Evolution Doesn't Have Just One Direction
In everyday language, the words "evolution," "progress," and "improvement" are used interchangeably. But when it comes to biological evolution, things don't always work that way. It's just as possible for an organism to evolve new features and abilities as it is for an organism to lose them. That's what some people call "devolution," or "reverse" or "regressive" evolution. You probably shouldn't, though.
There's No Such Thing As "Devolution"
Let's make one thing clear: regressive evolution, or "devolution," is a misnomer. "From a biological perspective, there is no such thing as devolution," biologist Michael J. Dougherty told Scientific American. By definition, any change in the genes of a group of organisms is evolution—for better or for worse. (Nature, for its part, doesn't distinguish one from the other). The idea that lungs are better than gills or opposable thumbs are better than paws comes from centuries-old beliefs about humans being the pinnacle of evolution. In fact, we're just one result of millions of adaptations that help organisms survive their environment to bear offspring. If circumstances make it so an organism with flippers produces more children than one with feet, that organism's population thrives. Match point, evolution.
Nature Is Full Of So-Called Reverse Evolution
Perhaps the most obvious example of so-called "regressive evolution" is the penguin. It's clear to anyone, but especially to biologists, that if you go back far enough in the penguin's evolutionary tree, you'll find a bird that can fly. Fossil evidence shows that penguins lost the ability to fly more than 60 million years ago, and recent studies point to why: as their wings became more efficient tools when diving for prey, they became less efficient at getting them off the ground. Evolution always comes at a cost—you can be good at swimming, you can be good at flying, but rarely can you be both. Still, the inability to fly opened the door to other adaptations. They got bigger, which made them better at withstanding the cold, and their bones got denser, which kept them from immediately floating to the water's surface. Penguins didn't "devolve," they just got really good at what they do best.
There are plenty of other examples—snakes once had legs, birds once had teeth—but most surprising might be the fact that the whale's ancestors started in the sea, evolved legs and walked on land, then went back into the water and lost them again. At each step in the evolutionary chain, these unique adaptations helped the organisms eat more, live longer, and have more babies. Think about it this way: whales share an evolutionary ancestor with the hippo, which kept its legs. Watching the way each live in their respective environments, you'd never say the hippo is "more evolved" than the whale, would you? We hope not.
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