Optical Evolution, Not Limbs, Could Have Helped Fish Evolve Into Land Creatures

Rodiano Bonacci
Marzo 9, 2017

Risky crocodile-like creatures used the power of sight to make easy meals of insects on land, and later evolved legs to walk. The expanded visual range of seeing through air may have eventually led to larger brains in early terrestrial vertebrates and the ability to plan and not merely react, as fish do.

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. Their computer simulations of the animals' visual environments (such as clear or murky water in the daytime or above water in the daytime and the nighttime) show that the benefit of increased eye size would be realized when an animal is seeing through air, not water.

Eyesight, rather than legs, prompted large animal life to venture onto the land for the first time some 385 million years ago, according to a new study.

The researchers who constructed the theory studied 59 fossil specimens spanning the time before, during and after the water-to-land transition.

The researchers measured the size of each fossil's orbits, or eye sockets, and head length.

Professor Malcolm MacIver looked a fossil records along with evolutionary biologist Professor Lars Schmitz and found that animal's eye size almost tripled before they left the oceans.

We found a huge increase in visual capability in vertebrates just before the transition from water to land.

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", MacIver said.

'But larger eye size is very valuable when viewing through air. Strikingly, before the transition the eye sockets were only 13 millimeters in size but around the time of transition or 13 million years later the average length nearly tripled to 36 millimeters. "What's the point? Here we think the point was to be able to search out prey on land", he said.

MacIver's team believes that fish evolved into fishapods and later into different animal types during the Devonian period as the animals adjusted to the decreasing oxygen levels of the era by developing breathing holes atop their heads and behind their eyes. In evolution, it often comes down to a trade-off.

Before the water-to-land transition, the average orbit size (eye socket) was 13 millimeters and around the time of transition the average size was 36 millimeters.

'Is it worth the metabolic toll to enlarge your eyes? Limbs then developed after the eyes significantly increased in size, and these let them cover greater distances on the ground. Bigger eyes enabled them to see 70 times farther in the air than underwater, increasing their visually monitored space up to a million times. Freed of the water's murkiness, early tetrapod ancestors could now concentrate on improving their cognition and transition from reactive to active - the strategic skill that to this day many terrestrial animals rely on for their survival but which much modern fish still lack.

'The tripling of orbit size took 12 million years, ' Dr MacIver said.

'This is the timescale of evolution, which boggles our mind, ' he said.

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