Theory: Fish Grew Legs Because Their Eyes Got Big Enough To See Snacks On Land

By Holly Brockwell on at

For the first time, a study has persuasively argued that our water-based ancestors made the move from sea to land because their eyes grew and suddenly they could see all the tasty goodness on the shore.

A collaboration between Northwestern University; Claremont McKenna, Scripps and Pitzer colleges; the study took a careful look at the visual capabilities of aquatic creatures in the fossil record. It found that just before the shift to land, the fish's eyes tripled in size and moved from the sides to the tops of their heads. The physical movement of the eyes increased their visual range by a factor of 70, while the size change increased their view a millionfold.

Previously, it was assumed the croc-like creatures evolved their legs and then fortuitously used them to wander onto the land. Now, it seems they evolved the legs because they could see why they needed to get onto land. Basically, the same reason modern humans do anything: snacks.

Neuroscientist Malcolm A. MacIver of Northwestern comments:

"Why did we come up onto land 385 million years ago? We are the first to think that vision might have something to do with it.

We found a huge increase in visual capability in vertebrates just before the transition from water to land. 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."

Paleontologist Lars Schmitz of Claremont McKenna, Scripps and Pitzer colleges elaborates:

"Bigger eyes are almost worthless in water because vision is largely limited to what’s directly in front of the animal.

But larger eye size is very valuable when viewing through air. In evolution, it often comes down to a trade-off. 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."

This all occurred millions of years before fully land-based animals appeared, and the eye-tripling alone took twelve million years to complete. Those were some seriously patient fish.

However, not all the specimens studied liked their dry new neighbourhood. At least one animal group went back to the water and their eyes reverted to the old fish-style. Some people just prefer the old ways. [Northwestern]

Main image: Magdeleine