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A long-standing suspicion seems to have been confirmed: mammals like us spent their first hundred million years in the dark, and only came out in daytime when the dinosaurs disappeared. It is the first time we have had a firm date for this change. The first mammals to truly embrace the daytime were simians: our ancestors.

The first mammals evolved over 100 million years ago, but most remained small while dinosaurs ruled. Many palaeontologists think early mammals were “nocturnal”, only coming out at night.

Nowadays many mammals are active in the day – “diurnal” – yet most have eyes and ears adapted to darkness. For instance, most mammals have a thin reflective layer at the back of the eye that helps them see in the dark, and which causes the “eyeshine” of cats caught in car headlights. This is thought to be a hangover from nocturnal ancestors. However, this idea is hard to test, because eyes don’t fossilise.

at Tel Aviv University in Israel and his colleagues compiled data on the activity habits of 2415 living mammals. They then aligned this with the mammal family tree and reconstructed the likely activity patterns of modern species’ extinct ancestors. For instance, if two closely-related mammals are nocturnal, their common ancestor probably was too.

Step into the light

Moar found daytime activity only appeared 65.8 million years ago – within a few hundred thousand years of the mass extinction 66 million years ago that killed all the dinosaurs, barring birds.

That supports the “nocturnal bottleneck hypothesis”: the dinosaur extinction opened up new opportunities for mammals, particularly daytime foraging.

Because the team used “the largest dataset on extant species yet published”, they could date when many mammal groups became diurnal, says at Midwestern University in Glendale, Arizona.

Cathemeral activity – both at night and in the day – started to appear first. Nobody knows why, but there is evidence that long before their final doom – so mammals may have emerged into the day gradually.

The first cathemeral species was a hoofed mammal, which was the ancestor of cows, deer and hippos, plus whales and dolphins. It probably ate plants, and daytime grazing may have helped it digest food by warming up its stomach bacteria.

Emerging from darkness

However, cathemerality seems to have been unstable, says co-author at University College London, UK. “You see high rates of transition out of that state, but not into it,” he says. “Perhaps… they were just starting to exploit the daytime niche.”

The first mammal group to become exclusively diurnal was the simian primates, the group that includes monkeys and apes, about 52.4 million years ago. This may be why they are the only mammals with a visual system adapted to daytime foraging. In particular, they can distinguish red and green, which may help them spot ripe and unripe fruit.

Only one simian group returned to the dark: the of South America, which have large eyes to cope.

Simians’ sunny lifestyle may have facilitated the emergence of social behaviour like ours, says Moar. “I think it’s very difficult to be social when you’re nocturnal, because it’s hard to communicate between the parts of the group.”