The first mammals emerged during the reign of the dinosaurs, adopting a nocturnal lifestyle to stay safe. It was only until the dinosaurs were wiped off the face of the planet that certain mammals began to assert themselves during the daylight hours, according to new research.
New evidence published in Nature Ecology & Evolution suggests that the first diurnal (daytime) mammals emerged at some point between 52 to 33 million years ago. The timing would suggest that mammals had been “stuck” in a nocturnal mode of existence during the reign of the dinosaurs, which ended some 66 million years ago, and that the sudden disappearance of dinos allowed mammals to assert themselves during the day. The new study improves our understanding of mammalian evolution, but other researchers say more evidence is required for us to be absolutely certain about the timing of this important transition.
Mammalian fossils dating back to this period are hard to come by, but even if paleontologists and evolutionary biologists had tons of fossils, it would still be hard to discern an animal’s behavior from a pile of bones—including whether or not an animal was active during the day or night. That said, scientists are reasonably confident, based on the scant fossil evidence they do have, that the first mammals were nocturnal. They’re just not sure when our distant ancestors first started to emerge from the darkness.
To get a better sense of when this happened, a team of researchers from the University of College-London and Tel Aviv University constructed multiple family trees (phylogenies) based on the 2,415 species of mammals alive today.
Both of the phylogenic timelines constructed by the researchers pointed to the same conclusion: mammals switched over into diurnal mode only after the dinosaurs were gone. The change was gradual, however, requiring millions of years of evolution; the “big switch” happened at some point between 52 to 33 million years ago (that’s a big window, even for evolutionary biology).
“We were very surprised to find such close correlation between the disappearance of dinosaurs and the beginning of daytime activity in mammals, but we found the same result unanimously using...alternative analyses,” explained lead author Roi Maor in a statement.
Of significance, ancestors of simian primates, such as gorillas, gibbons, and tamarins, were among the first to reject the night life. The new analysis is consistent with the fact that simian primates are the only mammals to have evolved adaptations to seeing well in daylight.
That said, Polly said the paper is based on a phylogeny that’s controversial in terms of which different subgroups of mammals arose.
David Grossnickle, an evolutionary biologist from the University of Chicago, also not involved in the new study, agrees with Polly, saying a study based solely on phylogeny has its limitations.
Biologist Stephen Brusatte from the University of Edinburgh said the team did a “great job” in mapping out the patterns of modern mammals on a family tree, and by running a variety of statistical tests to estimate when different types of activity patterns, like daytime behavior, evolved.