Artist’s impression of the mouse-sized Xianshou Songae, a tree-dweller in Jurassic forests. (Illustration by Zhao Chuang)

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.

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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.

“The study uses the evolutionary relationships to determine when mammals started moving around in the daylight,” P. David Polly, an Adjunct Professor of Biology and Anthropology at Indiana University who wasn’t involved in the new study, told Gizmodo. “These scientists wanted to know whether this group, which includes us, evolved eyesight that functions well in broad daylight before or after the dinosaurs became extinct. At the heart of the question is whether the dinosaurs held back the diversification of mammals.”

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).

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“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.

“It’s very difficult to relate behaviour changes in mammals that lived so long ago to ecological conditions at the time, so we can’t say that the dinosaurs dying out caused mammals to start being active in the daytime,” said study co-author Kate Jones. “However, we see a clear correlation in our findings.”

A lot of what the study shows was already known, said Polly, namely that the ancestors of mammals were nocturnal and that the first groups to utilize twilight or full daylight probably arose after the extinction of the dinosaurs. “The nice thing about the study is that the authors addressed the question quantitatively using data from many living mammals in combination with an evolutionary tree, or phylogeny.”

That said, Polly said the paper is based on a phylogeny that’s controversial in terms of which different subgroups of mammals arose.

“Both trees used by the authors put many of the splits in the Mesozoic period, before dinosaurs became extinct, whereas many scientists, especially paleontologists, would put those splits after the extinction based on the [fossil record],” Polly told Gizmodo. “For example, [the researchers] assume that the group of primates that includes monkeys, apes, and tarsiers originated before the extinction, which is highly unlikely given the quality of the fossil record of primates. If correct, the dates [given by the researchers] for the first daytime mammals would be pushed younger if their own analysis was adjusted.”

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Despite this, Polly says their estimate of 52 to 33 million years is not unreasonable, and that “the diversification in sight specializations may have occurred over a more compressed interval of time than their analysis suggests.”

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.

“A majority of the mammalian species from the Mesozoic (or Age of Dinosaurs) went extinct and have no living descendants,” Grossnickle told Gizmodo. “Basically, much of the mammalian evolutionary tree is not represented in studies that only use data from modern species, such [as this study]. With this kind of data it’s difficult to infer behaviors of mammals tens of millions of years ago, because most species living tens of millions of years ago simply don’t have living representatives... I think their study is a strong contribution to our understanding of early mammal evolution, but I recommend caution when inferring traits of ancient mammals from modern mammals alone.”

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Also, and as Grossnickle pointed out to Gizmodo, this new research contradicts another recent study arguing that diurnal behavior in mammals first arose about 10 million years after the extinction of dinosaurs. “[The] question of when diurnal behavior arose remains unsettled, although the recent research has made great strides toward addressing the question.”

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.

“But until we find a way to look at fossils and directly figure out how these extinct animals were behaving, it will still be a prediction,” he told Gizmodo. “It may be that the end-Cretaceous extinction caused a big shift from nocturnal to diurnal mammals, but it wouldn’t surprise me if some mammals living alongside the dinosaurs were also active during the day and we just haven’t found a good way to determine that yet. That will be the next big step in testing these results.”