It used to be cosmic rays, but now we have cellular RF radiation unleashing forces that will hasten evolutionary change. Ever wonder how evolution occurred? The amount of water in your system is a distinct disadvantage in evolution towards smaller and smaller creatures apart from those living in water or in caves. Would human beings survive better as one inch tall beings? That would certainly save resources. Problem with RF radiation is that it can go right through you. Creatures in caves might survive. Ever wonder why humans have little hair?