Optical Evolution, Not Limbs, Could Have Helped Fish Evolve Into Land Creatures

It was about 385 million years ago when fish first evolved into land animals. This took place as their fins were slowly but surely transformed into limbs, with the limbs coming with feet that allowed them to easily feast on meals on land. However, a new study suggests that it wasn’t the limbs that allowed prehistoric fish to transform into the first land creatures, but rather their eyes.

According to EurekAlert, Northwestern University neuroscientist and engineer Malcolm MacIver and Claremont McKenna, Scripps, and Pitzer evolutionary biologist and paleontologist Lars Schmitz co-led a study which hypothesizes early crocodile ancestors first saw their meals on land before growing limbs to walk to that prey. The researchers analyzed fossil records, which was where they saw a rather unique twist to existing theories on fish evolution – eye size had tripled before, and not after these early fish transformed into land animals. This also came at a time when their eyes moved from the side to the top of the head, which gave them a wider range of vision, and ultimately larger brains. The study also suggests that this evolution process helped animals make plans and not simply react to stimuli like fish normally would.

“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,” said MacIver in a statement. He added that his team’s research is the first to suggest vision was what drove fish evolution and helped them transform into early land animals. These creatures then evolved into tetrapods, who, in turn, evolved into amphibians, reptiles, and mammals.

Previous studies had singled out early tetrapods having substantially larger eyes than their fish ancestors as one of the main takeaways. A report from The Atlantic quoted MacIver, who said that it was “ignorance” fueling the above expectation. But when he was able to study several fossils belonging to so-called “fishapods,” or prehistoric animals who emerged in between fish and tetrapods, he realized he and many others were wrong, as he found that the bigger eyes came before the ability to walk with legs.

With the animals’ eyes also moving to the top of their heads and gaze out of the water like today’s crocodiles, the researchers also believe that this longer line of sight and improved ease in catching land-based prey could have been what pushed them to completely leave the water and further evolve.

“The gateway drug to terrestriality was being like a crocodile.”

Possibly the earliest known example of a “fishapod” is Tiktaalik roseae, which, according to the University of Chicago’s homepage, was first found in the Canadian Arctic in 2004, and thought to have existed about 375 million years ago, 12 million years before tetrapods first arrived. While it has a lot of similarities to early fish, it shows signs of evolution into a completely different kind of animal, with some features closer to those of prehistoric amphibians.

Commenting on this fishapod, MacIver said that Tiktaalik might have likely behaved like a crocodile as it ventured onto land and sought out prey such as centipedes and millipedes.

“I imagine guys like Tiktaalik lurking there like a crocodile, waiting for a giant millipede to walk by, and chomping on it. No invertebrate on land would have been a match for it.”

MacIver’s team believes that fish evolved into fishapods and later into different animal types during the Devonian period as the animals adjusted to the decreasing oxygen levels of the era by developing breathing holes atop their heads and behind their eyes. As they adapted to their environment, they made it to the surface more regularly, and while their sight may have been blurry at first, the increases in eye size allowed them to see more clearly over time. Limbs then developed after the eyes significantly increased in size, and these let them cover greater distances on the ground.

University of Chicago researcher Neil Shubin discovered Tiktaalik over a decade ago, and while he wasn’t involved in the new fish evolution study, he commented to The Atlantic that MacIver’s team “took a very creative and new approach to (solving) an old problem.”

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