New Theory: Eyes, Not Legs, First Brought Fish Out of Water

What’s the best way to motivate a baby to crawl? Ask someone she adores to stand where she can see them, just out of her reach. Now imagine that the baby is a prehistoric fish, and her loved one is a teeming mass of mouthwatering bugs. That’s the controversial theory published this week in the Proceedings of the National Academy of Sciences: that the sight of the smorgasbord available on land inspired early vertebrates to grow legs and get out of the water.

Scientists have long believed the opposite. The theory is that fish shimmied themselves onto dry land, growing legs as they went, evolving big, land-critter eyes only when they needed them. To test this idea, researchers examined the fossil record of our distant fishy relatives, charting the growth and development of their eyeballs.

What they found surprised them. The ancient creatures’ above-water vision definitely improved as time went on—but the big eyeball boom happened before, not after, the animals made the transition to land 385 million years ago.

Co-author Malcolm A. MacIver is a neuroscientist and engineer at Northwestern University. He and his colleagues assert that the newfound visual acuity showed vertebrates a world ready for the taking. “Our hypothesis is that maybe it was seeing an unexploited cornucopia of food on land—millipedes, centipedes, spiders, and more—that drove evolution to come up with limbs from fins," he said in a statement.

Little eyes may be fine underwater, MacIver said, but bigger is often better for looking through the air. “In evolution, it often comes down to a trade-off,” he said. "Is it worth the metabolic toll to enlarge your eyes? What's the point? Here we think the point was to be able to search out prey on land.”