Life on Earth began in the water. So when the first animals moved onto land, they had to trade their fins for limbs, and their gills for lungs, the better to adapt to their new terrestrial environment.

A new study, out today, suggests that the shift to lungs and limbs doesn’t tell the full story of these creatures’ transformation. As they emerged from the sea, they gained something perhaps more precious than oxygenated air: information. In air, eyes can see much farther than they can under water. The increased visual range provided an “informational zip line” that alerted the ancient animals to bountiful food sources near the shore, according to Malcolm MacIver, a neuroscientist and engineer at Northwestern University.

This zip line, MacIver maintains, drove the selection of rudimentary limbs, which allowed animals to make their first brief forays onto land. Furthermore, it may have had significant implications for the emergence of more advanced cognition and complex planning. “It’s hard to look past limbs and think that maybe information, which doesn’t fossilize well, is really what brought us onto land,” MacIver said.

MacIver and Lars Schmitz, a paleontologist at the Claremont Colleges, have created mathematical models that explore how the increase in information available to air-dwelling creatures would have manifested itself, over the eons, in an increase in eye size. They describe the experimental evidence they have amassed to support what they call the “buena vista” hypothesis in the Proceedings of the National Academy of Sciences.

MacIver’s work is already earning praise from experts in the field for its innovative and thorough approach. While paleontologists have long speculated about eye size in fossils and what that can tell us about an animal’s vision, “this takes it a step further,” said John Hutchinson of the Royal Veterinary College in the U.K. “It isn’t just telling stories based on qualitative observations; it’s testing assumptions and tracking big changes quantitatively over macro-evolutionary time.”

Underwater Hunters

MacIver first came up with his hypothesis in 2007 while studying the black ghost knifefish of South America — an electric fish that hunts at night by generating electrical currents in the water to sense its environment. MacIver compares the effect to a kind of radar system. Being something of a polymath, with interests and experience in robotics and mathematics in addition to biology, neuroscience and paleontology, MacIver built a robotic version of the knifefish, complete with an electrosensory system, to study its exotic sensing abilities and its unusually agile movement.

When MacIver compared the volume of space in which the knifefish can potentially detect water fleas, one of its favorite prey, with that of a fish that relies on vision to hunt the same prey, he found they were roughly the same. This was surprising. Because the knifefish must generate electricity to perceive the world — something that requires a lot of energy — he expected it would have a smaller sensory volume for prey compared to that of a vision-centric fish. At first he thought he had made a simple calculation error. But he soon discovered that the critical factor accounting for the unexpectedly small visual sensory space was the amount that water absorbs and scatters light. In fresh shallow water, for example, the “attenuation length” that light can travel before it is scattered or absorbed ranges from 10 centimeters to two meters. In air, light can travel between 25 to 100 kilometers, depending on how much moisture is in the air.

Because of this, aquatic creatures rarely gain much evolutionary benefit from an increase in eye size, and they have much to lose. Eyes are costly in evolutionary terms because they require so much energy to maintain; photoreceptor cells and neurons in the visual areas of the brain need a lot of oxygen to function. Therefore, any increase in eye size had better yield significant benefits to justify that extra energy. MacIver likens increasing eye size in the water to switching on high beams in the fog in an attempt to see farther ahead.

But once you take eyes out of the water and into air, a larger eye size leads to a proportionate increase in how far you can see.