According to Christopher Boehm, in his book Hierarchy in the Forest: the evolution of egalitarian behaviour. (Harvard 1999) the answer to this question is "Yes". Humans are neither nice egalitarians nor nasty Hobbesian despots, but both - and that is the secret of our success, as what we might call a "militantly egalitarian" species.
Humans have a *capacity* for forming dominance hierarchies, as other primates do, but have evolved very good counter-dominance strategies for cutting upstarts down to size - which accounts for the overwhelmingly egalitarian nature of hunter-gatherer and most tribal societies. i.e., human egalitarianism is a highly dynamic affair, in which a very high value is placed on individual autonomy. People who might become upstarts are cut down to size by a whole range of techniques ranging from pleasant mockery to ostracism to (as a very last resort) homicide. People of exceptional ability or strength, who have the potential to become upstarts, are at pains (unless they are psychopaths) to pre-empt criticism by practising self-deprecating humour and modesty (lest they be thought boastful, etc).
Boehm reckons that (apart from the last couple of thousand years) humans have been so effective at maintaining equality that natural selection may even have had time to make this predisposition part of our genetic makeup. But whether it is genetic or not, egalitarianism is something humans do very well - until they fall under the control of invincible despots from whom they cannot escape. At which point Paul Gilbert's work on depression (The Evolution of Powerlessness) becomes very relevant, I think. Gilbert and Wilkinson ought to get together to discuss this - both have offices at Nottingham University after all.
I recommend Boehm's book very strongly indeed. Also (but from a quite different angle) Robert Axelrod's The Evolution of Co-operation (Basic Books, 1984).
After reading Boehm, I suddenly saw my late father in a new light. He went through WWII without rising above the rank of temporary lance-corporal despite being a very capable, intelligent man with a good university degree. He declined promotion partly because he hated the officers, but also because he felt the other men would resent his elevation, and make his life a misery. This, I think, was an example of a human society functioning well, despite the unpromising circumstances (and maybe it was what got them through).
But do read Boehm. Worth it.
PS - On the environmental impacts of inequality: In his book Feast: why humans share food (OUP 2007) Martin Jones says that the onset of hierarchical societies in the Bronze Age coincides with first signs of environmental degradation due to human activities. This was also the period when humans became shorter and unhealthier: "An average bronze age male farmer from the eastern mediterranean would stand 167cm (5'6"): 6cm shorter than his ruler, and 10cm shorter than his hunting ancestors." (p248)
