Around 385 million years ago, fish started hauling themselves onto land. Over time, their flattened fins gradually transformed into sturdy legs, ending in feet and digits. Rather than paddling through water, they started striding over solid ground. Eventually, these pioneers gave rise to the tetrapods—the lineage of four-legged animals that includes reptiles, amphibians, and mammals like us. 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.
But Malcolm MacIver from Northwestern University was more interested in eyes.
The earliest tetrapods had much bigger eyes than their fishy forebears. MacIver always assumed that this enlargement happened after they marched onto land, allowing them to see further and to plan their paths. “That was an expectation fueled by ignorance,” he says. Actually, after studying the fossils of many fishapods—extinct species that were intermediate between fish and tetrapods—MacIver found that bigger eyes evolved before walking legs.
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 traits 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 that ability could have given 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. Perhaps, as MacIver puts it, “the gateway drug to terrestriality was being like a crocodile.”