Complex sentence: It is generally assumed that colour vision is less effective than monochromatic vision under low-light condition, so that the basically nocturnal nature of mammals may have lead to the evolution of dichromacy as the basal mode of vision in placental animals.
Split into simpler sentences: It is generally assumed that colour vision is less effective than monochromatic vision under low-light conditions. The basically nocturnal nature of mammals, therefore, may have lead to the evolution of dichromacy as the basal mode of vision in placental animals.