Scientists from Tel Aviv University, in Israel, and University College London suggested that the majority of mammals that currently live among us in the 21st century also lived with dinosaurs millions of years ago. However, their survival instinct made them become nocturnal creatures to avoid being eaten by the biggest and fiercer predators.

The study published Monday in the journal Nature Ecology & Evolution suggested that only when dinosaurs were suddenly wiped off the Earth, the mammals started to live in the open areas by daylight. This could bring the scientific community more knowledge about the mammalian evolution, but there’s still a lot of research to be done if experts want to reach an absolute conclusion.

Mammals in Mesozoic, Mammals and dinousaurs, Mesozoic Era
One of the first placental mammals that lived through the Mesozoic. Image credit: Carl Buell via Live Science

The researchers wanted to explain the reason why some animals have extremely-suitable ears and eyes for when everybody’s asleep. Most of them don’t follow a daytime-active  or “diurnal”  lifestyle. But those who indeed live the majority of the time by day are still capable of adjusting their senses in late hours at night.

“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…alternative analyses,” the lead author Roi Maor said in a statement.

David Polly told Gizmodo that the scientists used the “evolutionary relationships” to approximate the moment when the mammals left their comfort-zone, started to move around the area during daylight, and if they did those things before or after the dinosaurs were extinct. According to Polly, an Indiana University professor and Adjunct Professor of Biology, who was not part of the study, the heart of the question is “whether the dinosaurs held back the diversification of mammals.”

Most mammals still see when the sun hides

According to the lead researcher, Roi Maor from the Tel Aviv University, the majority of warm-blooded, milk-producing animals are nocturnal and possess “adaptations to survive in dark environments.” Thanks to the study, the scientists could realize about a nocturnal “bottleneck” in the evolution of these creatures, who had to hide in the dark due to the fear of being eaten or competing with others for territory.

“Monkeys and apes (including humans) are the only diurnal mammals that have evolved eyes that are similar to the other diurnal animals like birds or reptiles. Other diurnal mammals have not developed such profound adaptations,” Maor said.

Primates are the only mammals who have a part of the eye called the fovea, which is full of photoreceptor “cone” cells that let them see in color during high light. Instead, the other mammals possess high levels of something called “rod” cells, letting them perceive low amounts of light in vague conditions  although it gives them a very low-resolution sight.

Also, other mammals who live during daylight  squirrel, dogs, antelope and many carnivores  have extremely-heightened senses of hearing and smell. These conditions are much needed for animals that tend to live at night.

The fact of animals easily adapting to the environment at night made paleontologists and evolutionary biologists always believe what this study could prove. This throws light on the previous misunderstanding about some mammal’s eyes that are only able to function in low light, as well as some developed, sensitive-ears that let the animal catch the sound returning after producing it.

Scientists don’t know when mammals stopped being nocturnals

The exact moment when animals started to live under the daylight is still unknown and extremely hard to discover. Scientists need more than fossils to understand the behavior of ancient creatures. However, they could infer by studying the shape of eyes sockets and nasal cavities, as those senses were the most important for the animals. Of course, the results can be misleading.

“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,” said study co-author Kate Jones. “However, we see a clear correlation in our findings.”

To further understand the beginnings of the mammals, scientists from the University of College-London and Tel Aviv University examined many family trees from the 2,415 different species of mammals  including us  that are living today. They used advanced technology to develop an algorithm and construct the possible behaviors of every species and their ancestors.

Mammals in Mesozoic, Mammals and dinousaurs, Mesozoic Era
Researchers can’t tell if the mammals started living in daylight because dinosaurs went extinct. Image credit: Highlights for Children/Oxford Scientific/JupiterImages

By performing the study, the scientists were able to find the first ancestor who lived between 220 and 160 million years ago. This species evolved from a reptilian forebear and was probably a nocturnal creature.

On the other hand, they reassured previous studies saying that cold-blooded dinosaurs preferred to live during the day so their bodies could warm up  exactly like any other reptile does today.

The experts suggested that mammals lived throughout the Mesozoic until a fatal event occurred in the Earth. The most accurate theories suggest it was caused by an immense asteroid that struck our planet and ended with the dinosaur’s era.

However, the mammals for that moment  who were very tiny and able to hide in very small-spaces  survived and developed until what we know today. Cats, elephants, and cows, for example, have preserved very well their senses to live during both day and night.

Source: Nature Ecology & Evolution