To survive in the rough and tumble world of the Mesozoic Era, mammals had to quickly learn that dinosaurs ruled the sun-filled hours. So these furry creatures adapted to a nocturnal existence, developing traits like whiskers and acute hearing. But the key adaptations took place in their eyes, reports Sarah Kaplan at The Washington Post.
A new study, published in the journal Developmental Cell, shows how the light-sensing rods crowded out the color-sensing cones, allowing mammals to operate by the dim light of the moon and stars. This change was a critical step in the dominance of the mammals, co-author of the paper Ted Allison tells Helen Briggs at the BBC.
Since the 1940s, most paleontologists have subscribed to the "Nocturnal Bottleneck Theory," the idea that in order to avoid dinosaurs, which also evolved during the Mezozoic Era, mammals adapted to life at night. That theory has gained even more support in the last few years. But this latest study shows how the evolutionary adaptation likely happened.
“We're learning how mammals evolved their vision to survive at night-time and avoid dinosaurs. That's what allowed mammals to diversify and become abundant in the world,” he says. “They did that by switching their daytime vision in the cones to allow night-time vision using their rods.”
Most animal species—including fish, frogs and birds—have eyes dominated by color cones. But the researchers wondered how those eyes transitioned to the mammal's rod-dominated version, which opens up sight in the dark hours of the night, the study's lead author Anand Swaroop says in a press release.
In previous studies, Swaroop found that rods and cones all come from one type of precursor cell. Left alone, the cell will develop into a cone. But if a type of protein called NRL is present, it suppresses certain genes, which forces the precursor cells to develop into rods.
Swaroop and his team looked at embryonic mice to understand exactly how these rods develop in mammalian eyes. At two days old, the rodents had developed cones that could detect short-wave lengths, which allow the mice to see ultraviolet light. But by ten days, rods dominated their retinas.
“Early mammals changed one type of cell from capturing UV light—which isn't necessary at night—to something that is just extremely sensitive to light,” Swaroop says in the release.
When the team looked at developing zebrafish eyes, however, they found that their rods developed differently, and did not start out as UV-detecting cones. Swaroop tells Kaplan that this likely means rods could have evolved twice during evolutionary history, once early on and a second time in mammals.
Even so, the development of night vision could have its downsides, Kaplan reports. Fish and other animals with cone-dominated eyes can regenerate these photoreceptors if damaged—mammals cannot. “One can imagine that maybe we could change some of the rods to cones and find some ways to have that regenerative potential in our retina again,” Swaroop tells Kaplan.
Swaroop has already successfully used NRL to help mice that are going blind keep some of their cone cells. But a similar therapy for humans is still a bit out of sight.