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You can get a $100 60-day credit on a new Linode account at linode.com/scishow. [♪ INTRO] Picture fish transitioning to life on land to eventually become four-footed vertebrates. Ask yourself what adaptation they need to do that.

Are you thinking of legs? I’m thinking of legs. But land critters aren’t just special as walkers, they’re also special as blinkers.

Because, yeah, news flash: fish don’t blink. Unlike all major groups of tetrapods, the four-legged vertebrates, which do blink So scientists have set out to determine why blinking evolved, and whether it was directly related to our move to land. And they did this by studying adorable amphibious fish called mudskippers.

The benefits of blinking range from protecting yourself from an incoming poke to keeping things moist. And you’d think it would be helpful to blink underwater too, since moisture isn’t a problem there, but getting poked definitely is. But fish live without ever blinking.

Even the closest relatives of tetrapods don’t blink. And that’s a hint that moving to land was likely a major factor. After all, virtually all land vertebrates do it, from salamanders to turtles to us.

We don’t all blink the same way, though. We humans blink by lowering our upper eyelids, while ducks blink by raising their lower lids. Plus we don’t even all have the same number of eyelids!

Now, we also, unfortunately, don’t have a ton of blinking-related fossil evidence, so it’s tough to draw conclusions about when and how this widespread adaptation evolved. To figure out the likely origins of blinking, researchers stared into the eyes of mudskippers, a little fish that weirdly spends a lot of time on land. These charming amphibious fishes blink, but not in the way we do.

Mudskippers basically bloop their eyes down into their head, while a membrane simultaneously sweeps upward. The authors of the study, published in April 2023 in the journal PNAS, proposed that blinking in mudskippers might do one of three main things: moisten the eyes, clean away debris, or protect them against injury. Protection makes a lot of sense, because look at these guys!

Their beady little eyes stick way up to peek out of the water, much like early tetrapods. Several other groups of fishes can retract their eyes too, so this blinking-adjacent adaptation could point to its evolution as protection from injury. Yet we don’t see eye retraction in gobies, the closest relatives of mudskippers.

And even mudskippers forgo blinking while underwater, unless they straight up bump their noggin. So while it does seem to help protect their eyes, we have to conclude that protection isn’t the number one reason for blinking. In the end, this study showed that mudskipper blinking served all the same functions as in tetrapods.

Not only do they blink to avoid potential injury, a single blink clears their eyes of essentially all fine debris. Also they blink to keep their eyes moist, likely spreading mucus secretions from their head over their eyes since they don’t have tear glands. And if conditions get too dry, they adorably roll around in the water to spread it over their eyes.

Not only does it appear the evolution of mudskipper blinking is directly tied to life on land, but one function didn’t appear more important than the others when it came to blinking benefits. And all signs point to our ancient relatives evolving to blink as part of their quest to become terrestrial, much like it did in mudskippers, even though they evolved separately. This kind of convergent evolution makes it seem like you have to blink to adapt to life on land, at least the way vertebrate eyes work.

Might not be as flashy as legs, but this tetrapod is very glad to have eyelids. And I’m very sorry if your eyes are all itchy now. This SciShow video is supported by

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