As we know our hunter gatherer forebears lived in rigorously egalitarian societies: some of our instincts even now are that we should similarly be living in such egalitarian manner. No one has much more than the other and peace and harmony should prevail. We do certainly know that excessive (a changeable measurement, to be sure) inequality can lead to great unhappiness in a society.
So, if this is so, how come the inegalitarian, unequal, societies, won out over those more equal? No, not oppression, not the "capitalists", at least not according to these researchers:
Agent-based simulation results show that in constant environments, unequal access to resources can be demographically destabilizing, resulting in the outward migration and spread of such societies even when population size is relatively small. In variable environments, stratified societies spread more and are also better able to survive resource shortages by sequestering mortality in the lower classes.
That doesn't sound very fun (that "sequestering mortality" means the poor die) but what we seem to have here is Darwinian evolution of societies (no, please don't write in, this is an analogy, not a direct comparison). The environment is changeable and it appears that unequal societies are able to thrive in such changes, while the more equal ones are not.
I would go further than this research myself: no proof, just my own prejudices. Two things: firstly, it's necessary to move beyond the hunter gatherer technology before there are things with which it is possible to be unequal. Certainly inequality in the physical goods sense requires stationary and thus agricultural living.
The second is that those hunter gatherer societies are vehemently, violently egalitarian. Place can only be measured by positional goods thus there is always a battle for them, for status, for the prettiest woman and so on. So much so that, as Stephen Pinker points out, such societies are not just much more violent than our own, murder isn't just the most common cause of death for men, in some it is the majority cause of death.
In essence, I'd argue that this inequality is beneficial: we may have an inequality of physical goods but squabbling about keeping up with the Jones' stops us all from murdering each other over the zero sum game of who gets to be top dog.