The Real Reason Fish Grew Legs

Some 385 million years ago, our watery ancestors evolved into land mammals, their fins slowly evolving into limbs. But a new study out of Northwestern suggests that when it comes to fish evolution, it all comes down to the eyes. Fish could see far better above the water line, and were likely tempted by tasty prey on land, the Atlantic explains. That clear-eyed goal perhaps prompted fish to grow bulging peepers that migrated to the top of their heads. With that evolutionary task ticked off, the shoreline was theirs to conquer and limbs followed. At least that's the theory laid out by scientists in this week's journal Proceedings of the National Academy of Sciences. (See a video about it.) The team measured fossilized eye sockets from 59 species from fish to tetrapods with legs. Over 12 million years, the eyes nearly tripled in size.

This boosted the ability of crocodile-like early tetrapods to spot shore-based snacks like centipedes and millipedes without having to expend lots of energy moving around. "Long-range vision gives you a free lunch," lead researcher Malcolm MacIver tells the Atlantic. "No invertebrate on land would have been a match for it." Other changes followed, MacIver reasons, such as breathing holes behind the eyes to sample surface oxygen. Fins morphed into flippers and then limbs, permitting the creatures to make their first foray out of the water. That process could also explain brain development as the tetrapods began to navigate terra firma. "Now you’re choosing the best thought for your environment, which looks like a kind of proto-consciousness," MacIver says. (Your jaw may come from this ancient fish.)