The earliest tetrapods had much bigger eyes than their fishy forebears, and those bigger eyes evolved before walking legs:
Eyes don’t fossilize, but you can estimate how big they would have been by measuring the eye sockets of a fossilized skull. MacIver and his colleagues, including fossil eye expert Lars Schmitz, did this for the skulls of 59 species — from finned fish to intermediate fishapods to legged tetrapods. They showed that over 12 million years, the group’s eyes nearly tripled in size. Why?
Eyes are expensive organs: it takes a lot of energy to maintain them, and even more so if they’re big. If a fish is paying those costs, the eyes must provide some kind of benefit. It seems intuitive that bigger eyes let you see better or further, but MacIver’s team found otherwise. By simulating the kinds of shallow freshwater environments where their fossil species lived — day to night, clear to murky — they showed that bigger eyes make precious little difference underwater. But once those animals started peeking out above the waterline, everything changed. In the air, a bigger eye can see 10 times further than it could underwater, and scan an area that’s 5 million times bigger.
In the air, it’s also easier for a big eye to pay for itself. A predator with short-range vision has to constantly move about to search the zone immediately in front of its face. But bigger-eyes species could spot prey at a distance, and recoup the energy they would otherwise have spent on foraging. “Long-range vision gives you a free lunch,” says MacIver. “You can just look around, instead of moving to inspect somewhere else.”
Those early hunters would have seen plenty of appetizing prey. Centipedes and millipedes had colonized the land millions of years before, and had never encountered fishapod predators. “I imagine guys like Tiktaalik lurking there like a crocodile, waiting for a giant millipede to walk by, and chomping on it,” says MacIver. “No invertebrate on land would have been a match for it.”