THE EVOLUTION OF THE EYE - How Anatomy Can Change Our Perception
INTRODUCTION
“Only a tiny fraction of the sun’s light and energy bathes the earth, but it is all we need. Virtually all life on earth depends directly or indirectly on the sun’s radiant energy. Thus, the beginning of photoreception on earth is close, at least geologically, to the beginning of life.”
The eye is the second most complex organ in our body following the brain and for vision to take place both structures need to be involved and cooperate. Considering nearly 70% of all the receptors in our body are in our eyes is no wonder vision is the one sense we rely on the most.
Eyes are especially important for birds. Their good ability and speed in receiving and elaborating the visual clues of their surroundings is essential, not only for a safe flight, but for detecting preys at a great distance.
But not two eyes are the same. Different species evolved different types of eyes, depending on the needs of their environment. Did you ever stop to notice that some animals have eyes on the side of their head? Why ours are so frontal? And why the eyes of a fly look so different?
In this article I'll break down the basic process of their evolution, this writing is divided in two parts: the first one will be dedicated to the physical evolution of a “standard” eye ( the ones we consider similar to our own) and its internal structure, also exploring some of the different mutations we find in other animals; in the second part we will move on a larger scale and see how the visual system can affect our perception of the world.
ORIGINS
Light is composed of electromagnetic waves, every time we see something, what we are actually seeing is light bouncing off objects. The purpose of the eye is to convert electromagnetic waves into electric signals that the brain can recognize, the key components of this process are the photoreceptors that we find in the back part of the eyes. Before light hits the photoreceptors though, it needs to be manipulated.
In short, when light passes through the front of your eye it meets first with the cornea, the outermost lens where light refracts and focuses. Following we find the iris, a muscular layer that can close and open the pupil, adjusting the amount of light that finally will enter the eye. Next light travels through the lens, which helps focus the image even more, finally, we arrive at the retina where the photoreceptors lay.
The eye is then connected via the optic nerve to the visual cortex in the back of the brain where the electrical impulse can be better elaborated.
Photoreceptive cells
“The mystery of the beginning of all things is insoluble by us, and I for one must be content to remain an agnostic.”
It would be unsound to think that at some point of our evolution all the different parts that make up our eye somehow collapsed together by random doing (although it would make for a more interesting story).
If we want to investigate the origin of the eye, we need to start removing each component until we are left only with the unit essential for the detection of light.
In this case, we are talking about the photoreceptor cells.
As with evolution in general, the very origin of the eye still remains obscure.
The earliest trace of these cells can be found even in unicellular organisms, a group of photoreceptive proteins is called an “eyespot”. These cells are the critical component in the formation of the eye, as they can respond to light by initiating a nerve impulse (which will be connected to a nervous system in more complex organisms).
The earlier and simplest form of an eye can be found in the Euglena gracilis. This unicellular creature has the ability of photosynthesis, it needs sunlight to properly retrieve energy. Its eyespot is connected to the flagellum (a simple propulsion system utilized for movement). The photoreceptor cells can detect the amount of light in the surroundings, while the flagellum responds to this input by moving until it finds a brighter zone. Hence increasing the chances of survival.
An eyespot is limited in his capacity of understanding its environment. It can determine the intensity of light in a particular area, but that's pretty much all there is.
Getting into shape
With the basic component for detecting light, we are now able to understand the amount of light that surrounds us. Still, we can't perceive from which direction these photons are hitting from, let alone trying to “see” shapes of potential predators or prey.
Pushed from evolutionary pressure, animals needed to strengthen their visual acuity.
The next step in the chain consisted not in modifying the eyespot itself, rather in changing the shape where we find our photoreceptors. Let's try it, first person.
Imagine fixing eyespot not on a flat surface, but at the bottom of a cup. Now take a flashlight and keep it pointed to the eyespot (using that as a pivot) start moving the source of light in any direction. What you will see is that not all the photoreceptors are hit by the light, with a little bit of processing from the brain you can evaluate where this light is coming from.
This is exactly what happened in a long stretch of time, the surface slowly invaginated the optic cup, allowing it to better detect the direction and intensity of the incoming light. Over the millennia, different animals slowly dug this cup more and more. At the same time, the opening grew smaller, leaving but a pinhole. The smaller the opening, the higher the resolution.
