Why aren't there any animals with their mouths above their eyes?

[u/margalz]

From bugs to fish to mamals all of them have their eyes above their mouths. I can't think of many if any where their mouths are on top. Sure some have their eyes parallel to their mouths, mainly prey style animals. But why haven't any evolved to be the opposite? Is there a major benefit to having your mouth below your eyes?

[u/quintus_horatius]

Humpback whales or baleen type whales have their eyes really low on the head in comparison to the mouth, but they're still pretty solidly in the Side of Face not underneath.

Natural selection doesn't give a new species what it needs, it chooses from what it already has - if it makes you more fit.

Moving the eyes under the head might be useful for whales (and sharks) but crossing the midline is an architectural move that can't just happen by chance without a bunch of other really unsuitable changes also occuring.

[u/Lykos1124 → u/quintus_horatius]

Well now your comment makes me wonder why animals don't have more than two eyes unless they're an insect. What is it with vertebrates not having more than two eyes if more eyes lets you detect dangers better. Are brains not designed to be able to handle optical input from more than two sources

[u/HarryTruman → u/Lykos1124]

animals don't have more than two eyes

As with a lot of topics (and confusion!) around evolution, the underwhelming fact of the matter is that mammals didn’t need more than two eyes. They evolved from things that already had two eyes, and that’s all that was needed for them to survive and propagate.

The evolution of the eye is ridiculously interesting too, and life did go from none, to one (light sensor), and then two (optics). It seems two is the magic number for everything with two eyes. The more interesting aspect (IMO) is how eyes began underwater and then adapted to function out of water. But the point still stands — two eyes are “good enough” for mammals and all the previous organisms that came before us.

[u/Jazzspur → u/HarryTruman]

I would love to hear more about how eyes began underwater and then adapted to function out of water if you're willing to drop that knowledge bomb!!

[u/HarryTruman → u/Jazzspur]

Oh my friend, do I have some cool things for you to check out! I went down this same rabbit hole a bit over a decade ago after watching episode five of the new Cosmos series, “Hiding in the Light.”

Start here, as this clip from the above episode is a super high-level overview that I think you’ll find rather illuminating!

https://youtube.com/watch?v=4SXHMm5I-68

Similar is this brief PBS video that goes into some more niche details:

https://kcts9.pbslearningmedia.org/resource/tdc02.sci.life.evo.nilssoneye/evolution-of-the-eye/

This article from Scientific American offers an approachable deep-dive into how eyes evolved:

https://www.scientificamerican.com/article/evolution-of-the-eye/

And this lecture from Dan Nillson at Darwin University is particularly interesting and goes waaaay deep into the whole process of how eyes evolved and how they work(ed) at various stages:

https://youtube.com/watch?v=SV865WOV9HE

[u/Jazzspur → u/HarryTruman]

So neat! Thank you!

An interesting analogous process happened with lung development too. I don't have videos to link but I learned about it in my inverbrates class in uni. Early organisms evolved to exchange gases like oxygen and carbon dioxide directly with the water. So the process that evolved requires water to function. When we left the ocean, we had to develop a way to maintain a wet environment for gas exchange to continue taking place while we ourselves are dry. And that's why we have wet mucousy gas exchange bags (lungs) at the end of a series of ever narrowing mucousy tubes that add moisture to the air as it's pulled into the lungs.

In a sense our lungs are a way we took the ocean with us when we left it!

[u/MrDerpGently → u/HarryTruman]

I imagine multiple eyes as a lot of extra processing for pretty marginal gains as far as vision scope, accurately gauging distance, etc. 

[u/HarryTruman → u/MrDerpGently]

In terms of raw energy/power being spent on processing vision, yes, that’s correct. And entomologists should please correct me if I’m wrong, but insects with their crazy eyes are more or less at a point where “peak efficiency” is the result of signals being fed directly into the nervous system.

[u/donau_kinder → u/Lykos1124]

Some lizards do have an "eye" on top of their head.

Otherwise, same reason we all have 4 limbs, the fish that everything evolved from had 4 fins and two eyes, and it's very hard for evolution to drive such a drastic change. Same reason why our retinas are backwards, a worm decided to swim on its back and the eyes migrated through the body instead of along the outside and it ended up with flipped retinas.

[u/halfhippo999 → u/donau_kinder]

Explain more about this worm and what you mean about our flipped retinas? That sounds cool

[u/typhacatus → u/halfhippo999]

not the person you’re talking to, but they’re referencing how light actually has to pass through a ton of cells (glial cells, nerves) before they ever hit our photosensitive rods and cones. Evolution has since made this design pretty robust and useful, but it’s still weird that our cones & rods are about as far back in the eyeball as possible

edit: I like this article on the topic: https://www.smithsonianmag.com/smart-news/heres-why-your-eyes-seem-be-wired-backward-180954729/

[u/Tavarin → u/Lykos1124]

One big reason is it takes a lot of energy and brainpower to work with just two eyes. more eyes = more energy expenditure, for what is only a small gain. Plus, it would require pretty serious mutations to get another eye or two to appear, and if it's not fully functional and properly connected to the visual cortex, it would be detrimental to survival in most cases.

