However, humans and human societies are currently near some evolutionary equilibrium.

I think there's plenty of evidence that human societies are not near some evolutionary equilibrium. Can you name a human society that has lasted longer than a few hundred years? A few thousand years?

On the biological side, is there any evidence that we have reached an equilibrium? (I'm asking genuinely)

It's very possible that individual intelligence has not evolved past its current levels because it is at an equilibrium, beyond which higher individual intelligence results in lower social utility.

The consensus among biologists seems to be that social utility has zero to very little impact on evolution. See http://en.wikipedia.org/wiki/Group_selection

In fact, if you believe SIAI's narrative about the danger of artificial intelligence and the difficulty of friendly AI, I think you would have to conclude that higher individual intelligence results in lower expected social utility, for human measures of utility.

Higher levels of human intelligence result in a lower expected social utility for some other species (we are better at hunting them). It does not result in lower expected social utility for humans as we are generally good to other humans. Higher levels of individual intelligence have brought us the great achievements of human kind with very few downsides. The concern with AGI is that it might treat humans as humans treat some other species.

If anything, the reason we don't see a rapid rise of intelligence among human beings is that it does not provide much evolutionary benefit. In modern societies, people don't die for being dumb (usually) and sexual selection doesn't have much impact since most people only have child with a single partner.