Basking in the sun can be a great way to spend an afternoon while replenishing your vitamin D. However, too much time in the sun can lead to a nasty sunburn and an increased risk of skin cancer. Why are we so vulnerable to sunlight when it’s all around us? A new study seeks to explain this evolutionary conundrum, and it might all be thanks to the dinosaurs.
The sun emits electromagnetic radiation across the spectrum, but the visible portion is the only part we can see. There’s also ultraviolet rays, and this part of the spectrum has enough energy to damage your DNA. When you slather on sunscreen, that’s to block UV from damaging your cells.
Humans need sunscreen because we’re part of a wide-ranging group known as placental mammals. We also don’t have fur to protect us from sunlight like many other mammals. Placental mammals lack a genetic feature called photoreactivation DNA repair function. Organisms that do have such an ability can activate DNA repair mechanisms in response to sunlight. That’s how plants and most animals can spend all day in the sun without frying their DNA.
According to the new study, there’s one animal other than placental mammals that lacks this mechanism: the Somalian blind cavefish, Phreatichthys andruzzii. As its name implies, the cavefish lives in underwater caves where there is no light, so it doesn’t need eyes. After millions of years of evolution, it also lost the photoreactivation DNA repair function. By analyzing the fish’s DNA, scientists found the genes that should control those DNA repair abilities, but they are no longer functional because of mutations. The team theorizes that Phreatichthys andruzzii is in the early phases of the same process that affected our mammal ancestors millions of years before.
The study suggests placental mammals have experienced relaxed selection in our DNA repair mechanisms. When an organism’s conditions change, traits that once were very important can be lost over time as mutations in them no longer confer an evolutionary disadvantage. This can leave an organism with vestigial features like a whale’s pelvis or the human tailbone.
It comes back to dinosaurs when you look at the history of our mammalian ancestors. They evolved at a time when dinosaurs were sauntering around all day, looking for small morsels to eat. This may have caused a “nocturnal bottleneck” as mammals survived for generations by hiding underground during the day and only coming out at night. So, they lost the photoreactivation repair mechanism over time because it wasn’t useful.
More teams will need to go over this work and look for clues in mammal genetics before we can be certain. Still, if it’s the dinosaurs’ fault, we can at least take solace in the fact they’re all dead, and we aren’t. Take that, T-Rex.