Mammals, as it turns out, got their start as the origonal "night life." Scientists have discovered fossil evidence to prove that the earliest of mammal ancestors may have started out as nocturnal creatures, suggesting nocturnal vertebrates evolved almost 100 million years before they were previously thought to. The paper was published in the Proceedings of the Royal Society B: Biological Sciences. 

Scientists previously believed that mammal's transfered to a nocturnal lifestyle around the same time that they started appearing, around 100 to 200 millinon years ago. The previous theory's logic was that this change occured as a way to avoid larger dinosaurs who were more active in the day. It turns out, however, that this shift to nocturnalism came much earlier in the evolutionary timeline. The creature that produced this evidince is called a Synapsids.

Dehli Daily News reports that Kenneth Angielczyk, a curator at The Field Museum, the lead author of a paper said "Synapsids are most common in the fossil record between about 315 million years ago and 200 million years ago. The conventional wisdom has always been that they were active during the day (or diurnal), but we never had hard evidence to say that this was definitely the case."

Scientists made the discovery by studying ring-shaped bones embedded in the eyes of Synapsids. The bones, called scleral ossicles, reveal the Synapsids' eye's dimensions, which in turn enable predictions about light sensitivity, which in turn indicate if an animal is most active in day, night, or twilight conditions.

The eyes of ancient synapsids likely spanned a range of light sensitivities, some suited to nighttime and others favoring daylight. The oldest synapsids possessed eye dimensions consistent with night activity. Predators were more likely than herbivores to be nocturnal.

VOA News reports Angielczyk said, "The study does give us new insights into the daily lives of some of our most ancient relatives," said Kenneth Angielczyk, a paleontologist with the Field Museum of Natural History in Chicago.

The researchers scoured museum collections around the world and found 38 specimens comprising 24 species, mostly from the United States and South Africa but also from Russia and Brazil.

"Specimens with well-preserved scleral rings are rare, so it took a lot of searching," Angielczyk said.

"Nocturnality comes with advantages and disadvantages," said another of the researchers, Lars Schmitz, a biology professor at Claremont McKenna, Pitzer and Scripps Colleges in California.

"It's cooler at night, which may be beneficial for some species. As a hunter, it may be easier to approach prey. On the other hand, the dim light levels make it difficult for animals. Keen senses are beneficial," Schmitz added.

The researchers found that the Dimetrodon had eye dimensions that pointed to activity at night.

Newsledge reports Angielczyk said, "The idea of a nocturnal Dimetrodon [kind of Synapsid] was very surprising, but it shows how little we really known about the daily lives of some of our oldest relatives.

"This is the first time we can make informed predictions about the activity patterns of synapsids," added Schmitz. "As we discover more fossils, we can continue to test these predictions and start to address questions such as how many times nocturnality evolved in synapsids and whether the synapsids most closely related to mammals were also nocturnal."

The major findings were detailed in a press release. Here's the big three. According to the study, the specific findings were : 

"The eyes of ancient synapsids covered the full spectrum of light sensitivities seen in living aninmals, with some species having eyes best suited to activity under bright conditions during the day, others having eyes best suited to low-light conditions at night, and still others having eyes suited to activity under twilight conditions.

2) The eyes of the oldest synapsids species considered in the study, which are about 300 million years old, are predicted to have been best suited to activity under low light conditions at night. These animals are about 100 million years older than the oldest fossils of mammals.

3) The common ancestor of all synapsids (including living mammals) may have been nocturnal (active at night).

4) Most plant-eating synapsids that were included in the study are predicted to have been active during the day.

5) Nocturnality may have evolved multiple times in synapsids."

"Today, predators are largely active in dim light conditions," Schmitz told New Scientist, when "it's easier to attack prey."

He thinks synapsids probably used cover of darkness to kill, not to escape being killed. But if there is a direct link between nocturnality in early synapsids and mammals, we may have misunderstood why mammals adapted to night life. Dinosaurs only appeared 240 million years ago, so it's unlikely the synapsids became nocturnal to avoid being eaten. 

This evidence may impact how we understand the early evolution of mammals in a new light, and at the very least provides an interesting excuse as to why you are late to work after spending all night binge watching Netflix.