How Did Fish Evolve To Walk On Land? They Used Their Eyes First, Says Study


Exciting, new evidence on how animals first came out of the water challenges everything we knew about the terrestrial vertebrates's evolution. A compelling study by the joint efforts of prestigious universities suggests that vision — and not limbs — was in fact paramount in the leap from water to land.

The collective research offers a new approach in understanding this evolutionary threshold, providing key information on what drove early aquatic vertebrates to conquer land. According to the study, it was evolved eyesight that prompted this momentous change, and came first both chronologically and in importance.

Neuroscientist and engineer Malcolm A. MacIver, from the Northwestern University, partnered up with evolutionary biologist and paleontologist Lars Schmitz, from the W.M. Keck Science Department — in a joint program of Claremont McKenna, Scripps and Pitzer colleges — to analyze fossils and their evolutionary changes.

They discovered substantial adaptations in visual range and were the first to ascertain this was how transition from water to land was made possible.

Their findings, featured in the journal Proceedings of the National Academy of Sciences on March 7, explain that the ability to see better outside the water and spot an abundance of food on land later enabled our ancient aquatic ancestors to evolve limbs and gain access to a new world of bounty.

"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," says MacIver.

Eyesight Evolutionary Changes And How They Led Us Here Today

A thorough examination of 59 fossil specimens revealed important changes in the cranial structure before our ancient aquatic ancestors first emerged on land. Fossil record from three important evolutionary periods — before, during and after the transition — showed eyes almost tripled in size prior to the leap from water to land.

Measurements of each fossil's orbits, eye sockets, and head length indicated an average orbit size of 13 millimeters (or about 0.5 inches) before the fish emerged on land, which changed to 36 millimeters (or about 1.4 inches) during the transition.

Apart from growing in size, eyes also changed their location by moving from the side of the cranium to the top of the head. This allowed early vertebrates to expand their visual range from water-adapted eyesight to seeing through air.

Bigger Eyes Specifically Tailored For Life On Land

With eyes now placed in the center of the head, fish could peek above the water line and enjoy a larger field of vision than when remaining submerged. Bigger eyes enabled them to see 70 times farther in the air than underwater, increasing their visually monitored space up to a million times.

Schmitz points out that enlarged orbits were particularly developed for animals that ventured on land. This evolutionary adaptation is inconsequential for aquatic vertebrates, whose eyesight can't penetrate too far in the distance.

The scientists discovered that an animal group which appeared after the transition actually took to the aquatic lifestyle again, and were surprised to find out the creatures's eyes regressed in size, adopting the proportions we normally see in fish.

A larger eye size, however, is substantial for seeing through air. Researchers believe this important adaptation — which occurred 12 millions of years before the appearance of fully terrestrial animals — may have led in turn to larger brains, capable of planning strategies instead of just reacting to visual stimuli.

Study authors argue this is where the human ability of prospective cognition (the skill to weigh down options and make calculated choices) eventually originated.

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