Why did we evolve to see the part of the light spectrum that we can see? We evolved to see the range of the light spectrum that we can see because it was the most available and easily detectable. If our planet had a different atmosphere and our sun was hotter or colder, we may have evolved to see the ultraviolet or the infrared part of the light spectrum.
The fact that we can see with the part of the light spectrum we use is proof that it is useful to us. If it wasn’t , we most likely wouldn’t have evolved to use it. Some animals have evolved to see ultraviolet light that we cannot because it is useful for them. The vast majority of animals have evolved to detect light in some form. That makes sense because it is the easiest way to get information about the world around us. The other options are chemically, as in smell, or aurally, as in sound. Light is the easiest because for at least half of the day we have an enormous source of electromagnetic radiation bathing the part of the planet we are standing on. It makes sense that we would evolve to use it.
Anything outside the 400 to 700 nanometers range that we can see doesn’t cause a reaction in our eyes and is invisible to us. When a light photon hits our eyes, it travels to the rods and cones at the back of the eye in the retina. There, it imparts its energy and causes a chemical reaction, which produces an electrical signal that goes to the brain. Photons of light that have a longer wavelength than this range have less energy and don’t produce any chemical reaction. Photons that have a shorter wavelength and more energy than this would cause too much damage.
The main reason we use this range of light to see is that the Sun produces enormous amounts of radiation in and around the visible range. The surface of the sun is about 5,800 ℃, and when something is that hot it puts out electromagnetic radiation across a wide range of wavelengths. However, it emits some of the wavelengths more than others and its temperature determines where most of the radiation falls. The peak is at a wavelength of about 500 nanometers, which is close to the middle of the light we can see, and then it falls off in both directions. It is no coincidence that we have evolved to use the light radiation that is the most abundant. When eyes first evolved, they would have been nothing more than simple light detectors. It would have been easiest for them to detect the most common light radiation. And our eyes evolved from there.
The second reason we use this range of light to see is that our atmosphere lets most of that radiation through. Much of the ultraviolet radiation from the sun is absorbed high up in the atmosphere. This is lucky for us because it would be very harmful. Then gases in the atmosphere absorb a lot of the infrared radiation. Visible light falls in the band of radiation that can make it through the atmosphere. This means that not only is it the most abundant band of radiation from the sun, it is also the most abundant band that makes it through our atmosphere.
The third reason is that this band of light is able to pass through water. This mattered for early life and the early eye because life developed in water. Because early animals evolved in water, wavelengths that penetrate water were particularly useful for vision. Conveniently, water is relatively transparent to much of the visible spectrum. And the eye itself is largely made of water. The longer infrared wavelengths would be absorbed by water and would be no use.
The fourth reason is that the visible light spectrum reaches our eyes without much background noise. If we were able to see infrared light, it would be almost impossible to see. Everything with a temperature above absolute zero emits thermal radiation, and objects at ordinary Earth temperatures emit much of it in the infrared. Our eyes themselves would also be giving off heat. The whole world would be bathed in infrared and it would be very difficult to make out detail, especially during the day. It would be like trying to see an object when the sun is shining in your eyes. As the energy of the photons gets lower, it becomes harder for our eyes to tell the difference between a photon and the heat that is already there.
So, we evolved to see the part of the light spectrum that we can see because it was the most advantageous. It is the best way to get information about our surroundings on our planet. However, if we were orbiting a cool red dwarf rather than our sun, we would get more long wavelengths and more infrared. This star would be cooler than ours, so perhaps the whole world would be cooler. Maybe we would have evolved a lower body temperature and if there was less thermal noise, it might have been possible for us to evolve infrared vision. And this is what I learned today.
Sources
https://en.wikipedia.org/wiki/Visible_spectrum
https://learn.genetics.utah.edu/content/senses/vision
https://www.animations.physics.unsw.edu.au/jw/light/why-vision.htm
Photo by Erica On The Go: https://www.pexels.com/photo/bright-sun-and-silhouette-of-mountains-under-blue-sky-13719515/