This evolved eye allows the owner to seek shade, or cover, to escape a potential predator. It can detect with precision the direction of the light and even perceive shapes. Although this is a great achievement from the earlier stages, the imaging produced at this point will still have a poor resolution compared to what we are used to.
Adjusting Focus
We are starting to get somewhere, we have a solid basic structure that we can refine.
The cup eye can make more sense of the world that surrounds it, but it’s still far from being able to recognize faces or focusing on a ball mid-air.
The key step to building what we categorize as a camera-style eye is the creation of a cornea with the lens (the closing sector of our eye).
The cornea is the transparent front surface of the eye. For humans, it has typically a diameter of 11/12 millimeters and a thickness of half a millimeter.
This layer is believed to have evolved at first with the purpose of avoiding infections. In conjunction with filling the now-closed cavity with a liquid, the cornea helped to prevent bacteria from flourishing inside our optic cup.
The lens, along with the cornea, is what allows us to put objects into focus.
The lens is biconvex in shape, suspended in place by a ring of fibrous tissue just behind the iris.
By changing its curvature, the lens can adjust to focus the incoming light into a single point and it will hit the retina without losing the intensity of the stimulus, allowing a sharp image of the object.
This adjustment is called accommodation and it works similarly to the focus mechanisms of a camera through the movement of its own lenses.
EVOLUTION'S BRANCHES
Fish eye
Different creatures have different needs in their struggle to survive and they will adapt to their circumstances, that is the base assumption of the theory of evolution. It makes sense to assume that fishes will have their own particular flavor of eyes. Although their base structure of the eye is very similar to the one of other vertebrates (and to ours), they do indeed vary in a couple of interesting ways, especially considering what we talked about so far. Oddly enough, both of these mutations are the result of water being the environment they live in.
Flat cornea: our cornea is, as you might have noticed, rounded.
The extra refraction that a round cornea gives is helping the lens focusing power, help that is much needed due to the contrast between the different media that light must travel through. In our case from outside the eye (air) to the inside (liquid).
Fishes do not rely on their corneas for this extra step because the incoming light has a similar index of refraction of water. All their refracting power must come from the lens and their cornea is, as a result, flat.
Accommodation: since the cornea is not helping in the process, the lens of fishes must be stronger or better than ours in some way, but that is not the case. Fishes and mammals evolved two different solutions to adjust the lens. As explained before, mammals will change the shape of the lens rather than moving it, this allows for a more accurate refraction to take place.
Fishes adopted another solution, they move their lens to switch focus on distant objects. In a relaxed state, or at its “neutral point”, the lens is closer to the cornea and able to focus on their vicinities, they can then retract their lens further in the back when looking in the distance.
This method of accommodation is a bit clumsy and definitively less fine-tuned then ours, but good enough for underwater vision. Due to debris and loss of light, looking in the far distance would be impeded anyway, so the majority of sea creature will default to proximity as their strong point and rely more on their other senses.
The Compound eye
The compound eye is not as much a mutation on the eye as the mental image we have of it, but it might be considered like a different structure altogether.
The early development of the compound eye is speculated to have been aquatic, now it is found on a bigger scale in arthropods (insects, crustaceans...)
This eye is composed of small single “eyes” (called ommatidia) spread adjacent one another, each armed with its own individual lens. All ommatidia contribute to a small area of the field of view of the animal, causing the formation of a mosaic image of the world once the different stimuli are connected from the brain.
As the number of ommatidia increases, the resolution of the image becomes higher, much like the working of pixels in a monitor. But even then the ability of the compound eye in elaborating images remains limited, the most precise compound eye is estimated as 1/60 of the human quality.
On the counterpart, as an object crosses along the field of view of an arthropod, the reception of a light hitting a singular ommatidia turns on and off, making so that the compound eye has an easier time perceiving moving objects rather than stationary ones.
Night vision: Tapetum Lucidum
We all know that some animals are better suited to see in low-light conditions (like cats, owls, tigers, deep sea fishes etc...). The elasticity of the pupil certainly plays a role, as the specific structure of the retina does, but a great deal of help comes from a layer of tissue hidden in some animals (humans are NOT included): the Tapetum Lucidum.
The tapetum lucidum (Latin for bright tapestry) is nothing but a mirror layer found right behind the retina. This mirror reflects any miss-absorbed light back to the retina at the same angle of the source, giving it a second chance of being absorbed by the photoreceptors in the retina, making the most of every beam of light. It is important for the reflection to happen at the same angle of the light-source, otherwise the image would be registered out of focus.
