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Eh, similar answer, your basic body plan goes back really far in your genetic history and is a really early part of embryonic development. All of your relatives going back to a common ancestor ~500Ma have two eyes. It’s a quick internet answer not an essay, the exact correct common ancestor / taxonomy group is a bit difficult to pin down.

Bilateria is the reason the number is even… some common ancestor after you split with spiders is the “exactly two” reason, which i think is all vertebrates now, or at least close to that level.

> (Also, I find it interesting that most of internal organs are not symmetrical)

And they can sometimes grow 'mirrored': https://en.wikipedia.org/wiki/Situs_inversus

>2 is not the only even number for this.

No, but it's a very common number in more advanced organisms, and the smallest possible that satisfies bilaterality/symmetry.

'External symmetry is an adaption to environment' seems to be a promising starting point. You can see this at the micro level too: cell externals and internals.