I don't have a fully fleshed out model in my head just yet, but I imagine it like this: humans seem to have a pretty fixed emotional and cognitive makeup. What changes is the landscape in which we operate - what technologies are available, what customs exist in our times, what are the core "cultural memes". Kind of like water flowing downhill - a simple process in itself, but where it goes and what shape it takes depends on the landscape it's flowing through, and the flow itself shapes that landscape over time.

Another way of viewing this: we humans are like somewhat fixed neurons, but it's the connections we make with each other that change. Connections both to our contemporaries through complex social structures, as well as to the past, through knowledge accumulated in books and traditions, and other artifacts of past cultures.