups with a high number of males, and most killings were carried out by males against other males from neighbouring groups, supporting the assumption that this behaviour is related to adaptive strategies.
Dr Hobaiter said, "It was incredibly exciting to be a part of such a ground-breaking study. Within any one chimpanzee community these events are quite rare, so individually it is very hard to draw any systematic conclusions.
"Here, by combining decades of data from every major research site, we were finally able to start to answer some of the key questions about the origins of this fascinating behaviour."
The study was published by Nature.