In one of the documentaries he's featured in, Neil deGrasse Tyson talked about the evolution of the eye.


The part that's always intrigued me is this:

Our eyes originally evolved to see in water. ... For land animals, the light carries images from the dry air into their still-watery eyes. That bends the light rays, causing all kinds of distortions. ... Our vision has never been as good since [our distant ancestors left the water to live on land]. ... 375 million years later, we still can't...discern fine details in near darkness, the way fish can.

I can think of at least two world building applications for eyes that evolved in air. In a fantasy setting, creatures that evolved in an elemental air plane, such as the Aarakocra in some retellings of the D&D multiverse, might have very different eyes from ours due to the relative lack of water in their home environment.

In a sci-fi setting, transhumans who want to "correct" our water-based vision to see better might re-engineer the evolution of human eyes.

So, what would eyes that evolved in a "dry" gaseous atmosphere be like in comparison to the ones humans have now?

Edited for clarity:

I'm looking for how plausible and informative the response is regarding the physical structure of eyes that evolved in air instead of water. Thus the emphasis on what they would "be like," and the suggestion to compare them to the eyes we have now.

I specifically left out anything having to do with bacteria in deep time because, while I'd certainly consider such details supportive of the overall answer, the history of how and/or why the structures evolved seems to be its own unique question, and the site generally frowns on more than one question per... question.

  • $\begingroup$ Pick any animal that didn't evolve from the sea (or eyes while in the sea), and has eyes, and look at those. Aside from that, we really can't say which one it'd be, making this question POB. $\endgroup$ – Aify Jul 13 '18 at 23:40
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    $\begingroup$ The original Darwin theory was that all life began in the sea. Meaning the practical answer to your question is "we have no perspective on that." New discoveries suggest life might have began in mud, but that's still water and we're still descended from, originally, water-dwelling critters. Can you be more specific about what you're asking for? If an eye evolved specifically for an avian hominid, would it be different than ours today? I'm not sure what you're asking. $\endgroup$ – JBH Jul 13 '18 at 23:43
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    $\begingroup$ @JBH: This is everything that Darwin wrote about the beginnings of life: "there is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved." (Origin of species, 6th edition, 1866.) His theory supposes that life already exists. $\endgroup$ – AlexP Jul 14 '18 at 1:48
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    $\begingroup$ Maybe I' getting this. "Imagine a world with small, shallow seas where the majority of early evolution occured on land rather than in the seas. How might eyes evolved under those conditions be different than ours?" However, even if I'm right, how can we give you an answer? Can you also provide instructions for how you'll judge the best answer? $\endgroup$ – JBH Jul 14 '18 at 6:04
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    $\begingroup$ Re "...we still can't...discern fine details in near darkness...", who is "we" here? Humans? But humans are basically diurnal, so have no reason to evolve to discern fine detail in near darkness. How about cats? $\endgroup$ – jamesqf Jul 14 '18 at 18:01

They would look like pit viper pits--or at least, one kind of eye that can evolve in air looks like pit viper pits--because that's exactly what a pit viper's IR-sensing pits are. They are a real-world example of a new eye evolving in air, rather than water.

It basically goes the same way as evolving an eye in water. You start out with a simple light-sensitive patch. Then it develops into a cup, or pit, to allow for better directional sensing. This is where pit viper evolution has stopped, so far. But the next stages would still be pretty much the same to get to more advanced eyes. Either you develop a lens, enclosing the eye and filling it with pressurized gas (rather than liquid) in order to maintain its shape, or you split the retina and develop a mirror arrangement (like scallop eyes), in which case the whole structure can remain completely open. It is still possible for such a structure to be enclosed anyway, and supported by internal pressure, but otherwise it would simply be made out of more rigid materials.

There's also the example of arthropod compound eyes. Those happen to have originally evolved in water, but they didn't have to. Compound eyes are not filled with liquid, and their evolution would proceed identically in air.

Incidentally, pit vipers had a real evolutionary pressure to evolve an entirely new set of in-air eyes, rather than just continuing to adapt their existing water-filled eyes, for two major reasons--they rely on seeing other animals' body heat for hunting, but water-filled eyes cannot see infrared, because water absorbs infrared.

  • $\begingroup$ I like the fact that this is applicable to multiple sff species - Lizardfolk, Yuan-ti, anyone insectoid. $\endgroup$ – KernelOfChaos Jul 14 '18 at 18:21

To maintain the highest degree of clarity and sensitivity, the effective refractive index of ocular elements would be as close to one as possible. That would mean the equivalent of a lens could not depend on thinning and thickening to focus light on the focal plane. So, I think they might be in the form of a Fresnel lens with adapting ridges to permit focusing at different depths.

The lens itself would likely be multi-layered to keep the size of the eye down. The higher the index of refraction, the more the light can be bent and focused. So a constant index of refraction would result in a wide and deep eye.

The photoreceptors might be very different too. Rather than setting on a curved surface, they might just be separate planes of transparent for each wavelength of the creatures visual spectrum. These would be similar to our cornea, it's a transparent covering densely covered with nerves -- I cut mine once and it hurts. In these creatures though, each plane of nerves would only absorb energy at a specific range of wavelengths. This would give them a very sensitive vision that could see threats and prey from great distances and even sometimes by their IR emissions, IR shadow, or UV reflections.


Our eyes is basically a layer of photosensitive cells together with a diaphragm to control the amount of light passing through a lens, which focus the light on the photosensitive layer.

From a mechanical point of view, having the space between the lens and the photosensitive layer filled with liquid helps keeping the shape of the entire device, and makes also more precise to control the lens via muscles: try squeezing a bottle filled with air and one filled with water, and see where you have it easier.

Not using the liquid would probably mean choosing between two paths:

  • sensitive layer with no lens: would not go further than detecting movement and light/shadow difference, I am afraid.
  • use bones or cartilage to build a container for the eye.

One possibility could be eyes that look like that of Chameleons. The smaller surface area would reduce the rate of loss of water from the corneal surface, while the unique monocular focussing might keep the quality of vision of similar quality as that of normal eyes.

  • $\begingroup$ Welcome to Worldbuilding.SE! When you have a moment, please take our tour and visit our help center to learn more about us. This was a clever first answer! Thanks!. $\endgroup$ – JBH Jul 14 '18 at 15:55

Our eyes are filled with liquid because we evolved from water.

A life that evolved in air will likely have eyes filled with gas.
So basically a sac filled with transparent gas, surrounded by muscles to move it and change its shape. I know fish have a "gas bladder" to help with depth control, so air-filled sacs are feasible.


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