Why can (most) eyeballs be controlled to go in the same direction as well as cross-eyed, but not in separate directions?
In: Biology
To facilitate depth perception. We achieve depth perception by having slightly different views in each eye and our brain “calculates” the difference between those 2 images to give us a sense of “depth” (how far away something is in relation to something else).
In terms of evolution, this was likely more useful for us than being able to move our eyes independently (and thus losing depth perception whenever they aren’t “synchronized”), or it was a good enough compromise that there wasn’t enough pressure to evolve something better.
Going cross-eyed still counts as going in the same direction (at least as far as maintaining depth perception goes), but optics being what they are, the more versatile an optical system has to be, the more complex it has to be. At some point you hit diminishing returns where increasing complexity doesn’t increase versatility that much; at that point it isn’t worth “investing” the resources needed for a more complex system.
Generally, evolution is a cost/benefit ecuation.
Other animals (usually prey animals) have far less depth perception since for them it was more valuable to have a wider field of view (by having eyes that don’t overlap, or not as much as our eyes).
Other animals do have independently moving eyes (like Chameleons) and benefit from both a wider field of view and depth perception when needed. Likely, for these animals, there was enough pressure to evolve this system, and therefore the benefit likely outweighed the cost.
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