I've been reading some books on the subject that claim the contrary, also based on archaeological proofs (and on identifying faults with the existing studies). So I wouldn't state that with such confidence.
See RoboTeddy's comment for some details on this POV.
And also, if we look at bonobos, another primate closely related to us, we might get a completely different perspective on life in the distant past. It's just that we discovered chimpanzees first.
It appears that some people prefer to focus on a very specific behavioral trait pinned on bonobos (having sex with group members to diffuse conflicts) just because it fits their preconceived notion of an ideal outcome involving absence of conflicts in general and war in particular. Some people even pin the name "hippie ape" in what looks like a desperate attempt to validate their personal ideals. Yet, bonobos are indeed known for organizing attacks on neighboring primate groups for the express purpose of killing and even eating them.
But could you please shed some light on this sentence:
> Yet, bonobos are indeed known for organizing attacks on neighboring primate groups
Are those primates other bonobos? That could be called going to war. If not, it's called hunting: those other primates are just food. It's not like we go to war against whatever other animals we are eating.
To the parent's comment though, bonobos don't actually form patrols and they tend to avoid contact with other groups, so intra-group conflict is lower for bonobos than chimpanzees.
Could you please cite the names of one or two of those books? I'll start by citing "War Before Civilization" and "Social Conquest of Earth" as in the camp of "our forbears lived very violent lives".