Hmm, while this the assumption (in some pretty flawed) early anthropology, I think the modern consensus is that hunter gatherer societies were relatively peaceful, both amongst themselves[1] and between each other[2].
Of course, successful/stable hunter-gatherer societies are pretty geographically isolated, since encounters by tribes w/ more "advanced" (post-agrarian, colonial, modern) societies usually has meant obliteration.
Recently I read Pinker's "Better Angels" [1]. Using archeological records, he argues that dying from a violent death was extremely likely in hunter gatherer societies.
Not an anthropologist, so take my casual observations with a grain of salt, but I can think of quite a few examples of latter day stone-age societies that are notably bloodthirsty. The few remaining indigenous tribes in the amazon, for one. 'Headhunters' in Indonesia, for another.