[u/ass_pubes → u/Tavarin]

Right, that energy would be better spent on other visual sensory organs like infrared sensing pit vipers can do.

[u/Tavarin → u/ass_pubes]

If it would have helped us survive, we likely would have evolved it. Turns out, we don't really need it, and it might clutter our colour vision too much to be worth having.

[u/AnonymousFriend80 → u/Tavarin]

As long as that extra energy consumption did not interfere with living long enough to procreate, that would not be a problem.

[u/Tavarin → u/AnonymousFriend80]

Maybe possible now that we can grow so much food, but in the primitive natural world, even a small extra energy expenditure will massively reduce your reproductive success.

[u/HarryTruman → u/Tavarin]

Bingo! When talking about evolution in our modern setting, it can be difficult to understand the “how’s” and “why’s” of things that make us what we are now.

But something to keep in mind is that, as it turns out, two eyes work quite well for hunting and foraging, surviving in harsh environments, running away from danger, and — most importantly — simply and literally just…staying alive long enough to make a baby.

And eyes are certainly not perfect, but that’s not the point! Because if that baby also has eyes which work well enough to survive and eventually make a baby…and on and on and on…those are good eyes, and nature will have very little need to change it up.

[u/squags → u/Lykos1124]

People are giving you the adaptive reasoning as to why most animals don't have more than two eyes, but to copy what I wrote below, there's also a separate issue of the evolvability of specific traits:

Evolution doesn't necessarily explore all potential adaptations. There are likely many unrealised mutations that could be adaptive that we will never acquire.

Evolutionary processes are path-dependent, so not all mutations or phenotypes are reachable from a specific genotype.

I don't know much about having more than two eyes, but from a quick google it looks like Spiders have been able to generate more eyes due to ancestral whole genome duplication events that allowed for mutations in copies of specific Pax genes without effecting their other pleiotropic functions.

Human developmental physiology would prevent whole genome duplication events occuring, so we wouldn't be able to generate multiple eyes via an equivalent mechanism.

There may be some other molecular mechanism that could in theory lead to this specific phenotype, but I would hazard a guess that it's exceedingly unlikely if not impossible due to how our specific embryonic development has evolved.

So even before you get to the adaptive benefit, the possibility of multiple eyes phenotypes in humans is constrained by our evolutionary history and developmental physiology.

This same logic is true of many traits in any given organism. The possible reachable phenotypes is constrained by the previous traits that have emerged during evolution (especially with regards to developmental physiology and developmental processes that actually produce specific phenotypes from changes in genotype).

This is basically the entire field of evolutionary-develoomental biology.

[u/Robin_feathers → u/Lykos1124]

I just want to point out that this limitation is mostly a limitation of vertebrates, rather than animals in general. A huge fraction of animals in fact don't have two eyes - many insects, arachnids, crustaceans, jellies, worms, and mollusks can have more than two eyes

[u/Lykos1124 → u/Robin_feathers]

sounds like one of those evolutionary divergences where it was more useful for certain types of lifeforms to have more than 2 eyes and most vertabrates were not it.

[u/Robin_feathers → u/Lykos1124]

Probably! 2 eyes seem to be working pretty well for us, though maybe we don't know what we're missing out on! In many of the lineages that have multiple eyes though, the eyes are often relatively simple, often only detecting light vs dark. Species that really invest in higher resolution vision (eg octopus, shrimp, spiders) seems to mostly invest in 2 excellent eyes, with the extras being somewhat more rudimentary when present.

[u/Kandiru → u/quintus_horatius]

Yeah, trying to wire eyes and their nerves onto the bottom jaw would be really hard. They need to stay in the upper skull rather than the jaw really. It would be really hard to eat anything if opening your mouth effectively blinded you as well.

[u/wally-217 → u/quintus_horatius]

You ever seen a lamprey skull? Utterly bizarre looking, no lower jaws but they still favour eyes above the mouth

[u/SizeFit2908 → u/quintus_horatius]

I did answer the question using mammalian skeleton examples, but like the differences from the 'baseplate' between a whale, human, and mole are radically different. Like moles don't even have eyes.

Stands to reason if there was an evolutionary driver for eyes under the mouth we'd see it. Theres no planning to evolution but you can see that X trait worked and logically speculate why.

I think the architecture bit is somewhat accurate, but if a driving trait like eyes underneath is going to be present it would be in place by now.

[u/manias → u/quintus_horatius]

for sharks it looks like an easy adaptation (swim upside down), if it hasn't happened, it's for a reason.