“Why Human Eyes Are Sensitive To Only Visible Light”. These processes are arbitrary, which is somewhat more technically referred as “contingency”. Its not just for the case of vision that this arbitrariness is present, but all forms of biological properties. eg A conch shell is spiral in a certain way and this is only specific to the snail-likes. Fur is different for different animals. Arms and limbs are different for different animals. This is because of such arbitrariness which are present in 100s of different ways depending on exactly how a life form develops. ( — We are all describing this on general terms, here in this discussion).

Technically, adaptation has to do with exposure. One has to be exposed to the kind of situation one is borne in. Human beings are not borne deep down, on the ocean bed, but on ground with optimum temperature and other life conditions. So, there has to be optimal amount of heat present. Human Beings can’t survive huge temperatures. But many animals can, so they are exposed more harshly to Infra Red (IR) etc. Its the whole body which evolves and then different parts might evolve differently. [w.r.t. different thermodynamic and other life defining conditions]

Just a wild guess: may be some animals were borne [created by contingent chances] very close to volcanic eruption, eg snakes, which is why they could be sensitive to IR type of vision. etc etc. On planet earth, optical light comes only from Sun, (in its profusion, can turn off the sun during night or moon some nights and its total darkness). This sun-light in optical radiation window is extremely intense in visible spectrum but with IR spectrum its intense only during summer. This intensity going from one place on planet earth to another can vary between say 10% – 60% during a year. That means the likelihood of birth can be reduced by that factor towards IR exposure, if it were to come from sun but not from volcanoes. The actual condition on earth might … More…