To see you better: why did life come out of the water to land?

Life on Earth began in water. Therefore, when the first animals came to land, they had to exchange their fins for limbs and gills for lungs in order to better adapt to the new terrestrial environment. Recently, a new study appeared, which showed that the transition to the lungs and limbs does not reveal the full picture of the transformation of these creatures. When they left the sea, they received something, perhaps even more valuable than oxygenated air: information. In the air, eyes can see much farther than under water. According to Malcolm McIver, a neurologist and engineer at Northwestern University, the increased visibility range provided the animals with additional information about rich food sources near the coast.

Makiver had an interesting hypothesis, but he was lookingevidence. Therefore, he teamed up with Schmitz, who had experience in interpreting the eye cells of four-legged fossilized tetrapods (one of which was tiktaalik), and two scientists thought about how best to test Makiver’s idea.

McIver and Schmitz first carefully analyzedfossil record to identify changes in the size of the eye sockets, which indicate corresponding changes in the eyes, as they are proportional to the size of the sockets. Scientists collected 59 skulls of the first four-legged, which took place during the transition period between water and land, which were intact enough to measure both the eye orbit and the length of the skull. Then they downloaded this data into a computer model in order to extrapolate the change in the size of the eye socket over many generations and get an idea of ​​the evolutionary genetic drift of this feature.

It turned out that a noticeable increase in eye sizereally observed - three times, in fact - during the transition period. The average size of the eye socket before the transition was 13 millimeters, and after - 36 millimeters. In addition, for those creatures that came out of the water to land and returned to water - like the Mexican cave fish Astyanax maxicanus - the average size of the eye orbit decreased to 14 millimeters, that is, it almost returned to its previous state.

These results had one problem. McIver originally suggested that the increase occurred after the animals became completely land, since the evolutionary benefits of long-distance vision on land would increase the size of the orbit. But the shift occurred before the transition from water to land was completed, even before the creatures developed rudimentary limbs on their fish appendages. How could being on land push a gradual increase in the size of the orbit of the eye?

When Makiver and Schmitz analyzed data onthe size of the eyes in the fossil record, they noticed that the orbits changed position during the transition period, shifted from the lateral parts of the skull to the top, where they were fixed in the bony protrusions. They also noticed tiny incisions near the ear region - the spiracles - which helped the four-legged breathe air. In short, these creatures began to resemble crocodiles. Suddenly, everything fell into place.

“I did not expect these creatures to useaerial vision while still floating, ”says McIver. "I assumed that aerial vision is equal to being on land." This is not true. Rather, transitional tetrapods were supposed to hunt like crocodiles hiding in shallow water on the shore, when only their eyes peered above the surface in search of tasty prey.

In this case, “it seems like hunting, likea crocodile was a gateway to land, ”says McIver. "Just as the information precedes the action, the appearance on land was probably due to the huge gain in vision due to the eyes sticking out above the water, which can see an untouched source of prey, and limbs were found after that."

This view is consistent with the work of Jennifer Clack,a paleontologist from the University of Cambridge, which is famous for the fossil Pederpes finneyae, is the oldest known “foot designed for walking on land,” which was not completely land. Although the early tetrapods were predominantly aquatic, and later the tetrapods became clearly land, paleontologists believe that this creature spent time both in water and on land.

By determining how large your eyes are,Makiver decided to calculate how far the animals could see with big eyes. He adapted the existing ecological model, which takes into account not only the anatomy of the eyes, but also other factors, such as the environment. In water, a larger eye only increases the visual range from just over six meters to almost seven meters. But increase the size of the eyes in the air - and the improvement will grow from 200 to 600 meters.

Makiver and Schmitz did the same thingmodeling in a variety of conditions: daylight, moonless night, starlight, clear water and even muddy water. “It doesn't matter,” says McIver. - In all cases, the increase in the air was huge. Even if they hunted in broad daylight in the water and went out only on moonless nights, it was still beneficial for them, from the point of view. ”

Using quantitative tools toexplaining patterns in the fossil record presents a completely new approach to the problem. More and more paleontologists and evolutionary biologists like Schmitz are using these methods.

“Paleontology is largely a studyfossils, followed by descriptions of how these fossils might fit into a particular environment, ”says John Long, a paleobiologist at Flinders University in Australia, who studies how fish evolved into tetrapods. “This article contains very good experimental data to test vision in a variety of environments. And these data correspond to those patterns that we see on the example of these fish. "

Schmitz highlighted two key events inquantitative approach that have occurred over the past ten years. First, more and more scientists are adapting the methods of modern comparative biology to the analysis of fossil records, studying the relationship of animals with each other. Secondly, there is great interest in modeling the biomechanics of ancient creatures in a way that can actually be checked - to determine how fast dinosaurs could run, for example. Such a model approach to the interpretation of fossils can be applied not only to biomechanics, but also to sensory function - in this case, explain how the exit from the water affected the vision of the first four-legged animals.

“Both approaches bring something unique, thereforemust go hand in hand, ”says Schmitz. “If I were to analyze the orbit of the eye itself, I would not understand what it really means. The eyes are bigger, but why? ” Sensory modeling can answer these questions qualitatively, not quantitatively.

Schmitz plans to explore other transitions fromwater on land in the fossil record - not just the first four-legged ones - to look for corresponding increases in eye size. “If you look at other transitions between water and land, between land and again water, you will see similar pictures that could potentially confirm this hypothesis,” he says. For example, marine reptile fossils that rely heavily on vision can also show an increase in the orbit of the eye as they land.

Think in a new way

The experience of McGiver-neurologist inevitably forced himto think about how all this could affect the behavior and cognitive abilities of the four-legged when moving from water to land. For example, if you live and hunt in water, your limited range of vision — about one body length in front — means that you live in a “reactive mode”: you only have a few milliseconds to react. It all comes down to one scheme. You will either eat or be eaten, and it is better to make a decision quickly.

But for a land animal the ability to seefurther means that he has much more time to assess the situation and develop a better strategy for action, whether it is a predator or a victim. According to McIver, probably the first land animals began to hunt land production reactively, but over time, those that could go beyond the jet mode and learn to think strategically got an evolutionary advantage. “Now you have to evaluate several future outcomes and quickly choose between them,” says the scientist. “This is a mental journey through time, which has become an important part of our own cognitive abilities.”

Other feelings probably also playeda certain role in the development of a more developed consciousness. “It's extremely exciting, but I don’t think that the ability to plan suddenly appeared only through vision,” says Barbara Finlay, an evolutionary neurophysicist at Cornell University. As an example, she points out how salmon rely on their sense of smell during their upstream migration.

Hutchinson agrees that would be helpfulto consider how many sensory changes during this critical transition period are combined with each other, and not to study vision alone. For example, “we know that smell and taste were first associated in the aquatic environment and then separated,” he says. “But hearing has changed dramatically during the transition from an aqueous to a dry environment, along with the development of a proper outer ear and other features.”

This work has implications for future evolution.human knowledge. Perhaps one day we will be able to make another evolutionary leap, overcoming what Makiver jokingly calls "the paleoneurology of human stupidity." People can understand the consequences of short-term threats, but long-term planning - for example, mitigating the effects of climate change - we practically do not digest. “Perhaps a number of our limitations in strategic thinking go back to how different conditions affect planning,” he says. "We cannot think on a geological time scale."

His work can help us identify our own blind spots.