Using genetic analysis of modern species, researchers have confirmed what has long been assumed to be the case – the early mammals that evolved when dinosaurs were roaming the land became nocturnal early on, most likely to avoid the reptiles that were snacking on them.

“This method is like using the genome as a fossil record, and with it we’ve shown when genes involved in night vision appear,” says Liz Hadly, co-author of the study published in Scientific Reports, in a statement. “It’s a very powerful way of corroborating a story that has been, up to now, only hypothesized.”

Both mammals and modern reptiles (encompassing snakes and lizards) share a common ancestor, before going their own separate ways around 300 million years ago. During the preceding period, dinosaurs rose to become the dominant beasts, with the early mammals scurrying around their feet trying to make a living, while at the same time trying to avoid being gobbled up.

By looking at the skulls of early mammals, it was suggested that these small critters had an advanced sense of smell and hearing. This, coupled with the fact that most mammals today are nocturnal, implied that these early ancestors of ours were most likely active during the night, no doubt trying to avoid the reptiles that were their main predators during the day.

Yet there has been no hard evidence to prove this. To try and change that, researchers turned to the genetics of modern animals. They looked at the genes involved in night vision across a broad selection of reptiles and mammals alive today – including snakes, lizards, alligators, mice, platypuses, and humans – to try and determine when these genes first appeared and when they became refined.

They found that before the split of the two main groups, the earliest common ancestor of both mammals and reptiles did not have particularly good night vision, meaning it was likely active mainly during the day. But once the tree split, both groups quickly took divergent routes. Mammals started to enhance their night vision.

“Early mammals coexisted with early reptiles in the Age of the Dinosaurs and somehow escaped extinction,” explains co-author Yonghua Wu, who first identified the night vision genes in owls. “This research further supports the hypothesis that diurnal reptiles, such as lizards, snakes and their relatives, competed with mammals and may have led them to better adapt to dim light conditions.”

The researchers hope they can now use this technique to reveal other characteristics of our ancestors that are not preserved in the fossil record.