Fish started hauling themselves onto land around 385 million years ago. As the time passed, their flattened fins gradually transformed into sturdy legs, ending in digits and feet. Instead of paddling through water, they started striding over a solid ground. Eventually, these pioneers gave rise to the tetrapods, the lineage of four-legged animals that includes mammals, reptiles and amphibians. This transition from water to land is an evocative one, and for obvious reasons, people tend to focus on the legs. They are the organs that changed most obviously, that gave the tetrapods their name, and that carried them into their evolutionary future.
The earliest tetrapods had much bigger eyes than their fishy forebears. MacIver always assumed that this enlargement happened after they were walking onto land, allowing them to see further and to plan their paths. As the eyes swelled in size, they also moved to the tops of their owners’ heads, allowing them to peer out of the water surface like crocodiles do today. These attributes enabled them to see further than their aquatic ancestors and to look over a much greater range of space, allowing them to snatch up prey from the shoreline. And this ability gave them the impetus to leave the water entirely, driving the evolution of their vaunted legs. Perhaps eyes, not legs, led the invasion of the land.
Although eyes don’t fossilize, you can estimate how big they would have been by measuring the eye sockets of a fossilized skull. MacIver and his colleagues and a fossil eye expert Lars Schmitz did this for the skulls of 59 species from finned fish to fishapods and from fishapods to legged tetrapods. They showed that over 12 million years, the group’s eyes nearly tripled in size.
Eyes are expensive organs. It takes a lot of energy to maintain them, and even more energy if they’re big. If a fish is paying those costs, the eyes must provide some kind of benefit. It seems spontaneous 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 were living 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 under the water, and scan an area that’s 5 million times bigger.
Those early hunters would have seen plenty of appetizing prey. Millipedes and Centipedes had colonized the land millions of years before and had never encountered fishapod predators. Maclver said, “I imagine guys like Tiktaalik lurking there like a crocodile, waiting for a huge millipede to walk by, and chomping on it.” He further added, “No invertebrate on land would have been a match for it.”