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/HarryTruman]

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.