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.” “This study helps explain how fishes built for life in the water could be lured to shore without losing their competitive edge along the way,” says Lauren Sallan from the University of Pennsylvania, who was not involved in the work. 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.” So here’s what MacIver thinks may have happened. In shallow, freshwater rivers and lakes, some fish gradually moved their eyes to the top of their skulls, to better exploit the down-welling sunlight. This happened during the Devonian period, when the Earth’s oxygen levels were declining. Struggling to get enough oxygen in the water, creatures like Tiktaalik evolved breathing holes at the tops of their heads, just behind their eyes. They started surfacing more regularly, allowing them to tackle prey that wandered near the water. At first, they would only have seen the blurry outlines of their targets, but even small changes to their eyes would have produced enormous benefits. Over time, their eyes became bigger. And eventually, their limbs changed too, allowing them to make longer forays into this new world—a world of not only solid ground, but also large distances. ||||| EVANSTON - A provocative new Northwestern University and Claremont McKenna, Scripps and Pitzer colleges study suggests it was the power of the eyes and not the limbs that first led our ancient aquatic ancestors to make the momentous leap from water to land. Crocodile-like animals first saw easy meals on land and then evolved limbs that enabled them to get there, the researchers argue. 
 
 Neuroscientist and engineer Malcolm A. MacIver of Northwestern and evolutionary biologist and paleontologist Lars Schmitz of Claremont McKenna, Scripps and Pitzer colleges studied the fossil record and discovered that eyes nearly tripled in size before — not after — the water-to-land transition. The tripling coincided with a shift in location of the eyes from the side of the head to the top. The expanded visual range of seeing through air may have eventually led to larger brains in early terrestrial vertebrates and the ability to plan and not merely react, as fish do. 
 
 “Why did we come up onto land 385 million years ago? We are the first to think that vision might have something to do with it,” said MacIver, professor of biomedical engineering and of mechanical engineering in the McCormick School of Engineering. 
 
 “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,” MacIver said. (Invertebrates came onto land 50 million years before our vertebrate ancestors made that transition.) 
 
 The enlargement of eyes is significant. By just popping those eyes above the water line, the fish could see 70 times farther in air than in water. With the tripling of eye size, the animal’s visually monitored space increased a millionfold. This happened millions of years before fully terrestrial animals existed. 
 
 “Bigger eyes are almost worthless in water because vision is largely limited to what’s directly in front of the animal,” said Schmitz, assistant professor of biology at the W.M. Keck Science Department, a joint program 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. 
 
 Larger eyes were consequently selected for, whereas the study shows that in water, larger eyes led to negligible increases in visual range. In fact, one animal group that arose after animals came onto land went back to full-time life underwater, and their eyes went back to the smaller eye size normally seen in fish, MacIver and Schmitz found. 
 
 The study, “Massive Increase in Visual Range Preceded the Origin of Terrestrial Vertebrates,” was published today (March 7) by the journal Proceedings of the National Academy of Sciences (PNAS). 
 
 The massive increase in visual capability enabled by vision in air likely allowed early-limbed animals to evolve more complex cognition. These animals were no longer forced to react with split-second speed as was required by life in the vision-limiting water. Eventually, the researchers said, evolution led to the human capability of prospective cognition: the power to weigh options for the future and to choose strategically. 
 
 MacIver and Schmitz studied 59 fossil specimens spanning the time before the water-to-land transition, during the transition and after the transition. Their computer simulations of the animals’ visual environments (such as clear or murky water in the daytime or above water in the daytime and the nighttime) show that the benefit of increased eye size would be realized when an animal is seeing through air, not water. 
 
 The researchers measured the size of each fossil’s orbits, or eye sockets, and head length. From that, they determined the size of the eyes and the size of the animal itself. They found 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.” 
 
 Through an interdisciplinary approach, MacIver and Schmitz have been able to show that our aquatic ancestors followed an informational zip line provided by long-range vision to an abundance of food on land. Rather than limbs, it was eyes that brought our ancestors to land. 
 
 The research was partially funded by the National Science Foundation (IOS-ORG 1456830 and PECASE IOB-0846032 to Malcolm MacIver). 
 
 In addition to MacIver and Schmitz, other authors of the paper are Ugurcan Mugan and Todd D. Murphey of Northwestern University and Curtis D. Mobley of Sequoia Scientific, Inc.
– Some 385 million years ago, our watery ancestors evolved into land mammals, their fins slowly evolving into limbs. But a new study out of Northwestern suggests that when it comes to fish evolution, it all comes down to the eyes. Fish could see far better above the water line, and were likely tempted by tasty prey on land, the Atlantic explains. That clear-eyed goal perhaps prompted fish to grow bulging peepers that migrated to the top of their heads. With that evolutionary task ticked off, the shoreline was theirs to conquer and limbs followed. At least that's the theory laid out by scientists in this week's journal Proceedings of the National Academy of Sciences. (See a video about it.) The team measured fossilized eye sockets from 59 species from fish to tetrapods with legs. Over 12 million years, the eyes nearly tripled in size. This boosted the ability of crocodile-like early tetrapods to spot shore-based snacks like centipedes and millipedes without having to expend lots of energy moving around. "Long-range vision gives you a free lunch," lead researcher Malcolm MacIver tells the Atlantic. "No invertebrate on land would have been a match for it." Other changes followed, MacIver reasons, such as breathing holes behind the eyes to sample surface oxygen. Fins morphed into flippers and then limbs, permitting the creatures to make their first foray out of the water. That process could also explain brain development as the tetrapods began to navigate terra firma. "Now you’re choosing the best thought for your environment, which looks like a kind of proto-consciousness," MacIver says. (Your jaw may come from this ancient fish.)