The Eyes Have It

Eyes are complicated, right? You’ve got the ball full of fluid, with a pupil and an iris in the front and photoreceptors in the back, which feed information into the optic nerve and send it to the brain.

In very simple terms, I have just described human eyes. Or the eyes of a wolf, bird, or any other vertebrate on Earth. Are there any other kinds of eyes?

You bet. However, there are a surprisingly few types of eyes, given the variety of animal life on our planet.

Eyes started on Earth as very simple organs, and those super-simple organs are still around today. They consist of a few photoreceptor cells connected to an optic nerve. This allows that creature to know the difference between light and shadow, although they don’t know from which direction the light is coming from. And that allows the creature to regulate circadian rhythms and respond to shadows. That is the eye found in an earthworm, sea urchin larva, sea star larva... The earthworm, for example, doesn’t want to bake in hot sunlight, so if its eyes say it’s bright, it will wriggle around until it senses shade.

The next step in the development of eyes was that the photoreceptor cells were joined by pigmented cells, which allowed this creature to tell the direction of the light. These creatures - like the box jellyfish larva - could now determine which direction to move to get out of the sunlight (or into it) and could respond if a predator’s shadow moved across them.

The third step was actually a split. Not of the eyes, of the types of eyes that evolved, but this step in both types is called Low-Resolution Vision. Some animals developed a ‘cupped’ eye; an eyeball with photoreceptors lining all but the front opening. Other creatures developed compound eyes. Having Low-resolution vision meant the creatures could detect their own motion, avoid objects and find preferred habitats because they saw crude images of objects in the world around them.

Both types of eyes took one more evolutionary step - to High-Resolution Vision - by adding a lens, cornea and iris at the front of each eye to focus the light. With this higher resolution, a creature is able to identify their mate, a co-worker, an approaching predator or something they could eat.

Humans have High-Resolution cupped eyes. They might not stay ‘high-resolution’, and some just don’t work at all, but as a species, that is the type of eye we have. Of course, our high-resolution vision can’t really be compared to the high-resolution vision of eagles. But then, we don’t rely on seeing prey from miles away to keep our belly full.

Eyes are there to fill a need for the owner. If the creature only needs to know if it’s in sun or shadow, then that’s all the eyes will tell it. That species has no need for eyes any more complex than that.

I don’t know about you, but I’m thankful my eyes are so complex.