Why do we blink both eyes at the same time rather than winking each eye as needed?
Why would winking independently be better?
The benefit would be a minor improvement whereby a person would receive continuous sight of the area of vision shared by both eyes. I'd also assume that it would be more likely for the wink-as-needed mechanism to evolve than the blink-as-needed mechanism were there no benefit to blinking over winking.
Why then has evolution favoured blinking?
My guess at why is that the overhead on our brain of switching between binocular vision to monocular vision is more significant than the overhead of momentarily pausing our visual processing.
I also believe that some animals don't blink, but wink; e.g. chameleons. This supports my theory as their eyes operate independently, so their brains would have to cope with the variance in each eye's information anyway. That said, their wink is different; i.e. they roll their eye inside their head, rather than having an eyelid.
Research so far
I found a couple of Reddit posts:
- https://www.reddit.com/r/explainlikeimfive/comments/2kzx4s/eli5_why_does_blinking_take_me_almost_no_effort/ - explains why blinking is easier than winking; but only based on how we are rather than how we got there (i.e. why evolution would have resulted in different muscles for winking vs blinking).
- https://www.reddit.com/r/explainlikeimfive/comments/3mijgw/eli5_why_do_we_blink_with_both_eyes_at_same_time/ - doesn't have any hard fact answers; just discussion of opinion.