8 avril 2017

La vision est-elle le moteur de l'invasion terrestre des poissons?


Pourquoi certains poissons ont-ils quitté le milieu aquatique pour s'aventurer sur la terre ferme et, ce faisant, devenir nos ancêtres?

Cette étude apporte une explication possible:

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.

(...) 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.

(...) "Bigger eyes are almost worthless in water because vision is largely limited to what's directly in front of the animal," 

(...) 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.

(...) "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 zipline provided by long-range vision to an abundance of food on land. Rather than limbs, it was eyes that brought our ancestors to land.




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