A study suggests that it was the sighting of an easy meal on land that prompted our ancient aquatic ancestors during prehistoric times to evolve limbs to get themselves out of the water.
After studying fossil records, a team of scientists from Northwestern University and Claremont McKenna, Scripps and Pitzer colleges discovered that the eyes had nearly tripled in size before (not after) the water-to-land transition. This prompted them to propose that it was the sight of an easy meal on land that prompted the evolution from aquatic to land-based animals.
"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," said neuroscientist and engineer Malcolm A MacIver, from Northwestern University.
This enlargement of the eyes also coincided with a shifting of the eyes from the side to the top of the head, facilitating the animals to pop their eyes above the water line. This would have allowed them to see 70 times farther through air, compared to water.
“Bigger eyes are almost worthless in water because vision is largely limited to what's directly in front of the animal,” said evolutionary biologist and paleontologist Lars Schmitz of Claremont McKenna, Scripps and Pitzer colleges.
"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," he said.
Both researchers studied 59 fossil specimens spanning the time before the water-to-land transition, during the transition and after the transition. They measured the size of each fossil's orbits and head length, then determined the size of the eyes and the animal itself. Their computer simulations showed that the benefit of larger eyes would be realised when the animal is seeing through air, not water.
Based on their findings, they discovered that before the water-to-land transition, the average orbit size was 13 millimeters; and around the time of transition, the average size was 36 millimeters.
"The tripling of orbit size took 12 million years," MacIver said. "This is the timescale of evolution, which boggles our mind."