In this science article we explain how the eye evolved from ‘light sensitive’ spot to simple ‘camera eye’.
The ability to see is not as widespread as you might think in the ‘kingdoms’ inhabiting our planet; the ability to see is absent in the ‘kingdom’ of plants, ‘kingdom’ of fungi and ‘kingdom’ of bacteria.
In the ‘kingdom’ of animals, out of 38 different types of ‘body plan’, (or phylum) only 6 ‘body plans’ (phyla) have ever evolved eyes. Animals from those 6 ‘phyla’ that have evolved eyes include….sixty two thousand species of vertebrate….
eighty five thousand species of slug, snail and octopus and…..a million species of spider and insect.
In the ‘kingdom’ of animals, the 32 ‘phyla’ that have never evolved the ability to see include lesser known ‘glass sponges’ from Antarctica….
microscopic ‘wheel animals’ (Rotifera) inhabiting fresh water lakes and …
‘comb jellies’ (Ctenophora) which are found in most marine environments.
The evolution of sight- how it started
600 million years ago during the ‘pre Cambrian’ era, the most complex life forms consisted of multi celled animals called Ediacarans. These strange looking creatures lived on sea beds and had no heads, mouths or digestive organs. It is believed that they ‘ate’ by absorbing nutrients from the sea bed through their bodies.
Ediacarans had no eyes; this was of little consequence since other organisms had no eyes either. There were no predators around that could attack and eat them; neither could Ediacarans prey on other organisms.
This static state of evolutionary development was not destined to last; these primitive life forms were about to evolve into something very different.
544 million years ago there was an explosion in the diversity of life.
Over a period of only 10 million years during the so called ‘Cambrian Explosion’, a huge number of complex organisms appeared in our ancient seas; complex organisms that included many species with the ability to see.
Reasons for the Cambrian Explosion
There are two main reasons why so many different species evolved within such a short timescale.
1) Rising levels in oxygen
The first change was the rising levels of oxygen which took place both in the atmosphere and in the sea. Rising oxygen levels gave organisms the ability to grow larger and evolve more complex body forms. More complex body forms included the formation of the brain and eyes.
Brains of increasing complexity allowed light signals from the eyes to be converted into visual images so that organisms could ‘see’.
The bizarre looking Opabinia regalis had five eyes and a long forward facing proboscis. It is entirely probable that it used the claw-like structure at the end of its proboscis to capture its prey.
It was the rising oxygen levels that gave Cambrian organisms the energy to chase (becoming predators) and be chased (becoming prey).
2) The evolution of eyesight
Organisms with the ability to see the world around them either became hunters or the hunted. Predators with sharp eyesight could locate their prey more effectively than predators with less sharp eyesight.
This one meter long Anomalocaris, with its acute sense of vision, was a successful hunter and top predator of the Cambrian seas.
- So how did prey adapt to survive attacks by the Anomalocaris and other Cambrian predators?
The hunted organisms developed effective eyesight of their own so they could be alert to attack by predators. They also developed defensive attributes which enabled them to survive being attacked.
For example trilobites had both acute vision and hard exoskeletons; these exoskeletons made it more difficult for predators kill them .