Eyeshine is the visible effect of the tapetum lucidum, it gives their owner the characteristics glowing eyes when pointed with direct light (including the flash of a camera).
Visual fields
“If you change the way you look at things, the things you look at change.”
It is not only the internal structure of the eye that makes a quality gaze, the position of it in the skull plays also an important role in the way the animal understands his surroundings. From this point of view, we can divide the animals into two big categories, some evolved eyes in front of their face while others have them at the side. There is more than one theory on why they needed this specialization, the most interesting idea being the “visual predation hypothesis” (Matt Cartmill, 2005).
Animals with eyes on the side of the head are usually preyed (like cows or rabbits), this arrangement gives them a wide field of view (reaching almost 360° in some cases, like rabbits). Using this awareness they are able to scan the horizon quickly and be on the alert for possible predators and threats in their area.
Animals with eyes in front of their head, on the other hand, tend to be predators (like eagles, cats, or primates), with eyes close to each other, part of the two individuals visual fields overlap in front of them.
It might not look so significant but this detail is what enables the power of stereoscopic binocular vision, by processing two slightly different versions of the same image allows the brain to perceive depth, separating the object of focus from the background.
Predators use this tool to lock in on preys and prepare an attack.
On the other hand, predators with lateral eyes tend to be ambush predators, their method of hunting consists in waiting for a prey to come up to them (see crocodiles), for this reason, they benefit more from the panoramic vision that having lateral eyes provides, keeping a lookout gives them a greater advantage than performing a precise strike since their prey is at hand reach.
Primates might have evolved frontal eyes for different reasons, to navigate the branches of trees. Misjudging the distance from one branch to another when leaping could prove fatal not only for the injury that the fall might provide, but also for their vulnerability against predators that would happily take advantage of a free meal literally falling from the sky.
It is interesting to note in the case of primates having eyes so frontal leaves a huge blind spot in their visual field. Because of it, our ancestor had a stronger need for group cohesion to keep an eye on all the dangers that surrounded them, especially aerial predators.
Belonging to a social group increased their chances of survival, watching each other's back. Keeping track of a big society forced the brain to develop, due to the tightness of the group they needed to become more sensitive to each other's desires and even the slightest change in facial expressions. Seeing a scared face looking just behind you is a good indication of a threat that is approaching and that you need to react fast.
Eye Movements
Having a focused vision requires us to be able to shift our gaze in the direction we desire. On the macro level, we can see how our ocular muscles can aid us in this, but on the micro level, that is not the primal reason why we have moving eyes, instead, we have to take a look back at the photoreceptors that are the keystone of our vision.
Photoreceptors have an average processing time (the time is taken to transform a light input into an electric signal) that is relatively long, around 20 ms, during this period light must hit the same set of photoreceptor cells to have a clean and focus image, if the light flashes too quickly across the retina all we would see is a blur.
At the same time overstimulating the same area would exhaust the cells, much like a muscle, a totally still image in our retina would fade in a few seconds.
From these constraints, we had adopted a certain rhythm, a type of dance if you will, combining the stability necessary to register the light-source with the freshness that comes with a new stimulus.
Seen from this perspective we can divide the movements of the eyes into 2 categories, slow and rapid.
The slow movements are passive and generally out of our awareness, they help to fixate and lock on a point regardless of the movement of the body. It is impossible for us to move our eyes in a slow, linear way from left to right unless we are tracking an object, and even that object must not be too fast or too slow.
But you can see this reflex in action by looking at your eyes in the mirror and rotate your head left to right, up and down, the eyes perfectly compensating the changing angle of the head without you having to guide them.
Rapid eye movements (also called saccadic) are what we are more familiar with, they consist of quick rotations followed by a sudden stop. Micro-saccadic movements are of the same kind but so small as to be imperceptible, this small switch of focus makes sure that light doesn't strain the same group of photoreceptors.
Because the central area of our retina is the sharpest, when we look at an object we scan it piece by piece, glancing rapidly at its main features and contours until we have enough information for our brains to categorize it.
During the transition from one position to another, our eyes move so fast as to render us effectively blind, our brain disguises this blindness by substituting the empty space with the first input of the newly acquired image. It will be clearer with an experiment called the stop clock illusion.
