While scientists have long been believed that the evolution of fins into limbs is the main reason organisms were first able to come out of the water and up onto land, a new study from researchers at Northwestern University suggests that better eyes may have been just as important.

All four-limbed vertebrates come from a group known as tetrapods. Tetrapods evolved from early fish that slowly came up out of the sea and onto land. However, they were not the first animals to make this transition. A wide range of invertebrates including arachnids, crustaceans, and insects accomplished this about 50 million years before our ancestors.

To explain this, the researchers have come up with the buenva vista theory, which states our early ancestors crawled onto land only after they evolved eyes that allowed them to see the numerous food sources existing out of the water.

Why did we come up onto land 385 million years ago? asked lead author Malcolm MacIver, professor of biomedical engineering at Northwestern University, in a statement. 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 team came to this conclusion by looking at both the eye sockets and head length in 59 fossils dating back to the periods before, during, and after tetrapods evolved. They found that the average eye socket measured roughly 0.5 inches across before the shift and 1.4 inches after,New Atlasreports.

This is an important distinction because bigger eyes would have had no evolutionary advantage underwater. As a result, there must have been another reason the animals evolved that feature. Researchers tested this by running a number of simulations that showed larger eyes could see almost 70 times further through the air than they could in water.

In addition, eyes also moved up on the skull over time, placing them in an area where they see over the surface. This would have pushed natural selection in a way so the limbed animals that could access more food were favored.

Bigger eyes are almost worthless in water because vision is largely limited to whats directly in front of the animal, said study co-author Lars Schmitz, assistant professor of biology at the W.M. Keck Science Department. 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? Whats the point? Here we think the point was to be able to search out prey on land.

The team also found evidence that the transition onto land led to more developed brains. This is because, while fish have to react quickly as a result of their short visual range, better eyesight may have given land-dwelling tetrapods more ways to detect predators. Without having to spend as much time worrying about being hunted, they could have allocated more energy towards developing complex cognition.

The findingswerepublished in the Proceedings of the National Academy of Sciences.

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Optical evolution may have helped fish transition onto land - Science Recorder