Sight and Vision: From Protozoa to X-Ray
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Sight and Vision: From Protozoa to X-Rays

The first creature to have a prototypical eye would most likely have been a microscopic creature in the primordial oceans, with a nerve ending that happened to be light sensitive. From this, the very first view of the world, we (as living creatures) have come a long long way, with complicated fine instruments with almost unbelievable visual ability. Looking forward, what will vision and sight be like, in the future?

The compound eye

Simple "eyes" often evolved quite early, as compound eyes. To give a relatable example to the layperson, the fly's eye is a compound eye, comprised of many individual units that each have their own photosensitive cell. If one aspect of the eye is damaged, the others continue functioning, giving a huge advantage to the owner of a compound eye, over more complicated eyes, as found in vertebrates, for example.

The vertebrate eye

The vertebrate eye is comprised of a true lens, which may change shape, and an incredibly delicate pinhole that is actuated by a muscle, called the iris. Humans and other mammals all have this kind of eye. Other non-vertebrates have a complex eye such as this also, with some strange changes: the pupil of an octopus does not focus light to a singular fine point as our eye does, it's pupil is a curly "W" shape, and has 3D characteristics to it that bend and distort light, allowing the creature to see color in ways that we do not fully understand.

Motivating evolution

Creatures that can see have a huge evolutionary advantage over creatures that don't. While some advantages are obvious (being able to determine the location of food), let's explore some of the less-than-obvious advantages. When selecting a mate, an individual creature is trying to select for the mate with the best reproductive value. As such, many creatures use pheromones or other chemical signals to exchange information with each other about the quality of their diet, their levels of fat stores, and whether they are sexually reproductive or not at a moment in time. All of this information can be conveyed (literally at the speed of light) in a single glance at a prospective mate, without having to physically venture near and risk harm. Another advantage, especially in the days before predators could obtain a view from the air, or from above, was to be able to detect approaching danger from below, early, and from a long distance. This is especially important in the case of large, slow moving creatures, which represent a significant portion of the food chain.

The evolution of color vision

In the earliest days of sight, it would be very advantageous for an organism to be able to tell the difference between colors. It is theorized that detecting infrared (heat) or ultraviolet light (from the sun) may have been the first color sense of organisms, and it is still found in some species today, such as the mantis shrimp. This can be called "tetrachromacy", meaning, literally, "four colors". The term is used to describe organisms that have the ability to see more than three colors. The mantis shrimp can see an astonishingly large band of the spectrum. It is currently an open topic of research as to why the humble shrimp needs to see so far outside of the visible light spectrum.

The evolution of the retina

Retinal evolution is probably one of the most fascinating aspects of evolution. The retina is a layer of light sensitive cells that lines the back of the eye. Far from being a simple sheet of cells, the retina is organized in a way that is both complex and efficient. The retina has an extremely high amount of surface area, all contained within the eye.

The future of vision

While the evolution of the human eye appears to have stopped (at least to us, living our brief lives). However, the need to perceive beyond our natural eyesight is strongly motivated by the modern environment, and has prompted the invention of many devices and mechanisms to see things that have never been seen before. Beginning with simple lenses, we've improved our once-damaged vision back to it's original state, or better. With early telescopes, we've been able to see the surface of celestial bodies, faraway stars, and (closer to home) far away ships that may pose a threat to us. In modern times, we have X-rays to see inside of our own bodies without harm, and we even have devices to see through concrete, used to locate utilities underground. With the advent of augmented reality, we are even able to see things that aren't there, such as overlaying directions onto our vision, to help us navigate the world. With retinal implants, we are even able to restore sight to those people who have been blind from birth, or have lost their eyesight. In the future, direct brain implants may provide even more exciting means of seeing the world. Research is already underway and a race is being run in the research labs of universities and companies around the world.

Keywords

Eye, Biology, Evolution, History, Vision

Author(s): Coco Ming
Published at: 19 Mar 2021 06:11 GMT
Original link (login required): https://ilde.upf.edu/pg/lds/view/213653/