Re: We're all really Cave Men, aren't we?
People cannot be adapted to the present or the future; they can only be adapted to the past.
Hmm! Most animals are adapted to the present.
An alternative view might be that (ignoring cultural evolution) we would continue to evolve until any innate, ancestral, behavioural attributes that would have evolved when we lived in the EEA, which might cause individual people not to pass on their genes have disappeared. At least, that is, up until the point where the trait becomes rare enough not to be considered part of the normal human condition. At this point - once the optimal level had been reached - the selective pressure would maintain it at that level, and adaptive evolution would be "complete" until there was some change in the environment which might set up a new selective pressure to set it off again. If this view is the correct one, evolution will stop, once we are properly adapted to the world we would then be living in. Against this view is the argument that cultural evolution moves so quickly that the environment is constantly changing and selective pressures would never be stable for a long enough period to provide a consistent selective pressure. If this view is the correct one the cultural evolutionary "hare" is so much faster than the Darwinian evolutionary "tortoise" that the tortoise will never catch the hare.
I hope this makes sense; these are complex issues.
I think the human animal is definitely cosmopolitan rather than specialist. By this I mean that we are very adaptable and I don't think it could be argued that we are naturally polygamous or naturally monogamous, because different societies might well favour different cultural arrangements. What could never arise would be a society where adult inter-sex bonding was absent.
If we are to answer the question you set, I think that we have to define what is innate and what is culturally defined behaviour. I have heard some evolutionary psychologists and philosophers make (cringe making) claims about how Darwinian adaptive change might alter phenomena that, I think, are obviously cultural in origin. I think that it should be possible to define - at least provisionally - what the differences between evolved and cultural phenomena are.
Now, we certainly have an evolved capacity for rational thought, but (and this is a massively important "but") this is a general-purpose capacity. We evolved this trait to assess and decide between different options that are presented to our senses and which evoke a subjective or emotional response. (I am saying here, that we simply won't deal with any sensory phenomenon that doesn't evoke a subjective or emotional response. This is another way of saying that we won't deal with something if it doesn't interest us. There is an important caveat to the last sentence but this is getting complicated enough)
If we take our evolved capacity for rational thought out of the equation, we are left with emotions and subjective responses to the world. Now, I argue that this is where we should be looking for innate behavioural attributes. We react emotionally, or with a positive or negative subjective response, to certain elements in our environment. We react positively to a smile; we enjoy looking at evocative images; we like certain things in our environment and dislike others. If you think about it these are the things that make us happy or sad and are the basis for our value judgements and, I venture to say, provide all human motivation.
These elements can be very specific. And, sometimes, it is intriguingly difficult to understand why we might certain things evocative. Why do some people find steam trains evocative? Or a herd of wild, white stallions pounding through the surf? (I've proposed answers to some of these questions but there isn't space here)
My point is that we might find different cultural solutions, which satisfy our evolved basic urges. Monogamy or polygamy to satisfy our basic sexual urges or train spotting to indulge our passion for steam trains. (I wonder if there were chariot-spotters in ancient Rome - I'll bet there were.) Don't look for evolved capacity for things like monogamy, because you won't find it. Monogamy and polygamy happen in humans because we have an evolved capacity for sexual pair bonds and we will come up with different solutions to satisfy the need for pair bonds using our capacity for rational thought. (Most other animals just wouldn't have thought of choosing between the possible options - they just have the evolved response - not the enhanced capacity for rational thought that humans have.)