How Did Eyes Evolve?
We take our eyes for granted, like most things to be honest. It is said we only notice something when there's a problem with it.
Our eyes are crucial for us to navigate the world. There is a huge section of our brain dedicated to processing the information that comes in via the eyes which shows how crucial it is for our survival.
The First Light Detectors
The story of the eye begins with organisms that had no eyes to begin with, naturally. The basics of the eye begin with light detecting cells called photoreceptors. The only thing they could initially do was disinguish between light and dark.
This is extremely useful as it began to help the organism adapt to what time of day it is, and to help develop and regulate circadian rhythms. The internal biological clocks run processes according to the cycle of day and night. Evolution can then build on this very simple, but powerful concept.
Next, it was crucial to determine where the light was actually coming from. And then to distinguish objects that are moving against a background. With this new information, the organism could make better survival choices. As it increased the chances of survival, evolution would continue to favour light detecting cells and develop them further.
The Cambrian Explosion
Around 500 million years ago during the Cambrian period, eyes became much more sophisticated. Two major types of eye developed along different evolutionary paths. One was the compound eye. The other was the camera-style eye.
Compound eyes are made from many small visual units, each containing structures that detect light. They are found today in insects and many other arthropods. They provide a wide field of view and are particularly useful for smaller animals.
The other design was the camera-style eye. This is the type of eye that humans have. It uses a lens to focus light onto a layer of light-sensitive cells called the retina. This type of eye actually evolved independently in several groups. Octopuses and squid, for example, have camera-style eyes despite being very distantly related to humans.
The Human Eye
Our eyes contain millions of photoreceptor cells and there are two main types, rods and cones.
Rods are particularly sensitive to light and are important for seeing in low-light conditions and detecting movement. Cones are responsible for colour vision and work best in brighter conditions.
Humans have three main types of cones which respond most strongly to different ranges of wavelengths. These include cones for detecting blue, green and red wavelengths of light. They work together to allow us to distinguish a huge number of different colours.
The signals from these cells are then sent towards the brain. The eye detects the light and wavelengths of light, and the brain creates the experience of seeing.
Why Eyes Were Worth The Investment
Our brains are expensive in energy terms and the area that governs sight is big. There is a clear advantage for us and other species to be able to 'see' the environment we are in. By seeing we can detect predators, find food, judge how close or far things are and recognise familiar safe or dangerous environments.
These are all incredibly useful for pure survival and of course many mating rituals also depend on sight.
On balance, it was worth the investment for evolution to develop these highly sophisticated cells and parts of the body, for us to be able to navigate our environment successfully by simply detecting light.
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