In the middle of the Devonian period, about 385 million years ago, our early ancestors made those crucial first steps out of the water and onto dry land. But while the evolution of fins into limbs currently gets most of the credit for that transition, a new hypothesis out of Northwestern University suggests that evolving better eyes was the first step towards driving early tetrapods out of ponds in search of food.

All four-limbed vertebrates, including mammals, reptiles, birds and amphibians, stem from a group called tetrapods, which themselves evolved from early fish before plodding out onto land. But as big a step as that was for us, we weren't the first life forms to do so: invertebrates, such as insects, crustaceans and arachnids, beat us there by about 50 million years.

Northwestern University scientists have formulated a new theory they call the "buena vista" (good view) hypothesis, that suggests that as our ancestors' eyes evolved, they were better able to see that untouched smorgasbord of food sources, which provided the evolutionary motivation to kickstart the formation of limbs.

"Why did we come up onto land 385 million years ago?" asks Malcolm MacIver, co-lead author on the study. "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."

The researchers studied 59 fossils dating from the period before, during and after tetrapods transitioned from water to land, focusing specifically on the size of the animals' eye sockets ("orbits") and the length of their heads. About 12 million years before the transition, the average eye socket was 13 mm (0.5 in), but by the time the animals finally crawled out of the water, that figure had almost tripled, to 36 mm (1.4 in).

But what do enlarged eyes have to do with leaving a watery habitat? According to the researchers, bigger eyes would have no evolutionary advantage underwater, with only minor boosts to their visual range. Running computer simulations of different visual environments, including clear and murky water in daylight, and out of water both by day and by night, the team found that larger eyes could see 70 times further through air than water. By the time the eyes had tripled in size, the space animals could visually monitor through air had increased by a million times.

"Bigger eyes are almost worthless in water because vision is largely limited to what's directly in front of the animal," says Lars Schmitz, co-lead author of the study. "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."

Not only did the eyes grow threefold, but over the same time they migrated from the side of the head to the top like a crocodile's, perfect for peeking above the water line. That aided natural selection to preference limbed animals, who now had access to far more food, but the advantages may also have given rise to our more developed brains.

With their short visual range, fish have adapted to be quick to react to sudden stimuli. Since better eyesight generally gave land-dwelling tetrapods more warning of incoming predators or dangers, they instead developed more complex cognition. That eventually led to the ability to plan for the future and act strategically – a skill that many terrestrial animals demonstrate now, but which modern fish still lack.

The research was published in the journal Proceedings of the National Academy of Sciences and team describes the study in the video below.