Science confirmed that early mammals were "night owls", and dinosaurs were only active during the day after their extinction

During the Mesozoic period 66 million years ago, the ancestors of mammals were not as good at daytime and preying as their descendants. The daytime ruler dinosaurs made these generally petite mammals unable to avoid it, and had no choice but to choose nocturnal activities. This evolutionary journey of mammals is known as the "nocturnal bottleneck".

In the early morning of November 7, Beijing time, Nature ecology & evolution, a sub-journal of the international academic journal Nature, published the latest results jointly completed by Israeli and British scientists online, confirming the above "nocturnal bottleneck". It is revealed that the ancestors of mammals were good at nocturnal activity, and the Cretaceous-Tertiary genocide caused the rulers of the time, the dinosaurs, to eventually disappear, and the ancestors of these mammals turned into daytime activities.

The study mentions that many of the living mammals are daytime activities, when in fact most mammals have vision that is more similar to that of nocturnal reptiles and birds. Mammals other than primates lack a central fovea on the retina, which allows fish, reptiles, and birds to have high visual acuity and is an important feature of these daytime predators.

In addition, mammals have less variety of photoreceptors that acquire color during the day compared to all other vertebrates. Many daytime mammals, such as ungulates and carnivores, have rod cells dominating the retina of their eyes, which means they are actually more adapted to low-brightness environments like nocturnal.

In general, the light-sensitive cells of vertebrates, including mammals, are mainly made up of rod cells and cone cells. Cone cells are less sensitive to light, but are able to help animals distinguish colors by detecting peak wavelengths of different rays. Rod cells are more acute in low-light environments, but not sharper. The ratio of rod cells to cone cells in the retina is usually highly correlated with species.

Previous studies have shown that mammals with poor day vision compensate for their deficiencies through enhanced olfactory sensitivity, a wider hearing range, and more complex tactile perception.

The paper mentions that some current anatomical, morphological, and growing molecular studies have indirectly supported the "nocturnal bottleneck" theory. For example, 300 million years ago, some inferior pores (a suborder of paleonttomy and also considered the ancestor of mammals) adapted to nocturnal activity, which may indicate that nocturnal activity was characteristic of mammalian ancestors during the Paleozoic period.

Despite the various studies surrounding the above, so far there has been no scientific research that directly confirms the "nocturnal bottleneck" of mammals, let alone the time point when the "nocturnal bottleneck" will be lifted. The research team innovatively used phylogenetic comparisons to reconstruct ancestral behavioral traits to understand mammalian patterns of activity and the time nodes in which these patterns evolved. At present, this research method is limited to two mammals, primates and rodents.

The team collected data on the activity patterns of 2415 species of mammals (covering 135 families out of 148 extant families, all extant orders) to investigate the activity patterns of mammalian ancestors and the time of the emergence of diurnal mammals. The study defined a total of five activity patterns, namely nocturnal (only or most of the time in the dark), diurnal (only or most of the time during the day), no fixed regularity (day and night activity), shimmering (only near sunrise and sunset), and super-day (only in a specific few hours of circulation).

The study also used the reversible jump Markov chain Monte Carlo method (MCMC) to determine the conversion rate between different activity patterns, inferring the posterior probability of the characteristic state of each node of the phylogenetic development.

The results suggest that extant mammals most likely originated from nocturnal ancestors, who maintained this trait until 9 million years before the Cretaceous-Tertiary mass extinction or at the end of the Mesozoic Era. The study also suggests that although diurnal and nocturnal irregularities occurred in the late Cretaceous period, there is sufficient evidence to show that a complete diurnal shift occurred after the Cretaceous-Tertiary mass extinction.

The Cretaceous-Tertiary genocide, which occurred about 66 million years ago, led to the disappearance of all non-avian dinosaurs, marine and reptiles, most other vertebrates, avertebrates and plant groups. The Mesozoic period ended, and the dominance of the dinosaurs disappeared, thus transitioning to the Cenozoic Era dominated by mammals.

In addition, the study also pointed out that the ancestors of great apes such as orangutans were among the first mammals to evolve into a diurnal lifestyle. This is also consistent with the fact that their offspring' vision can adapt to daytime activities, and they have advantages such as tricolor vision that is different from other mammals.