What Goes Bump in the Day?

Mammals Were Nocturnal Until the Dinosaurs Disappeared, Scientists Show

The ancestor of primates was among the first to see the light, either 33 or 55 million years ago, but most mammals have remained denizens of the dark to this day. We just don't see them, explain the scientists

Leinkupal laticauda using its tail to fend off predators : Illustration
Jorge Antonio Gonzalez/ via Reuters

Life formed in the primordial slime billions of years ago, since when its different forms have been hunting and eating each other. Proto-mammals that began to evolve hundreds of millions of years ago defended their proto-furry little bodies from predators very simply: they were nocturnal. Early mammals only began venturing out into the light when the dinosaurs died out 66 million years ago (leaving the theropods that evolved into birds out of it), argues a new paper published in Nature Ecology & Evolution.

Scientists at University College London and Tel Aviv University's Steinhardt Museum of Natural History believe they have identified the turning point from night-life to daytime, based not on fossils but on computer algorithms that reconstructed the activity of the ancestors of 2,415 species of mammals living today.

One background datum is that a great many mammal species have sensory adaptations to nocturnal activity. This is thought to have resulted from a "prolonged nocturnal phase or 'bottleneck' during early mammalian evolution," the scientists write. The transition was probably gradual, over millions of years, but the marks of that night-crawling lifestyle have remained in our bones and behavior alike.

In fact, about 60% of mammal species living today are still nocturnal, says the lead author Roi Maor.

They are? "Yes," he says, pointing out that the diurnal Man is blind as a bat at night and probably just can't see them. He then helpfully lists these denizens of the dark, which apparently include not only bats but most rodents and a great many of the marsupials. And the monotremes, which split off from the mammalian tree earliest of all.

One exception to the nocturnal rule is monkeys and apes, of whom there are just under 380 known species, which are diurnal except for South American owl monkeys, says Maor.

Why use computers to recreate lost behaviours rather than check the fossil record? It's not just because fossils of early mammals are incredibly scarce.

Illustration of an early mammal with a baby in its mouth, venturing out into the daylight - where dinosaurs lurk. The artist envisioned an early turtle also looking warily a the dinosaurs. This explains why early mammals developed a nocturnal lifestyle.
Mark Witton

"Just looking at skulls and eye sockets, all mammals except for monkeys and apes seem to be nocturnally adapted," Maor says. "So morphology alone can't give us a true picture of an animal's behavior. There is a lack of correlation between the behaviour and the fossil record."

Prof. Tamar Dayan points out that post-dinosaur adaptations that enabled mammals to live in light were in our soft tissues, which almost never survive fossilization.

The first mammal

We can assume that all existing mammals today arose from a single "first mammal," but there's no consensus about what that was. If we think "what is a Jew" is a tough question, it pales next to "who was the first mammal."

"That would require defining what exactly we call a mammal, what were the characteristics necessary to be mammalian," says Maor. This creature may have lived any time from 220 million years ago to 160 million years. Most palaeontologists agree that it was about the size of a rat but many think it looked more like a shrew. Either way, it was probably adorable, nervous as a proto-cat, and nocturnal.

Actually, there is no consensus about the mammalian family tree. There are three main theses for these. For the purposes of their research, lacking tools to determine which tree they would climb, the team looked at the main two postulated mammalian trees.

Reconstruction of a Megazostrodon species, in the Natural History Museum, London. This, an early mammal, looks like a shrew with a long snout, and a ratty-type tail.
Theklan

(They ignored the newish family-tree theory that almost all the mammal species died out with the dinosaurs, and that all mammalian diversification today originated with that one proto-rat. "It's an Interesting theory but its supporters never prepared a family tree that could be worked with," Roee explains, adding that genetic and molecular analyses argue strongly against the theory.)

Their conclusion was that diurnality developed in the Cenozoic era (in English, after the dinosaurs), though they qualify that cathemerality (mixed day-night periodicity) may have appeared in the late Cretaceous. "We were very surprised to find such close correlation between the disappearance of dinosaurs and the beginning of daytime activity in mammals, Maor elaborates.

Among the first to see the light were early primates, they say, though based on one family tree hypothesis, this happened 33 million years ago, and based on the other, it happened 55 million years ago. "That range is an artefact of using the different family trees," Maor explains. "We can say, our results show to a high degree of reasonability that the ancestor of the simians was entirely diurnal. But we can't be sure when this ancestor lived – one tree says 55 million years ago, one tree says 33 million."

In any case, simians are the only mammals that adapted to see well in daylight. Their vision and color perception is like that of reptiles and birds, which never left the daytime niche, the scientists say.

How does the algorithm work? "We take the data of existing mammals – day/night or both – and for each couple of nearest species, sibling species, we can estimate what their ancestor was and when it was active. Now we go back one more stage – to its cousin – and see when it was active... All species ultimately contributed to estimate of when each ancestor species was active, until reaching the ultimate ancestor of all mammals." Whoever that was. 

Japanese macaques, like almost all monkeys and apes, are diurnal. Picture shows two macaques with gray fur, orange eyes and pink faces, looking straight at the camera.
Roi Maor