Ocular Evolution Initiated Limb Growth For Fish To Transition to Land Animals

Life on Earth went through, perhaps, its most important metamorphosis about 385 million years ago. This is when fish first evolved to grow legs and started to move onto the land. Obviously, it was the growth of feet that would let them stand on land and eat to survive (and move around) that allowed for this evolution, but scientists are now saying that the growth of feet was not the beginning of the change.

As a matter of fact, a study—encompassing research from Northwestern University and Claremont McKenna, Scripps and Pitzer colleges—suggests that it was not the growth of limbs but the power of sight that led the first fish out of the water. Essentially, early ancestors of the crocodile began to see easy food on land. And it was that improved eyesight that encouraged aquatic animals to begin making the trek inland.

Neuroscientist and engineer Malcolm A. MacIver, with Northwestern University teame up with evolutionary biologist and paleontologist Lars Schmitz of Claremont McKenna, Scripps and Pitzer colleges to study the fossil record to make this discovery.

And what did they discover? The pair says that the eyes of these creatures tripled in size before—and not after—they made the water-to-land transition; at least, that is what the fossil record suggests. In addition, the research team says that this tripling in size coincides with a shift in the location of the eyes to the top of the head from the side of the head (like fish). This migration of the ocular sense organs expanded visual range through air and might have also contributed to the larger brains that that earlier terrestrial vertebrates grew to have; a survival necessity on land.

It helps, too, that after the simple movement of the eyes to the top of the head, which took maybe 12 million years, these animals (like crocodile ancestors) could see 70 times farther onto land than animals whose eyes remained in the water (on the side of the head).
McIver inquires: “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.”

The McCormick Shool of Engineering professor of biomedical engineering and of mechanical engineering continues, “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.”

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