Many invertebrate species like insects and spiders were already there. But recent scholarship indicates vertebrates' transition to land occurred about 400 million years ago, with the first land-adapted, four-footed creatures appearing about 330 million years ago.
The findings suggest these bigger-eyed fish hunted much like today's crocodiles - watching from the shallows for prey before coming out of the water to hunt.
"As the eyes got bigger, they saw further and further and had more and more selective advantage toward getting the limbs that eventually let them live full-time on land," McIver said.
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The study was published this week in Proceedings of the National Academy of Sciences. McIver said the research bolsters what he called the "Buena Vista hypothesis" of evolution, which posits that better vision helped drive aquatic vertebrates onto land before limbs evolved.
Bigger eyes weren't much of a help underwater, said Schmitz, assistant professor of biology at California's Claremont McKenna, Scripps, and Pitzer colleges. But above the surface, they expanded the visual range of those species a millionfold.
That gave those fish a massive amount of data that likely boosted their brains as well. And with better vision, they could see potential prey - or threats - further off, giving them more time to respond.
"In evolution, it often comes down to a trade-off. Is it worth the metabolic toll to enlarge your eyes?" Schmitz said. "What's the point? Here we think the point was to be able to search out prey on land."