Daniel GallucciJun 19

Evolution of Eyes

A close up a man's left eye with a rainbow reflecting on his face.

Eyes are terrible at leaving behind evolutionary evidence. This is due to their gelatinous, environmentally sensitive makeup. Historically this created much debate and frustration among scientists including the Godfather of Evolution Charles Darwin,

“The idea of eyes having evolved seems absurd to the highest possible degree.”

But evolve they did.

Current evidence suggests that proteins of simple unicellular organisms were able to receive light approximately 800 million years ago. While this form of light reception can’t be called “vision”, a slow multi-step evolutionary process was well underway.

Opsins

Opsins are a type of “receiver of light” (receptor) protein that showed up 700+ million years ago. There are many types of opsins spread throughout the animal kingdom. They can inform a cell about light, dark, circadian cycles, lunar cycles, and even depth.

One problem with these opsins was that they did not work well into the depths of the ocean where light became almost nonexistent. The solution was the evolution of stackable membranes which basically allowed light to pass through and increased the cells light sensing capacity by seven orders of magnitude.

Lamprey stackable membranes scan.Lamprey stackable membranes

Low Resolution Vision

Improved light sensing ability then led to what we could consider the first “eye” between 700–540 million years ago. These were tiny eyes usually less than 1mm in size that you see in flatworms, starfish and jellyfish.

This basic version of vision allowed for simple approach and avoid type behaviors. These creatures could not “see” other animals, predators or even each other, but they could use low resolution vision to find dark places to hide. Or the minuscule ripples in their visual field indicating a potential source of food even when they can’t see the food itself.

High Resolution Vision

The slow and steady progression of visually guided behaviors put pressure on the visual organs to continue their evolution. Eyes needed to be bigger the same way brains needed to be bigger. The larger eyes that emerged allowed for high resolution vision that appeared during the Cambrian period 540 million years ago.

Visually guided predation was now born! Predators and prey became connected in a visually mediated battle for survival and co existence. Eyes then evolved into more than just predator prey dynamics. Over time the refinement of vision has made for the ultimate episode of The Bachelor as mate selection became a largely visual phenomenon.

A male peacock fanning his tail feathers.

Modern Eyes

It’s important to realize that vision was not inevitable. The evolutionary ability of a simple cell to process light from the sun and use it as a principal source of energy has led to the eyes we have today. It led to us being here today as we most assuredly would not be without vision.

Could we have come to vision another way? Could we have developed a crabs immovable eyes perched a top its head, or the vertically slit pupil of a crocodile that masterfully gauges the distance of unsuspecting prey?

Close up shot of an alligator's eye.A vertical pupil is great for depth perception

Maybe, but not likely. What most scientists used to think of as separate lineages evolving from respective branches on the tree of life, now seems to share much more common underpinnings. All eyes, in their exquisite diversity share a unity of both the molecular underpinnings and genetics used to create them. It’s like being dealt a great hand from the dealer in a poker game.

But playing that hand is not preordained; our eyes remain in a constant state of change. Not the rapid-type change that makes for a trending Instagram story or TikTok post, but change, nonetheless. Driven by how we play the hand we’re dealt.