On evolutionary explanations of behavior

The plasticity of primate behavior is quite high.  Baboon aggression may be cultural in origin.  So what to say of humans, whose behavior is so much more plastic than baboons?  Yet people look for the roots of murder and rape in human societies in evolution and genetics.   And they miss the more interesting thing that is to be explained: the origin of the plasticity of behavior.   Considering how stuck in a rigid determinism that these evolutionary behaviorists are, one would think that this would be their supreme puzzle: how do deterministic parts combine to produce an organism with such an enormous ability to learn?