You have often seen cooperation - consider which side of the road you drive on. I think that humans have acquired traits that lead them to form groups and will do so even if, intially, they do not know one another or if their culture has previously collapsed. The group they form is a hierarchy and competition is usually for place in that hierarchy.
I am not sure what you mean by "fit(s) quite nicely with the idea of natural selection." Social groups lead, so it seems to me, to social selection and one must consider the types of selection occuring within the information flows of a social hierarchy. The issue here is partly the unit of selection and partly whether it is even appropriate to talk about units of selection.
The fact of human cooperation does not fit very well with natural selection based on genes or even individuals as the units of selection but we have gene-culture coevolution and, if social evolution dominates, which I think it does in humans, then human genetic evolution will adapt to serve the needs of social knowledge. That will lead to humans adapting to function not just as competitors but to becoming actors within social evolution and, as such, they can be expected to exhibit cooperative and altruistic traits toward other members of their social group.
I disagree that agreement as to which side of the road to drive on is an example of cooperation. It is the result of a negotiation between opposing drivers who, but for their agreement, would run head on into each other. Each side gets something: the ability to proceed in the direction the desire, and each side gives up something: one half of the road surface.
Maybe it's all semantics, but I don't find that cooperation exists. I find it an illusion. Even soldiers in the same troop/unit negotiate for their respective survival by mutually agreeing to protect each other.
However, I agree that if a particular cultural behavior remains the status quo for a long enough period of time, then as with any other relatively stable environmental stress, the cultural behavior may help shape the genetic future of the organisms who exist within the culture.
This is what humans are currently doing. For example, we culturally believe that a certain class of person makes a better lawyer. So, we screen for that class of person by giving a standardized test that effectively permits only the target class to become lawyers. Then we pay those people, in large part, to run our government/society.
Consequently, we create legislation which represents the sort of thinking which is screened for in those tests, and we can observe the result in the way our laws are enforced and how this effects society.
250 years ago in the USA, our legislators and jurists were much more free thinking. The result was more novel legislation and enforcement, and more freedom for the individual. Today, those legislators and jurists are overwhelmingly cut from nearly identical cloth, and the result is that we can't tell one political party from the other, and further, we are increasingly more narrow minded in our legislating solutions to societal problems -- and ultimately, less free, as individuals.
Self-directed evolution in action, in my view.
sheesh Paul - remember, you sought THEM out. 
