Seeing Food May Have Lured Fish Onto Land

This side view model shows a fossil fish known as the Tiktaalik in murky waters 385 million years ago. It's looking out above the water line through eyes set on top of its skull. Malcolm A. MacIver, 2017

Imagine peering down the length of a football field and seeing something on the ground. Let's say it's about the size of a saucer, but you can't make out much other than its shape. While you might be curious, you're probably not motivated to run 100 yards (91.4 meters) to get it, right?

Now imagine you can see it perfectly — that little object is actually a delicious chocolate chip cookie. And you are starving. Suddenly, that dash seems like a decent goal. After all, you're only human.

Or a vertebrate, as the case may be, because some new research suggests that a substantial increase in visual range — and not the development of limbs — led animals out of the water during the process of evolution. Scientists from Northwestern and the Claremont Colleges started with the hypothesis that the eyes of vertebrates increased in size (and thus acuteness) after they made their way on to land. But what they discover after their research was quite different.

Before animals made the transition to land about 385 million years ago, their eyes tripled in size, it appears — and also moved from the side of the head to the top of the head. By having their eyes above water, suddenly animals could see all the delicious things crawling on the land — things like millipedes, centipedes and spiders. (Remember, invertebrates made their way out of the water 50 million years before vertebrates.)

The fact that these vertebrates' eyes got bigger is, well, huge. Because their eyes tripled in size, the area they could see grew a millionfold. But it wasn't just size that made the difference. The ability to see out of water made the most impact; it allowed them to see 70 times farther than they could below water.

The study researchers are the first to think that this new ability to see the plethora of food on land might've hurried some of the selectivity for longer limbs that made land-dwelling possible. Of course it didn't happen overnight. "The tripling of [eye] orbit size took 12 million years," study author Malcolm A. MacIver, a neuroscientist and engineer at Northwestern, says in a statement. "This is the timescale of evolution, which boggles our mind."