How does evolution rule out the possibility of humans or others fragile herbivores from having 2 pairs of eyes, one at the front and the other at the back of their heads? Why didn’t that ever happen?

  • $\begingroup$ I’m sorry but I’m not a biology student; currently pursuing mechanical engineering. So, I don’t have any knowledgeable viewpoint over the same. The idea was that you people could hopefully provide some intellectual insight. $\endgroup$
    – RedHelmet
    Commented Oct 1, 2017 at 13:31
  • $\begingroup$ “Given limited resources” - what does that mean ? $\endgroup$
    – RedHelmet
    Commented Oct 1, 2017 at 13:42
  • $\begingroup$ @user115962 A limited resource is any element an agent utilizes, in which the quantity, or frequency of use, of the element can not be fully assumed throughout a completely defined process. The most general form of any limited resource is free energy. $\endgroup$
    – user22020
    Commented Oct 1, 2017 at 13:58
  • $\begingroup$ On top of the question marked as duplicate, you might want to have a look at Why don't mammals have more than 4 limbs?. $\endgroup$
    – Remi.b
    Commented Oct 1, 2017 at 16:20
  • $\begingroup$ First, humans aren't fragile herbivores, they're omnivores, and quite successful hunters even pre-technology :-) Now that that's out of the way, the basic vertebrate body plan - four limbs, two eyes, &c - was fixed hundreds of millions of year. Herbivores, such as horses, have the eyes located at the sides of their head, and have nearly a 360 degree visual field. Predators tend to have front-facing eyes, the better to focus on potential prey. $\endgroup$
    – jamesqf
    Commented Oct 1, 2017 at 17:34

1 Answer 1


Vertebrates have ears which sense 360 degrees for a range of many meters, and they have social eyes, where they group many eyes and ears into one social group, it's easier to listen and live in groups, than grow eyes in the back of the head.

2 eyes exists in nearly all bilateral animals, arthropods, flatworms, sea-slugs, crabs, crickets, squids, flies. Arthropods have compound eyes and it would be easier for them to have 4 eyes, and only spiders and horseshoe crabs have more than 2.

It would be easier for a vertebrates to develop 4 nostrils and ears than 4 eyes because they are simpler than the orbs of the eye.

Same reason as 5 fingers has been the optimal numbers for all vertebrates since early lizards. in short, it's very easy to lose extra organs and to modify old ones, but it's very difficult to evolve new organs, and to relocate ones around the nerves and veins... centrally connected organs travel around the body and the circulation moves in front of them, i.e. if you did get 4 eyes and two of them moved to the back and sides, the nerves for your face expressions that are next to your ear would have to go in front, around, and through the moving eye pathways. enter image description here enter image description here

To simplify, we stopped evolving our symmetrical archetypes when we were related to starfish, ammonites and ediacaran fauna, and the fastest most versatile animals all had 2 sided symmetries, with entirely symmetrical nervous systems, and forwards moving at that point in time, using binocular compound vision, which did have 180, and that binocular compound vision afterwards became our 2 eyes. We share the kingdom Animalia with starfish, our common ancestor experimented with shape during the Ediacaran era.

To understand the difficulty of adding new eyes later, you should learn why nearly all vertebrates have 5 fingers, except some frogs which have 4. it

google: why do animals all have two eyes?

  • $\begingroup$ The mysterious downvoter is back I guess. $\endgroup$
    – Roni Saiba
    Commented Oct 1, 2017 at 12:08
  • $\begingroup$ I should post more to compensate then. $\endgroup$ Commented Oct 1, 2017 at 12:17
  • $\begingroup$ 2 eyes and 5 fingers has been the optimal numbers for all vertebrates this is fantastically wrong (might want to have a look at Why don't mammals have more than 4 limbs?). we stopped evolving our symmetrical radiation is wrong. when we were starfish sounds like you mean that the common starfish is our ancestor which is wrong. You seem to equate symmetry with binocular vision which is also wrong. Your only two links are google search! $\endgroup$
    – Remi.b
    Commented Oct 1, 2017 at 16:19
  • $\begingroup$ We share the kingdom Animalia with starfish, our common ancestors experimented with symmetries during the Ediacaran era resulting in worms, corals, urchins, bivalves. Terrestrial vertebrates have 5 fingers.... you send me to a page called "mammals have 4 limbs" I send you to 4 pages called "why nearly all vertebrates have 5 fingers" .... bi-focal vision is a common theme in planar symmetry or in radial? PLANAR symmetry. $\endgroup$ Commented Oct 2, 2017 at 4:34
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    $\begingroup$ I downvoted too. This is the very worst kind of answer for StackExchange; it's superficially plausible and written with great confidence and authority, so a naive reader will probably think that the author is deeply knowledgeable; yet the answer is simply bluster, mistaken stereotypes, and sweeping claims based on ignorance. If you don't actually understand the field, you shouldn't answer questions. The fact that you think you understand this is simply Dunning-Kruger in action. $\endgroup$
    – iayork
    Commented Oct 2, 2017 at 11:59

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