​When dinosaurs went extinct, many animals literally came out of the dark

The demise of dinosaurs was good news for mammals, whose numbers exploded in the aftermath. Now, a new study suggests that the behavior of mammals changed rapidly as well, as the first of our furry ancestors began venturing out in the daylight after living a primarily nocturnal existence. The switch may have even sparked the eventual evolution of our own humankind.

Paleontologists and evolutionary biologists have long thought that the original mammals were nocturnal, in part because mammals alive today still carry traits of their night-loving ancestors. Most mammals have eyes that function well in low light, for example. They also have highly developed senses of smell and hearing, and sensitive whiskers that allow them to feel what is in front of their faces—all traits that are useful in the dark.

Exactly when mammals started to venture out during the day has been a mystery, because behavior is hard to discern from fossils. Scientists rely on the shape of eye sockets and nasal cavities to infer which senses were important to an extinct animal, but those clues can be misleading.

Evolutionary biologists Roi Maor of Tel Aviv University in Israel, Kate Jones of University College London, and their colleagues decided to approach the question from a different direction. They noted the day or night preferences of more than 2415 species of living mammals and then used genetic data to draw their family trees, noting when the earliest day-active ancestors might have emerged.