It has been speculated that mammals were hiding in the dark when dinosaurs ruled the Earth and emerged only when these gigantic creatures were wiped out. Now, a new study supports that premise by pointing a precise age of when such change occurred.
Many mammals nowadays are what we call “diurnal” or active during the day, but there are some evolutionary hiccups that show signs of previous nocturnality. For example, cats have a reflective layer to help them see things in the dark; this is what causes a reflection when you shine a flashlight to their face.
So, to see what changed along the mammal family tree, Roi Maor at Tel Aviv University in Israel and his colleagues gathered data of behavior of 2415 living mammals, aligned them with the family tree and rebuilt the probable behavioural patterns of both the existing and extinct animals. That is, if two mammals are closely related and are nocturnal, chances are their ancestor is nocturnal too.
After rigorous mixing and matching, they found that it was only around 65.8 million years ago was when daytime activeness was observed--that's just a few thousand years after the extinction event that wiped the dinosaurs out!
Between that transition, some mammals had Cathemeral activity or had activity both during the day and night. Using “the largest dataset on extant species yet published”, the team found that the first mammal Cathemeral species were of the hoofed kind: the ancestor of cows, hippos, deer, as well as sea creatures dolphins and whales.
But the first mammal species that have become fully diurnal were the simians, the ancestors of apes and monkeys. This happened around 52.4 million years ago. And with that diurnal activity, they became the only mammals that have a sense of sight that can adapt to foraging in daytime, like seeing ripe fruit from unripe. And according to Maor, this change in lifestyle may have planted the seeds of social behavior that eventually lead to civilization. “I think it’s very difficult to be social when you’re nocturnal, because it’s hard to communicate between the parts of the group,” Maor said.
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