Evolution happens very quickly. people think it happens slowly but it depends on what kind of change. A fundamental change like growing an extra set of arms would take forever since it is creating something entirely new. But changes in size and intelligence can happen after a single generation. Something catastrophic like WW2 can wipe out certain genes within a population and leave the remainders as entirely inferior to the population only a single generation prior. Aboriginals Likely "dumbed down" after reaching australia due to the continent being so dangerous. Anyone with adventurers spirit and the will to cross the oceans would quickly die to poisions from strange sources like platapus stings. Curiosity would have died off fast and anyone who simple survived living a lazy life style would have been left over.