Mammals Were Nocturnal Until Dinosaur Extinction, Then Emerged Into Daylight

Wednesday, November 15, 2017

With enormous predators like Tyrannosaurus rex skulking around in the daytime it is not surprising that the first mammals chose to live under the cover of darkness.

In fact, a new study, from University College London has found that our ancestors did not emerge from the shadows until after the dinosaurs became extinct, around 66 million years ago.

Before then, all mammals were nocturnal, sleeping in the daytime and hunting or foraging at night, new data suggests.

Researchers used computer algorithms to analyse details from 2415 species of living mammals to reconstruct the activity patterns of their ancestors.

The ancestors of gorillas were the first mammals to become diurnal, which is why their eyesight is so good

They found that following the comet strike which killed off the dinosaurs, mammals shifted to an intermediate stage of mixed day and night living, before primarily venturing into the daylight.

“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 analyses,” said lead author, doctoral student student Roi Maor of UCL.

The team found that the ancestors of gorillas and gibbons were the first to give up their nocturnal activity, a discovery which fits in with the fact that their descendants – which include humans – are the only mammals that see well in daylight.

Their vision and color perception is comparable to those of diurnal reptiles and birds – groups which never left the daytime.

“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,” added co-author Professor Kate Jones.

“However, we see a clear correlation in our findings.”

Ancestral reconstruction is the extrapolation back in time from measured characteristics of individuals, or species, to their common ancestors.

For example, if  a mammal had long fingers and its sibling also has long fingers  it is likely that a parent had long fingers.

The research was published in the journal Nature Ecology and Evolution.