If you are not a morning person the reasons for it may be genetic and ancient. Roughly half of currently living mammals are nocturnal and many more are most active at dusk and dawn. Based on fossil records it has long been believed that this nocturnal behavior first evolved at roughly the same time as the first mammals, roughly 200 million years ago.

This theory is based on skeletal features and other remains. The large brains of mammals would have been good at processing information from senses other than sight such as touch, smell and hearing.

New evidence, however, suggests that nocturnal behavior may have evolved in the ancient ancestors of mammals, called synapsids, 100 million years before the first mammal.

“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,” said Kenneth Angielczyk, a curator at The Field Museum, in a statement.

In a paper appearing in the September 3 edition of Proceedings of the Royal Society B, Angielczyk and his colleagues lay out their case based on the results of an analysis of tiny bones in synapsids.

The scleral ossicles is a bone found in the eyes of many vertebrates. Currently living mammals lack this bone but they were present in many of our ancient ancestors including synapses.

“The scleral ossicles tell us about the size and shape of different parts of the eyeball. In turn, this information allows us to make predictions about the light sensitivity of the eye, which usually reflects the time of day an animal is active,” said Lars Schmitz, a professor of biology at Claremont McKenna, Pitzer, and Scripps Colleges, located near Los Angeles.

Examining the scleral ossicles in synapsid fossils was a difficult task, because they are small and delicate and are not preserved in many fossil finds. However, by scouring museum collections in the United States and South Africa, the researchers were able to find enough of the bones to conduct an analysis.

The analysis was then compared to a dataset of bones from currently living lizards and birds that have the scleral ossicles bone. The comparisons demonstrated that the ancient synapsid species had a wide range of light sensitivities. This indicates that some were active under bright daylight conditions and others were better adapted to low-light, night-time conditions.

The most significant finding was that some of the synapsids which would have been better adapted to nocturnal activity, date back 300 million years, which predates the first mammals by 100 million years.

“This is the first time we can make informed predictions about the activity patterns of synapsids. 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,” said Schmitz. .

This new research will help researchers in analyzing evolution of mammal visual systems. It may even force scientists to re-evaluate the theory that mammals became nocturnal to avoid competition with the dinosaurs.