How advantageous is sight? It seems a silly question, to ask how advantageous it is to see, doesn’t it? Obviously, being able to see things is a lot better than having no image of the world. Sometimes, seemingly silly questions can elucidate some interesting ideas if we just pause to indulge them. Light from the sun is actually a critical driver of natural selection. One example of this is the fact that most living cells on earth evolved DNA repair enzymes long ago to fix the damage done to DNA molecules constantly bombarded by harmful ultraviolet radiation from the sun.
Another way light can drive natural selection is through vision. Light allows an animal to develop a new sensory modality, appreciating its environment in a different away (sight), and giving it the ability to better navigate its surroundings to find food or escape predators. If this were true, we would expect to see different ways of seeing emerge. Not only that, but if a specific type of eye is particularly good at collecting and processing visual information, we would expect to see it evolve independently in many living organisms. The fact is that we do see this in existing living species.
A recent discovery demonstrating this was made by Dr. Michael Bok and his colleagues and they published it in the journal of Current Biology. The scientists were studying a species of fan worms (Megalomma interrupta) which lives on the sea floor. These worms extend their tentacles up to sample the water and collect food particles. Interestingly, they’ve evolved compound eyes on their tentacles that can detect shadows, and therefore motion, to protect themselves from any approaching threats. The researchers used a sequencing approach to study the genes expressed in the fan worm’s eyes known as transcriptomics. Remarkably, they found that the proteins involved in light-detection in this invertebrate more closely resembled proteins found in the retinas of many vertebrate animals. Their results suggest that this fan worm’s unusual eyes most likely emerged independently to suit their needs.
So let’s walk through the stages of the evolution of our eye, and see how many times a stage has evolved independently like the fan worm’s eye may have.
From a Small and Imperfect Eye to a ‘Perfect’ One
In the same way that being a tiny bit faster already gives you an advantage over others, being able to see just a little bit better can make you more fit. Darwin suggested in his book, On the Origin of Species, that if you were to start with a “small and imperfect eye,” then added more features to it gradually over long periods of time, you might end up with an eye as ‘perfect’ as ours. So what evidence do we have in the animal kingdom that could point to these evolutionary stages?
There is a small single-celled organism known as euglena. This entire animal is one cell and has a little flagellum (tail) which allows it to swim. If you looked at it under a microscope, it’d look mostly like a green blob. That green color is chloroplast, which is what it uses for photosynthesis and feeding. Photosynthesis requires the gathering of light, and you need to see it light in order to swim towards it, but euglena is single-celled and couldn’t possibly have an eye. Well, in a way, it does. Amidst all the green you’d see under the scope, there’s a small reddish pigment called an eyespot, which is sensitive to light. Euglena uses its eyespot to undergo a process called phototaxis, by which it travels in the direction of a light source. Having the ability to detect sunlight allows this organism to swim towards it and use it for photosynthesis. Simple eyespots like Euglena’s have evolved independently at least 40 times in the animal kingdom.
So that’s the simplest light detecting apparatus you can get – a simple light-sensitive spot that can detect very rudimentary directional information. We can imagine now that having a sheet of light-sensitive cells (photoreceptors) would be the next step up, because it would allow us to collect more light, thus increasing sensitivity. But how can evolution improve on a sheet of photoreceptors? If some random mutations somehow caused an indentation in that sheet, that would provide considerably more visual information.
The planarian worm is a flatworm that gives us insight into the next stage of evolution of the eye. The planarian eye is basically a bowl on either side the worm’s head. These eyes are clusters of photoreceptors that sit in what’s known as the eyecup – the result of a gradual indentation. These tiny worms use the information derived from eyecups to move away from a brightly lit environment where they’re most exposed, and to hide in the darker, less vulnerable areas. The cupping provides the animal with more directional information than an eyespot because it can now see shadows. It may be that developing this type of vision is more evolutionarily challenging since only six phyla of animals have this system. Though, it is a testament to how advantageous this kind of eye is that those six phyla account for over 95% of all living species on earth.
If this slow indentation were to continue, gradually deepening as the animal continues to evolve, what would the next stage be? Imagine, as the indentation gets deeper, the outer edges of tissue begin to close in an almost spherical formation. The deeper the eyecup gets, the more sophisticated the visual information it can collect, increasing the fitness of the animal at each step. This progress can continue until the eyecup becomes a fully enclosed circle, in which case no light can enter and the animal is left blind, or a tiny gap is left at the very front of the circle. That gap is called a pinhole.
The eye of the nautilus is exactly that, a pinhole eye. This mollusk’s eye is a simple hole through which water can freely flow, and it has no cornea or lens, meaning it lacks any refractory ability. Nevertheless, a pinhole eye is the highest resolution you can have without a cornea or lens, making it a dramatic improvement upon the eyecup. As is common with many things in evolution, however, having that resolution comes with a tradeoff. This eye can’t let in much light through its pinhole, and as such, nautilus has very dim vision. Such pinhole eyes exist in few species today other than nautilus, as most animals had gone on to evolve means of seeing with greater resolution and sensitivity.
The Perfect Eye
You probably have already figured out how to improve upon the pinhole eye. The next stage would be to develop some sort of thin layer of coating over that pinhole. Anything would do, really, even skin, so long as it’s relatively translucent and will let light through. That is the basis for forming what is known as the cornea, the outermost layer of the eye. But why would nature go through the process of covering the pinhole and beginning to develop a cornea; in other words, what fitness does it confer? Covering that hole does three things. First, it protects the eye from infection because it represents a physical barrier to bacteria. Second, it has the ability to refract light, giving the animal the ability to gather more visual information and potentially resolve it into a finer image. Third, closing that pinhole allows the organism to form a specialized humor within the eye. Because water can no longer simply flow in and out of the eye, a specialized fluid can gradually develop to protect the retina from harmful radiation, and a lens can eventually form behind the cornea to more finely focus light-derived information.
When Darwin wrote On the Origin of Species, he was ignorant of two concepts. First, Darwin didn’t know about genetics. Gregor Mendel, the father of genetics, discovered the laws of heredity and shortly after Darwin came up with his theory, but Darwin never knew of his work. Secondly, Darwin had no clue about DNA. It wasn’t until after genetics became a field that scientists of various disciplines raced to discover the substrates of heredity - the basic building blocks of life that we know now to be DNA. Yet his hunch came pretty close to matching reality. That’s the explanatory power of the theory. You needn’t always know details to be able to come up with an evolutionary explanation for something; you need only understand the theory itself.
We began with a simple light-sensitive pigment (an imperfect eye) and ended with a full chambered eye protected from any external pathogens. Now, nature can begin to experiment with different components of the eye, gradually increasing in sophistication to increase the animal’s fitness depending on its environmental needs. Our eye is such an eye, and it does a great job of allowing us to function properly in our environment.
Lest you think (as Darwin did) that ours is an “organ of extreme perfection,” a quick look around the animal kingdom will certify to you that there is no perfect eye. Each eye is beautiful and elegant in its own right, and each eye we look at offers us a new way to appreciate the wonders of evolution.
