You're disregarding the fact that society can produce it's own need for adaptation. Like internal competition. A tribe grows, it's habitat grows, it's hierarchy grows - and at some point the guys from left bank of the Big River and their leader Grug come to see their very tribe mates from the right bank of the Big River lead by Grog as more of a nuisance than any natural factors. So they pack their shit and leave, or take pointy sticks and make Grog's people leave. 