Why is there no large animal with more than 2 eyes but spiders have a lot of eyes?
In: Biology
The last common ancestor of all vertebrates had two eyes and that’s worked well enough. No mutation that gives more than two eyes has given a significant survival or reproductive advantage, so we’ve stuck with what works.
The arthropod body plan is a bit more flexible when it comes to numbers of things. You can have anywhere from 6 to 100 legs, plus 0 to 4 wings, 2 simple eyes, compound eyes, or 8 or whatever.
Arthropod eyes are way simpler than ours, so to be able to navigate their environment they need more “pieces” of ocular organs, we never needed to develop more “eyes” because we can see enough to survive with our two.
Technically the tuatara lizard has a third eye, complete with retina, lens, cornea and nerve endings but it isn’t used for vision.
Eyes are expensive organs. Luxury items for life in general. Good eyes are hard to make, use, and keep.
Increasing the number of eyes means producing the materials the eyes are made of during gestation.
The brain needs more capacity to accept the incoming signal from the eye and integrate it with the rest, potentially very difficult depending upon where it is looking.
The body will need to accommodate the eye in a way the keeps it safe but effective. Effort will need to be expended to protect and maintain them.
So what do we get?
Why do so many cars have 4 wheels? Not all do, but it’s the overwhelmingly dominant design. 1 can work and advantages can be seen at 6 or 8 even. Why 4?
Because it works so well. It costs more than fewer wheels, but it helps solve major daily problems of driving a car all the way up to 5.
At 5 you need to come up with an exceptional reason for adding that wheel. They exist, but they are uncommon.
A car with 6 wheels may be much harder to tip over, but it’s heavier and costs more and very few cars ever tip over.
So why 2 eyes?
At 2 eyes you get something for your brain to compare. 2 slightly different pictures each eye is seeing. Animals can use the difference between those two images to guess a lot of stuff about how big things are, far away they are and how fast they are moving.
You also get a redundant eye which is handy because one is none.
The cost is high but the new feature is gamechanging.
And at 3 or more eyes you don’t get a new feature or ability. You just get slightly better versions of this over a slightly greater area while paying all the cost every time. That’s a bad investment.
For these reasons, Animals that had 2 good eyes generally did much better than animals with 1 better eye or 3+ as good or worse eyes. So more of them had babies and so on.
This was such a dominant feature in complex life on Earth because this was such a big difference to get to two and such a complete dud at 3 that it was decided before we left the oceans.
[But there are!](https://en.wikipedia.org/wiki/Parietal_eye)
It’s not the kind of eye that you were imagining, but many currently living species (reptiles, frogs, and more) have ‘primitive’ eyes on top of their heads. Our common ancestor with those species also had the eye, but mammals and birds have lost it.
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