Our Mammalian Ancestors Were Nocturnal Until the Dinosaurs Blew Up

Sixty-six million years ago, an asteroid slammed into Earth and triggered a chain of chaotic events that decimated most dinosaurs. The explosive first impact roasted any creature that was close enough to see it, and flooding, toxic air, and dying plants led to a nuclear winter and the death of the dinosaurs. This was a game-changer for our mammalian ancestors, who existed alongside dinosaurs for about 150 million years before the asteroid’s blow, in more ways than one.

A study published Monday in Nature Ecology & Evolution reveals that the surprisingly diverse population of mammals that persisted after the disappearance of non-avian dinosaurs went through a major change that scientists previously didn’t realize: They transitioned from nocturnal animals to creatures that lived in light.

“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 several alternative analysis,” lead author Roi Maor, a Ph.D. student at Tel Aviv University’s School of Zoology, explained in a statement.

The theory that the common ancestor of all mammals was nocturnal has been around for a while — the tentative explanation is that they couldn’t compete with dinosaurs during the day — but this study is the first to try to put a date on when the transition began.

The analysis, run by researchers from the University College London and Tel Aviv University’s Steinhardt Museum of Natural History, examined data on 2,415 species of mammals alive today, using computer algorithms to reconstruct the activity patterns of the ancestors of these species. It produced two different mammalian family trees, both of which revealed that the ancestors of these mammals switched from a nocturnal existence to a daytime life after dinosaurs disappeared. The process happened in stages over the course of a million years.

“It’s very difficult to relate behavior 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,” study co-author Tamar Dayan, Ph.D., noted in a statement. “However, we see a clear correlation in our findings.”

It’s not exactly clear when on the evolutionary timeline this event took place, but the scientists determined that it was the ancestor of simian primate animals — like gorillas, gibbons, and tamarins — that first made the switch from nocturnal activity. Today, simians have visual acuity and color perception comparable to that of day-dwelling lizards and birds, whose ancestors always lived during the daytime niche.

Nocturnal animals alive today are primarily active in the night because of environmental factors, such as excessive heat preventing hunting during the daytime or the fact that there’s less competition for food at night. Whether or not ancient mammals actually kept to the night during the time of dinosaurs for the same reason remains, for now, just speculation — but it’s not hard to imagine that scurrying around looking for a bite probably wasn’t easy when massive predators were out, too. That is, unless you were a Repenomamus giganticus, a ferocious, dog-sized mammal who said “fuck it” and fed on dinosaurs anyway.

If you liked this article, check out this video about chickens with dinosaur noses.