3 ms·

Your reasoning about bilateral/symmetry implies there is an even number of things (like the 8 eyes spiders have) 2 is not the only even number for this. (Also,

• cgriswald 4y ago
  > (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 https://en.wikipedia.org/wiki/Situs_inversus
• colechristensen 4y ago
  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.
• eternalban 4y ago
  '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.
• coldtea 4y ago
  >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.