Maybe someone else had read it in the fluff somewhere but I find it funny that we as humans in the 41st millenium inhabit so many planets. Honestly the chances of us finding a habitable in reality is slim to none considering the unique conditions a world must have to support life and then that world having the exact same conditions as earth to support human life. But I'm sure there's some fluff (possibly involving the STCs) that supports how each of these planets were terraformed for human survivalbility.
That said as has already been mentioned it would take extreme changes and millions of years of time for a race of beings to evolve in some extreme necessity (like how Ogryns, Ratlings, and Squats developed). If anything due to science, technology, and well in this ficitional universe psychi abilities humans should be better off, to survive to such vast numbers and over such a wide range of space with the constant wars against alien races and chaos humanity must be one "tough nut to crack".