It was the power of the eyes — not the limbs — that first led our ancient aquatic ancestors to make the leap from water to land, according to a team of scientists led by Northwestern University Professor Malcolm MacIver and Lars Schmitz of Claremont McKenna, Scripps and Pitzer Colleges.
Ancient crocodile-like animals first saw easy meals on land and then evolved limbs that enabled them to get there. Image credit: Zina Deretsky, National Science Foundation.
Prof. MacIver, Dr. Schmitz and their colleagues studied the fossil record and discovered that eyes nearly tripled in size before — not after — the water-to-land transition.
The tripling coincided with a shift in location of the eyes from the side of the head to the top.
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.
“Why did we come up onto land 385 million years ago (Devonian period)? We are the first to think that vision might have something to do with it,” explained Prof. MacIver, who is the lead author of a paper describing the research, published this week in the Proceedings of the National Academy of Sciences.
“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. Invertebrates came onto land 50 million years before our vertebrate ancestors made that transition.”
The enlargement of eyes is significant. By just popping those eyes above the water line, the fish could see 70 times farther in air than in water.
With the tripling of eye size, the animal’s visually monitored space increased a millionfold. This happened millions of years before fully terrestrial animals existed.
“Bigger eyes are almost worthless in water because vision is largely limited to what’s directly in front of the animal,” Dr. Schmitz said.
“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? What’s the point? Here we think the point was to be able to search out prey on land.”
The team studied 59 fossil specimens spanning the time before the water-to-land transition, during the transition and after the transition.
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.
The researchers measured the size of each fossil’s orbits, or eye sockets, and head length. From that, they determined the size of the eyes and the size of the animal itself.
They found that before the water-to-land transition the average orbit size was 13 mm and around the time of transition the average size was 36 mm.
“The tripling of orbit size took 12 million years. This is the timescale of evolution, which boggles our mind,” Prof. MacIver said.
The massive increase in visual capability enabled by vision in air likely allowed early-limbed animals to evolve more complex cognition. These animals were no longer forced to react with split-second speed as was required by life in the vision-limiting water.
“Eventually, evolution led to the human capability of prospective cognition: the power to weigh options for the future and to choose strategically,” the researchers said.
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Malcolm A. MacIver et al. Massive increase in visual range preceded the origin of terrestrial vertebrates. PNAS, published online March 7, 2017; doi: 10.1073/pnas.1615563114
This article is based on a press-release from Northwestern University.
