Photons – the building blocks of light
Colour is light. Light is photons. Photons are elementary particles with no mass and so travel at the cosmic speed limit, the speed of light, 186,000 miles per second. It is a constant in the universe. Photons are emitted in various types of energy like radio waves or gamma rays. They are also emitted as energy in the matter and antimatter collisions, like those of the early universe which emitted immense amounts of energy in the form of protons.
See here for my blog on the first atom, which describe more in detail those first moments from the early universe. The Evolution of the First Atom
Waves
Gamma rays have high photon energy and short wave lengths. It comes from radioactive decay of atomic nuclei. The lowest photon energy is radio waves with long wave lengths. They are used frequently in our society to transmit communication. In the middle somewhere is visible light, the light which we have evolved to safely detect and process.
The chemical structure of the object determines its colour to our brain, as it will absorb certain wavelengths and reflect others back into our eyes. For example hydrogen gas H₂ is colourless and oxygen O₂ is a pale blue. Depending on the structure of the object chemically and other properties like the temperature, it can change the perceived colour. The source of the light itself can change the way colour is perceived. A red can look very different at different times of the day and in different artificial lighting.
The evolution of the eye
The eye evolved before the brain. There is no need for a brain before there is information to process.
Early eye
The eye would have begun as a simple light detector using photoreceptors. There would have been enough to distinguish between light and dark, good for establishing circadian rhythms. These rhythms are essential to organisms as they determine the best time of day for certain processes to occur. Circadian rhythms have evolved separately in each kingdom of life, animals, plants, fungi and bacteria. Photoreceptors can’t distinguish shape or direction of light.
This basic function also is to distinguish an object that is separate and moving against the background. This trait would require no memory bank to recognise objects, but just the ability to notice an object. Then it would have evolved to detect movement with a stationary eye like some molluscs. And then to accommodate organisms that moved freely in all directions, the eye had to adjust to irregular locomotion, detect new things and stay relatively stable.
Two main types of eyes for animals
There are two lineages with animals when they diverged in the Cambrian period, leading to two different types of eyes on the whole, compound and camera.
Compound eye. Today in adult insects, spiders and crustaceans. An array of identical imaging units, each has a lens or reflector and beams light to photoreceptors. Good for small animals, offering wide angle view. An evolutionary advantage for small animals in Cambrian period. Not efficient for larger animals.
Camera style eyes. A single light focusing lens, arranged as a sheet called retina, lining the inner surface of the eye. Squid and octopus share this eye type with us. There are two types of photoreceptors, cones and rods. Early fish around 500 million years ago, had this type of eye.
Timeline
600 million years ago – began as a simple light sensor for daily circadian rhythms
500 million years ago – optical organ with links to the brain
420 million years ago – our camera style eye had evolved
Rods and cones
The rods are responsible for light and dark, detecting movement in low-light conditions. There are around 100 million rods in our retina. The cones are responsible for colour perception itself. There are between 6 and 7 million cones. The pupil will natural adjust to suit the lighting conditions. We have three types of cones, one for red, one for blue and one for green. It is the combination of these that enable us to see the 17 million colours that we are capable of.
In other animals, their colour perception depends on the photoreceptors in their retinas. Many insects can see ultraviolet colour, some snakes can detect infrared and bats see no colour at all. Dogs only have two types of cones meaning they fall into the same category as colour-blind humans.
Primate vision increases brain size
Stereopsis evolved to enable an organism to judge the distance to objects in its environment. For primates, this may have evolved in order to grab branches better. This trait is straight forward for organisms with fixed eyes and developed well for organisms like toads, who stay in one place and catch prey. But for those which move like us, increased processing power is required. We are also able to estimate the size of the object in association with the distance.
Brain tissue is metabolically expensive so the evolutionary changes that have occurred within primates, giving us increased brain size to body size ratio, it had to be worth it and beneficial to increase the size of the visual processing area of the brain. We would then have evolved special areas for colour distinguishing and added meaning to those different colours, in order to aid survival.
Evolutionary benefits of seeing colour
Seeing colour has many advantages. Organisms are able to distinguish between food sources and the quality of that food. If something is going bad and rotting, it is an advantage to see the change in colour to determine this.
They are able to use colour for mating purposes, to attract mates with vibrant colours and displays of their colour and designs. Some organisms are attracted to a specific colour only, and when artificially exposed to that colour, will still go towards it.
For the advent of predator and prey, camouflage is an interesting evolutionary trait. To blend into ones surroundings using colour and pattern, colour needs to be visible to other organisms. This is good for both predator and prey. For prey to hide completely under the predator disappears, or for the predator to hide in preparation to pounce on prey.
Colour related conditions for humans
Colour blindness comes from one of these cones not working properly. A person will functioning colour vision will have cones that overlap slightly. Colour blindness means the cones overlap too much which limits their perception of colour.
Tetrachromacy is when a person has four cone types rather than the usual three (trichromatic). This can occur by genetic mutation and only in women due to the gene for our red and green cone types in the X chromosome. Very occasionally then, women can receive two different versions of the gene, as women as two X chromosomes and men only have one.
Synaesthesia is a condition whereby the individual has a heightened sensitivity. Some people see letters or numbers as inherently coloured. Some see colours when listening to music. The artist Kandinsky had this condition and wrote a lot about how it felt. Colour to him was combined with smell and touch.
Seeing colour really is a fascinating journey, the development of the eye itself to discern light and movement, to then seeing colour, applying meaning to what the organism sees and then sharpening that sight for various environments.
