Mammals Emerged From the Shadows When Dinosaurs Went Extinct

When dinosaurs died out, many mammals evolved from being nocturnal to diurnal, and the primate ancestors of humans were among the first to do so.

Not all was doom and gloom after dinosaurs went extinct around 65 million years ago. Before that time, when Tyrannosaurus rex and other large predators lurked, mammals were mostly small and scrappy creatures of the night.

After non-avian dinosaurs died out, mammals began to come out of the shadows. New research published in the journal Nature Ecology & Evolution reports that mammals only started to become diurnal — active during the day — when dinosaurs were gone for good.

“The first mammals who became active in the daytime may have done so for a variety of reasons, such as reduced predation risk in the daytime or increased ecological opportunity in the absence of dinosaurs,” lead author Roi Maor of Tel Aviv University told Seeker.

Maor and his colleagues Tamar Dayan, Henry Ferguson-Gow, and Kate Jones had a daunting task: The fossil record reveals little about the activity patterns of animals. Dayan said researchers observe a living mammal to see if it is active at night or in the day. But, he added fossil evidence from mammals often suggests that they were nocturnal even if they were not.

“Many subsequent adaptations that allow us to live in daylight are in our soft tissues,” Dayan said.

To get around that problem, the researchers analyzed data from 2,415 species of mammals alive today, their relatedness to other existent mammal species, and their connections to known ancestors. They then used computer algorithms to reconstruct the likely activity patterns of the mammals that lived in the past.

Several theories exist that seek to explain the origins of mammals and their family trees. The researchers, therefore, constructed two different mammalian family trees portraying alternative timelines for the evolution of mammals.

The results show the same phenomenon: Mammals switched to daytime activity shortly after the dinosaurs disappeared.

The scientists found that the ancestors of simian or higher primates, such as monkeys, gorillas and humans, were among the first to give up nocturnal activity.

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The discovery helps to explain why these primates are the only mammals that evolved adaptations to seeing well in daylight. This is largely because all other mammals lack a fovea, which is a small depression in the retina of the eye where visual acuity is highest.

“No mammals, except monkeys and apes, have more than two types of cones — sensitive to short and long light wavelengths — in their retinae, and their vision has been compared to a red-green colorblind human,” Maor said.

According to the “nocturnal bottleneck hypothesis,” Maor said mammals co-existed alongside dinosaurs thanks to the segregation of the two groups’ ecological niches along a temporal day–night axis.

In short, the first mammals essentially evolved to avoid diurnal dinosaurs.

“It follows that when dinosaurs went extinct, they freed niche space to be taken by other species, including mammals, for which the daytime would have been a novel niche,” Maor said. “According to ecological theory, entering a novel niche may allow a species to evade predators and-or competitors present in its original niche, giving it an advantage that may lead to evolutionary success.”

Maor stopped short at saying that this advantage might have given the ancestors of humans an evolutionary edge, such as setting the stage for the later emergence of our relatively big brains and sharp eyesight during daylight hours. The visual acuity and color perception of healthy humans and other simians is comparable to those of diurnal reptiles and birds — animal groups that never left the daytime niche.

It is important to note that not all living primates are diurnal. Slow lorises, many lemurs, and other primates are strictly nocturnal. In fact, diurnal species are still a minority within all mammals.

“In our data set, there are about 26 percent diurnal species and 60 percent nocturnal ones, and the proportions among all mammals would be very similar,” Maor explained.

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Most day-active mammals also still retain vestiges of their ancestors’ nocturnal past. Rodents known as golden spiny mice and fat sand rats, for example, are diurnal in the wild, but when they are transported to sheltered enclosures, they revert to a nocturnal lifestyle.

While it takes millions of years for a species to evolve from diurnal to nocturnal, or vice versa, it is possible that some animals alive today are slowly undergoing the former process due to Earth’s most formidable predators: humans.

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