First, find an analog clock, now look away from it for a few seconds and then look back, you will notice that in some instances, the first second of the pointer seems to last longer than it should (you might need to repeat the experiment a couple of times since you there is no guarantee at which fraction of the second you will look at it). This happens exactly because our brain replaces the blank travel time of the eyes with the first image he's able to focus on, covering our blindness.
From the big size of their eyes, birds (who have the most refined visual system in nature) and some insects have limited local mobility. These species had to compensate the handicap by finding new solutions to the same problems, in the case of birds we talk about head saccades, that type of snappy head rotation that is so characteristic of them, and the incredible ability to keep their head perfectly still like a Steadicam, even when moving their entire body.
With some species of insects, this goes even forward by moving not the head but the body with the same speed and precision that we possess in our eyes.
Eye and brain
“Consider thinking as an extended form of vision that allows us to see more of the world, and creativity as the ability to expand that vision beyond conventional boundaries”
EXPANDING VISION
During the second world war, the US military force was faced with a dilemma, bomber planes are very expensive to build and far too many of those who went on a mission did not return.
The military needed to reinforce their planes to avoid the enemy shooting them down. Problem is, you can't just armor a plane like a tank and pretend it will still lift off, so you need to understand where the plane needs more protection to optimize the weight added.
So the military recruited the mathematician Abraham Wald, a member of the mathematical division of the army, to help look for a solution.
Everyone noticed that the returning planes took damage in the wings and the center of the body. The answer from the military was pretty obvious: to add as much protection in those areas as possible. Abraham, on the other hand, thought he could see something that everybody else was missing, he told the engineers to add whatever armor they could to the parts of the plane that were NOT seen damaged.
Why?
Well, the planes that returned were survivors, the points where they got hit were areas where a plane can take bullets and still be able to make it home to tell the tale, this means that the opposite is true, the places where the returning planes were healthy were exactly the ones that needed more armor because they were the most indispensable. After his instructions were put into practice they could see the positive results of his thinking and improve the surviving chances of not only the planes but, more importantly, the crew members as well.
VISUAL PROCESSING
Birds have the best visual system on the planet. Because they depend so much on their eyes, they evolved a complex retina that can process the visual input, producing a much faster and instinctive response.
Humans have lower quality vision than birds, and our visual processing occurs in the brain ( about one-third of our brain is engaged in vision or visual processing), using the higher-end of our neural power makes the response slower, but, on the other hand, more conscious, carefully analyzing what we see. Because of this more deliberate analysis of what we see some camouflages are not effective on us, also we tend to crash into the glass in windows less often than birds do (although no one is perfect).
Why does this all matter?
Due to this particular configuration when we look at the world, our eyes glimpse only a tiny fraction of it, the rest is processed and interpreted by the brain, filling the blanks and blind spaces.
Depending so much on the brain we might look at less, but we can see a lot more than what is in front of us.
The same reasoning power that Abraham Wald applied in his field is no different from the hunter tracking his prey, the hunter might not see the target with his own eyes, but he can pick up clues in the environment (like marks on the ground) and figure out what those might mean.
Seen on a larger scale, reasoning in this way gives our species the opportunity to have a greater grasp on the real world, a world that is far more complex than our senses can perceive.
The biggest discoveries and breakthroughs in history follow exactly the same process.
Take as an example of the theory of evolution itself, evolution is not something we can witness by ourselves, our lifespan is too short for that.
Charles Darwin in his travels looked at the different life forms around him, he noticed patterns and speculated on what they might mean, making connections between different observations until he came through with the theory of evolution.
He imagined in his mind evolution as an irregular branching tree, all life starting from the same seed and spreading in different directions or branches, some of those branches are still growing and creating new ones, while others fell short and disappeared.
When it comes down to vision, you can't separate the eyes from the brain, without either of the two we are effectively blind, but consider imagination as a way of the brain to “see” that is not directly dependent on your senses, freeing itself to a certain degree from external inputs gives our brain the opportunity to expand and explore the boundaries of our perception. Detaching itself from his immediate reality, imagination is ultimately the backbone of new discoveries, arts, and inventions. A way of not only observe more accurately the world that surrounds us, but also a way to create something that will influence it.
In our day to day living we might not make big discoveries, but when we begin a new project the dynamic of imagination remains the same.
Despite the myths that surround it, an imagination of this kind is a very deliberate and conscious process applied to a specific situation. It is a bit different from the kind of daydreaming that a child might have.
