Well that really depends on how the environment on Earth changes. I've actually been discussing this concept with one of my ethics teachers and I'm glad I have a chance to bring it up here. So natural selection enables those with greater fitness to spread their genes around because those with less fitness are unable to pass on their genes. With the advent of rational human beings however, altruism assures advantageous genes are not spread like they once were. Because of this we do have a greater overall population but disadvantageous genes are allowed to remain in the gene pool.
There would have to be some extreme environmental changes to force us as a species into the mindset of abandoning the weaker of the species so only the strongest and smartest survived.
I suppose there do exist some counter examples like how humans have been steadily growing taller / larger as centuries have passed but that also owes itself to an environmental innovation. That innovation being able to supply more food, etc.
To sum this all up, I really don't see there being a significant change in the anatomy or physiology of humans on Earth. This is all assuming however that our environment stays fairly constant but in the universe of 40k that's just not a good probability.
Space marines, servitors, etc all prove that humans have developed ways of enhancing humans through biological and technological means themselves so environmental influence I'm sure matters very little when compared to evolution in our universe.