"Arguably most of our mental structures evolved before modern humans even existed."
If you mean instincts, then yes, but if you mean concepts and reason, then no. And after all, who gives a monkey's toss about instincts. The mental structures worth thinking about were created in reaction to human conditions, not evolutionary ones.
"I used to use these Marx-mots and dialectical cliches as a rule of thumb all the time.But I think that in the "human-centrism" of such statements they display a world view that is pre-evolutionary and thus in a deep sense anti-materialistic."
How very wise you were, but less so now, I think. To not be human-centric, that would be a human delusion, surely. Imagine a trilobyte that was not trilobyte-centric. To be human-centric properly, though, is not to be pre-evolutionary, but post-evolutionary. The laws of evolution were superceded once man began to consciously shape his environment. Pointedly, human evolution ceased at just that point - overtaken by human industry.