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, 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.
However in the kingdom of animals it is those species with the ability to see which dominate life on Earth today.
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.
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.
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 .
The Haikouichthys, a proto fish, developed a different strategy to help ensure the survival of its species; it developed a streamlined body so it could swim faster than its predators.
The evolution of sight may even have assisted the development of locomotion.
If you can see food on the seabed you need to be able to reach that food. The evolution of vision in the Fuxianhuia was accompanied by the evolution of legs.
Not all organisms that evolved in the Cambrian era developed sight. The Hallucigenia, a three centimeter long worm-like creature, appears to have had a head but no eyes or mouth. Perhaps it deterred attacks from sighted predators by growing those vicious looking spikes!
We assume that the evolutionary steps leading to the formation of the ‘camera eye’ were driven by natural selection. Every change, no matter how small, improved the chances of survival for those animals that inherited the change.
The first stage is the formation of light sensitive cells; these light sensitive cells are sandwiched between a transparent protective layer and a layer of dark pigment. A single type of light sensitive cell evolved in a common ancestor of all vertebrates and invertebrates.
These light sensitive cells become recessed into a ‘pit eye’.
An organism that possessed ‘pit eyes’ could not create any visual images. However patterns of light and dark hitting the light sensitive cells could give an organism a sense of direction.
Patterns of light and dark hitting the light sensitive cells could alert organisms to the presence of approaching predators.
There are organisms alive today which possess recessed pit eyes. One such organism is the aquatic Planarian flat worm.
The pit in the eye deepens. A deeper ‘pit’ allows the organism to identify the location of predators with greater accuracy.
The proto ‘retina’, including both the light sensitive cells and underlying layers of cells with darker pigments, becomes enlarged. A transparent layer of gel protects the retina at the bottom of the pit.
An organism with these eyes has no chance of seeing any meaningful images; the rays of light hitting the retina would be too jumbled and chaotic to make any sense.
The problems caused by too many jumbled and chaotic images is solved by reducing the amount of light hitting the retina. An ‘aperture’ begins to form at the same time as the retina continues to grow.
The advantages of a light reducing ‘aperture’ become fully apparent at stage 5.
The ‘aperture’ is now fully formed and the ‘pinhole eye’ has evolved.
The ‘pinhole eye’ has one big advantage over the ‘pit eye’. Whereas the ‘pit eye’ can only distinguish light and dark, the ‘pin hole eye’ can distinguish what an object truly looks like.
The pinhole aperture works by directing small amounts of light through the pit onto the retina.
This is what an image looks like after being captured by a pinhole camera underwater. The image has a surprisingly high resolution.
The pinhole eye still exists in a few species living today. The nautilus, a type of mollusc distantly related to the octopus, is one such species.
There is one big drawback for the nautilus with its pinhole eyes.
With a limited amount of light entering the eye cavity, the images it sees are quite dark. In the image below we compare the view a nautilus might see with the view using a more sophisticated ‘camera eye’.
The nautilus’s pinhole eyes can distinguish the shape of objects in some detail, but the image that it sees is quite dark. Not being able to see particularly well in low light conditions is not a massive problem for the nautilus which relies on its well developed sense of smell to scavenge for food.
The next stage was the evolution of eyes that can see images in greater detail (higher ‘resolution’) and in all types of light conditions (greater ‘sensitivity’)
The answer lay in the formation of the lens; if an eye had a lens that could focus light onto the retina then the aperture could be wider and let in more light.
The lens develops from a thickening of the transparent gel already present in the pit cavity.
The earliest lenses were elliptical in shape. However these elliptical lenses were quite weak; they could not bend (‘refract’) light sufficiently to focus light onto the retina. As a result, all images remained blurred.
Despite the problem of blurring a rudimentary lens was better than no lens at all.
A camera eye with a rudimentary lens meant that hunted organisms could see blurred images of predators in low light conditions. This gave hunted organisms which possessed rudimentary lenses a distinct evolutionary advantage when it came to survival.
The aperture moves towards the retina and the lens becomes more spherical in shape. The lens is stronger and can now bend light at more acute angles.
Bending light at more acute angles (becoming more ‘refractive’) means that focal points move closer to the retina. Images are less blurred than previously.
At last, the perfect camera eye!
The lens is now powerful enough to refract light directly onto the retina to produce clear images of a high resolution.
The lens is covered by a transparent hard layer called the ‘cornea’. The cornea of an underwater ‘camera eye’, unlike the cornea of a ‘camera eye’ on land, refracts very little light; its main function appears to be to protect the eye and lens from damage.
The main drawback with the simple camera eye is that the pupil is fixed in size. The pupil of a simple eye cannot get smaller (‘constrict’) in bright light to restrict the amount of light hitting the retina. Neither can the pupil get bigger (‘dilate’) in conditions of low light.
The evolution of a more sophisticated camera eye, with a muscular iris controlling ‘constriction’ and ‘dilation’, is a topic for a future article!
Assuming that each generation of marine organism bred every year, it has been calculated that it would have taken less than 364,000 years for a recessed light sensitive cell to evolve into a ‘camera eye’! Incredible!