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If you've ever spent too much time in the sun and forgotten to put on sunscreen, you know how painful a sunburn can be. But for some animals, forgetting the sunscreen wouldn't be a problem because they can just produce their own!

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There’s nothing more annoying than coming back after a day at the beach or a sunny vacation and noticing that there’s a perfect outline of your sunglasses on your face. You’ve got yourself a nice tomato-red sunburn, which is never pleasant.

Not to mention the whole skin cancer thing. But some organisms don’t need to worry about sunscreen at all, because they just make their own. And a few years ago, scientists discovered that almost all fish, birds, amphibians, and reptiles at least have the genes for it.

For those of you keeping track at home, that’s every major group of vertebrates besides us mammals. And how we lost those genes is almost as strange to think about as the way we acquired them in the first place. In the 1960s, biologists discovered that some marine algae, bacteria, and corals could protect themselves from the ultraviolet rays in sunlight by making compounds called mycosporine-like amino acids, or MAAs.

Unlike many of the compounds in the sunscreen we use, which work by scattering or reflecting UV light away, these MAAs absorb UV light. Two decades later biologists found a similar compound, called gadusol, in fish and their eggs. Now, up until that point, scientists thought only microbes could make their own sunscreens and that fish got theirs by eating those microbes.

But then, in 2015, researchers found that zebrafish could make gadusol and figured out which genes were responsible for making it. Once they’d identified those genes, they searched for them in the genomes of other animals. That’s when they discovered that almost all birds, reptiles, amphibians, and other fish have them, too although it’s still unclear whether these animals actually express the genes and make their own UV-blocking compounds.

Since so many groups of vertebrates have these genes in common, they were probably passed down from some of the earliest back-boned creatures. And researchers think those animals got the ability to make gadusol around 500 million years ago, from algae. Instead of the genes being passed from parent to offspring, the algae would have passed their genes to animals through a process called horizontal gene transfer, which can happen between totally separate types of living things.

We don’t know exactly how it happened in this case, but the basic idea is that at some point, algae got into the cells of an animal, which then incorporated some of the algae’s DNA into its own genome. That DNA included the genes for making gadusol, which were then passed down to the animal’s offspring, and continued to be passed down for another few hundred million years in almost every vertebrate except us mammals. Over the course of our evolutionary history, we seem to have lost the genes.

And that’s probably because we weren’t always sun-loving creatures. We’re used to mammals dominating the earth, and it’s been that way since not long after the extinction that wiped out the non-avian dinosaurs 65 million years ago. There’s a reason paleontologists call this era “the Age of Mammals.” But around 250 million years ago, mammals actually spent most of their time in the dark, which helped them avoid the massive dinosaurs roaming around during the day.

Spending so much time in the dark is how mammals gained many of their distinctive features, like fur and a heightened sense of touch. These kept them warm and allowed them to hunt for food without too much light. But the darkness also meant mammals lost some features that weren’t so necessary for survival like being able to make gadusol.

A random mutation probably sprung up in the DNA of a few of these very early mammals, changing their genome so it no longer coded for making gadusol. If that had been a big deal for their survival, most individuals with that mutation wouldn’t breed and pass the mutation on. But since you don’t need to make sunscreen if there’s not much sun to shield yourself from, the mutation hung around.

Over time, more mutations built up, and eventually the gene cluster was lost. It’s the same deal with the West African coelacanth, one of the few non-mammalian vertebrates that doesn’t have the gadusol gene. It’s a fish that lives in a dark cave and feeds at night.

Of course, these days plenty of mammals have evolved their own ways to protect themselves from the sun. Elephants roll around in the mud. Hippos secrete their famous blood-colored sweat.

And most humans have at least some of the pigment melanin, which, like gadusol, absorbs UV light. Our ancestors evolved the ability to make melanin when they lost their dark fur, around 2.8 million years ago. It only absorbs about 3/4 of UV light at most, though, and people with lighter skin don’t have very much of it in the first place.

So it’s still important to slather on sunscreen if you’ll be spending time in harsh sunlight, at least for now. Researchers are looking at ways to make gadusol in the lab, and they’ve already had some success with genetically engineering yeast to produce it. The advantage is that unlike the compounds usually found in sunscreens, which can wash off our bodies and harm corals, gadusol is environmentally friendly because it’s already in the marine environment.

It could even come in pill form someday, which means your days of dealing with sticky lotion or accidentally spraying yourself in the eyes could be numbered. We may not be able to get these fish-sunscreen genes back, but we can at least find ways to use them to our advantage. Has all this talk of sunlight made you realize it’s been way too long since you had time to relax?

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