I watched a fascinating documentary about a french anthropologist who lived with (and actually ended up marrying) the Yononomi tribe in the Amazon rainforest. https://en.m.wikipedia.org/wiki/Yanomami Probably be difficult to find it on Google now, because of how they suppress information, but the gist of it was that these south American tribes only have oral history so he traveled the basin to figure it out. The history of the south American people is something like this (don't quote me as I'm going from imperfect memory). There are 6 people's in South America who have existed in a constant state of war since creation (but probably about 80,000 years). They periodically raid each other to kill men and capture females for breeding. All the tribes live this way and have a religion to explain why, all tribes (although they never interact except to kill each other) share the same religion, origin story, whatever. The anthropologist wanted to understand how the tribe were inter-related so he spent a lot of time recording who was related to who, who were the parents, uncles, children etc. Because they don't get married or record that information, and that is where the controversy surrounding him comes from. He discovered that in their culture the reproductive success of the males is directly linked to how violent they are. When you raid another tribe, kill men and steal their females you get to breed with them, he found that the number of descendants of the Yanomomi men directly correlated to the number of people they had killed - f you didn't kill anyone you didn't have any children, if you had killed a lot of men you had many children. This led him to conclude that their culture (the culture of the entire Amazon basin) had evolved over millennia to select for pure violence. Then the Spanish came. Unlike the Anglos they colonized into population centres and interbred with the natives giving us the modern multiracial south American (who often also have African genetic material). The appalling violence they display can't be explained just by taking cocaine (although that does make people violent) but most likely also by having that tribal killer DNA that originated in the heart of the jungle through thousands of years of constant tit-for-tat warfare. Combine that with African DNA lacking empathy and foresight, and with ruthless colonizer DNA and it's a potent mix.