TIL that Jane Goodall witnessed a four year long war between two chimp tribes. The tribes, which had previously worked together, decided to do battle. It only ended after four years because one of the tribes was completely killed off. When killings happen they are accompanied by brutal behavior. Eating the victim alive and such.
https://en.wikipedia.org/wiki/Gombe_Chimpanzee_War
Not really a "war"... (as you picture hundreds of chimps doing battle)... it was a tribe of 20 against a tribe of 10. At the end of four years, one tribe had 19 remaining and the other had none.
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The chimp is humans' closest animal relative; we are 98.8% genetically similar. The lines for chimps and humans broke off about 4-5 million years ago, a very short time, evolutionarily speaking.
So, chances are that we share these chimp instincts. To gather into tribes. To separate our group from others. And to find and bear such malice towards another tribe that we need them to be killed off completely and brutally. And to retain this collective malice over years and years, even without the aid of consciousness or language.
That is in all our DNA.
Well, good luck, everybody!