Why your eyes can rotate: an evolutionary explanation

Reader Bryan sent me a link to this video by Steve Mould discussing how our ability to rotate our eyes may be an exaptation (and a vestigial remnant) from ancestors who had eyes on the sides of their head. It’s a fascinating idea, and may well be true. Or, to quote the YouTube notes:

Torsional eye movement, known as cycloversion and cyclovergence have a fascinating evolutionary history via our evolutionary ancestors that had eyes on the sides of their heads. Your eyes twist in this way whenever you tilt your head!

After 10:07 there are ads, so you may wish to skip the last two minutes of the video.


  1. ThyroidPlanet
    Posted September 26, 2020 at 2:34 pm | Permalink


  2. jezgrove
    Posted September 26, 2020 at 2:56 pm | Permalink

    Fascinating – the My Little Pony popping up as an example was unexpected!

  3. Charles A Sawicki
    Posted September 26, 2020 at 3:20 pm | Permalink

    Interesting post and theory!

  4. davelenny
    Posted September 26, 2020 at 3:22 pm | Permalink

    Wonderfully clear exposition. And interesting!

  5. rickflick
    Posted September 26, 2020 at 3:27 pm | Permalink

    I didn’t know that.
    Come for the kitties, stay for the science. 😃

  6. Posted September 26, 2020 at 5:18 pm | Permalink


  7. C.
    Posted September 26, 2020 at 5:28 pm | Permalink

    I almost sent this as well. More people on this side of the pond should know Mould, one of the three members of the science-comedy trio who perform the Festival of the Spoken Nerd, Helen Arney and Matt Parker being the other two. They’ve got some neat stuff to sell, too, over at the Festival website and Maths Gear site as well, perfect for nerds of all ages.

    I generally enjoy his videos and even learn a thing or two. I will admit however that this video gave me the beginnings of a headache.

  8. merilee
    Posted September 26, 2020 at 6:04 pm | Permalink


  9. BJ
    Posted September 26, 2020 at 8:53 pm | Permalink

    I didn’t even know human eyes could do that! Very interesting. The idea that this could go back as far as our reptilian ancestors seems a bit weak to me, but I don’t know enough about evolution to know whether a “vestigal remnant” is likely to stick around for that long. Exaptation make more sense to me intuitively.

    • BJ
      Posted September 26, 2020 at 9:05 pm | Permalink

      I see now that you mean it’s both vestigial and an exaptation. But, to be vestigial, doesn’t something have to be less useful or useless? Or, in evolution, does it just mean “something that remains from a species ancestors,” regardless of whether or not its still useful? If it’s the latter, it seems like too much of our body could be somehow classified as vestigial, which is confusing to me.

      • Torbjörn Larsson
        Posted September 27, 2020 at 3:48 pm | Permalink

        My perception of the video was that there were two behaviors, one that had a (normally) suppressed vestigial template in some situations. (When bending your head front- or backwards.)

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