Research Suggests That Eyes Beat Limbs In The Evolutionary Race To Land

Nearly 400 million years ago, the first tetrapods crawled out of the water to make a stand on dry land. But why? A 2017 study makes a startling claim: it may not have been for an abundance of food or oxygen, but visual information. In the air, you can see much further than you can in water, and that can mean everything for the survival of a species.

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Northwestern University researcher Malcolm MacIver is the author of the study, which was published in the Proceedings of the National Academy of Sciences in January of 2017. In addition to being a neuroscientist and an engineer, MacIver also studies biology, paleontology, and robotics, which made him uniquely qualified to stumble upon this theory. He built a robotic version of the black ghost knifefish, which senses its environment during its nighttime hunts by generating electrical currents in the water. He then figured out how far in the water the knifefish could use its electrical currents to detect prey, and compared that distance with how far a fish that relies on vision could detect the same prey. Because generating electricity takes a lot of energy, he figured that the knifefish wouldn't be able to detect prey as far as a fish with vision—but he was wrong. Both animals could detect prey about the same distance away, which puzzled him.

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He eventually figured out why: underwater vision is lousy. Water absorbs and scatters light, so even in fresh shallow water, light can only travel a maximum of two meters before it stops producing a clear image. Compare that to air, which allows light to travel as much as 100 kilometers before it scatters. That means that for water-dwelling animals, there isn't much of a benefit to evolving larger eyes. But once their descendants emerged onto dry land, their eyes should grow to account for the huge vision improvement air brings—after all, seeing further means detecting both prey and predators sooner.

As organisms evolved, their eyes got larger and moved to the top of the head.

I Can See Clearly Now

That evolution of larger eyes is exactly what MacIver and his colleague paleontologist Lars Schmitz found—the fossil record shows that eye size tripled over the transition from water to land. But there was one difference between their theory and the results: organisms started evolving larger eyes before the transition was complete. If they were still in the water, why were their eyes getting bigger? In the study, the researchers suggest it's a result of the animals "surfacing their eyes above the water line and hunting like crocodiles." Cue Jaws music.

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Speculation is fine, but it's nothing if you don't have evidence to back it up. That's why MacIver and Schmitz adapted a computer model that predicted how much further larger eyes could see in water and in air—if the model's prediction matched their theory, they knew they had a case. Sure enough, larger eyes only increase the visual range in water from just beyond six meters to just shy of seven meters, but they increase the visual range in air from 200 meters to 600 meters. This was true no matter how much daylight there is. "In all cases, the increment [in air] is huge," MacIver told Quanta Magazine. "Even if they were hunting in broad daylight in the water and only came out on moonless nights, it's still advantageous for them, vision-wise." The team's "buena vista" hypothesis, as they call it, gives us an entirely new storyline for how life evolved to walk on land.

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Written by Ashley Hamer May 4, 2017

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