If we want to apply this to a new project we need to find the sweet spot in filling the blanks with our imagination, too little imagination and we will regurgitate the same common ideas, too much of it on the other hand and our conclusions will be too disconnected from reality.
No one said it would be easy of course, if it was we wouldn’t be celebrating these creative moments.
QUOTES
“To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I confess, absurd in the highest degree...The difficulty of believing that a perfect and complex eye could be formed by natural selection , though insuperable by our imagination, should not be considered subversive of the theory.”
“The limit of man’s knowledge in any subject possesses a high interest which is perhaps increased by its close neighborhood to the realms of imagination”
“Imagination is everything. It is the preview of life’s coming attractions.”
“This is the miracle of the human mind—to use its constructions, concepts, and formulas as tools to explain what man sees, feels and touches. Try to comprehend a little more each day. Have holy curiosity.”
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Bibliography and references
Schawab, Ivan. Evolution’s Witness : How Eyes Evolved. Published : 2011.
Parker, Steve. Color and Vision : The evolution of eyes & perception
Georg Glaeser, Hannes F. Paulus. The Evolution of the Eye
Trevor D. Lamb, Shaun P. Collin, and Edward N. Pugh, Jr. Evolution of the vertebrate eye: opsins, photoreceptors, retina and eye cup.
Greene, Robert. Mastery. Published : 2012.
Csikszentmihalyi, Mihaly . Creativity: Flow and the Psychology of Discovery and Invention. Published : 1996.
Jason G. Goldman. Evolution: Why do your eyes face forward? October 24, 2014
R. A. Barton. Visual specialization and brain evolution in primates.
David McRaney. Survivorship Bias. May 23, 2013
https://youarenotsosmart.com/2013/05/23/survivorship-bias/
Robert Greene: "Mastery" | Talks at Google
https://www.youtube.com/watch?v=J4v_34RRCeE
Amazing Evolution of Eyes(Nature Documentary)
https://www.youtube.com/watch?v=iuuan74brFM&t=1s
https://en.wikipedia.org/wiki/Prefrontal_cortexhttp://learn-math.info/historyDetail.htm?id=Wald
https://en.wikipedia.org/wiki/Binocular_vision
http://www.actforlibraries.org/why-some-animals-have-forward-facing-eyes-and-others-have-sideways-facing-eyes/
http://www.cogsci.nl/blog/bird-brains-and-fish-eyes/148-a-bit-about-the-evolution-of-eye-movements
https://en.wikipedia.org/wiki/Imagination
Amazing Evolution of Eyes(Nature Documentary)
https://www.youtube.com/watch?v=iuuan74brFM&t=1s
http://www.biology-pages.info/C/CompoundEye.html
https://en.wikipedia.org/wiki/Vision_in_fishes
Pictures:
Focus in an eye by Erin Silversmith https://commons.wikimedia.org/wiki/File:Focus_in_an_eye.svg
Eye anatomy diagram (modified) : https://www.publicdomainpictures.net/view-image.php?image=130389&picture=medical-eye
Euglena gracilis by fickleandfreckled
https://www.flickr.com/photos/fickleandfreckled/6793271762
https://www.flickr.com/photos/fickleandfreckled/6808789136/in/photolist-bmii2d-bnEPN9
Bug eye : http://maxpixel.freegreatpicture.com/Blue-Bottle-Fly-Insect-Blowfly-Eyes-Macro-Close-Up-2019364
Eye evolution By Zern Liew
https://www.shutterstock.com/image-vector/diagram-evolution-eye-122616334
Bird:
https://www.pexels.com/photo/bird-flying-zoo-beak-9291/
Cat by Krysten Merriman
https://www.pexels.com/photo/cat-whiskers-kitty-tabby-20787/
Human eye by Pixabay
https://www.pexels.com/photo/extreme-close-up-of-woman-eye-256380/
Cat eyeshine by Карма2
https://commons.wikimedia.org/wiki/File:Thai_cats_eyeshine.JPG
Snow monkey :
https://www.publicdomainpictures.net/view-image.php?image=223229&picture=snow-monkey
Lion:
https://www.publicdomainpictures.net/view-image.php?image=223222&picture=lion
Brown rabbit:
https://www.publicdomainpictures.net/view-image.php?image=24094
B-17 by Historicair
https://commons.wikimedia.org/wiki/File:B-17F.jpg
Glasses and journal :
https://www.pexels.com/photo/antique-blank-camera-classic-269810/
https://www.photojoiner.net/ for the photo edit