In the previous article, The Coming of Mammals II, we followed the journey of the mammals as they diversified following the Carboniferous Rainforest Collapse and came to dominate vertebrate life on land until the devastation of the Permian-Triassic Extinction Event. In that rapidly changing global landscape, thought to have been brought about by intense volcanic activity, a group of sauropsids known as the archosaurs swept to dominance and almost all therapsids died out.

The few that remained, unable to compete with the archosaurs withdrew from their ancestral niches and adapted to life at night, becoming burrowers and primarily insectivores, surviving by being elusive and nimble as the dinosaurs began their millennia-long reign.

The night is dark and full of…. scurrying mammals?

The idea that the earliest mammals evolved to occupy a nocturnal niche in the face of mounting threats from dinosaurs was first introduced by Gordon Lynn Walls in 1942, after examining the internal structures of the visual systems of various taxa, including reptiles, birds, and all three extant strains of Mammalia (eutherians, marsupials, and monotremes). He found that all mammalian eyes have adaptations or remnants of adaptations to lowlight conditions, even the eyes of mammals who are, in the present day, strictly diurnal.

This, he hypothesized, evidence that all living mammals evolved from a small ancestral group that was primarily or exclusively nocturnal i.e. a nocturnal bottleneck in evolution. Wells’ hypothesis was further strengthened by the fact dinosaurs were large reptiles, and reptiles are cold-blooded. They would have been obligated to operate during sunlit hours and leave the night to the mammals.

The idea that dinosaurs were strictly ectothermic (body temperature being regulated by the environment) is now rejected by most scientists. An interesting paper published in 2014 presents the idea that dinosaurs were neither fully endothermic nor ectothermic, but mesothermic, straddling the boundary between the two forms. This suggests that dinosaurs, like tuna and some shark species today, could metabolically regulate body temperature within a certain environmental temperature range, but revert to ectothermia at extremes. This would have made them faster and active for longer than other reptiles but require less food than a similar sized mammal.

While it is no longer accepted that dinosaurs and mammals had a strict temporal split in activity, where dinosaurs only roamed during the day and mammals only emerged after dark, the idea that mammals were, in their formative years, primarily nocturnal is certainly one worth looking at since it gives logical rise to features which we today consider diagnostic of the group.

Firstly, let’s touch on the feature that initially led Walls to explore the nocturnality of early mammals – the visual system. The earliest common ancestor of vertebrates, existing in the Cambrian Period, is believed to have 4 types of cones, the cells in the eyes which are tuned to distinguishing colours in brightly lit environments. This early ancestor would have seen the world in a gamut of four colours – a condition known as tetrachromacy. By the time of the dinosaurs, mammal eyes had lost two of the four cones, resulting in dichromacy. This movement away from light-sensitive cone cells suggests an environment where colour perception was not vital – such as nocturnality or burrowing. Indeed, most placental mammal classes alive today display dichromacy and see the world in a gamut of two colours.

There are a few exceptions to the dichromacy rule – the most obvious being you! Humans, and most higher primates and marsupials, have re-evolved a cone cell type in our eyes and are, therefore, trichromatic, seeing the world in a gamut of green, red, and blue. Marine mammals are believed to have lost yet another cone type and become monochromats, seeing the world in black and white. Interestingly, in some New World monkey species males are dichromatic while females are trichromatic.

Along with this loss of colour-sensitive cone cells, mammalian eyes generally have large cornea compared to birds with the exception, once more, being higher order primates. This is yet another adaptation for lowlight conditions, sacrificing sharpness of vision for the ability to take in more light.

As eyesight became less acute, other sensors evolved to ensure that the early mammals could navigate and hunt in the darkness and weren’t completely hapless. This is in stark contrast to the circumstances of myself, a higher order mammal, who, prior to undergoing LASIK surgery, was unable to navigate in the dark and rather hapless without my glasses. It is truly humbling that a rat-sized primitive mammal scurrying in the Triassic bush was better equipped for survival than me, who chose the word primitive to describe it.

Mammals generally possess a well-developed sense of smell, with a proportionally large olfactory bulb compared to other animals. Most mammals keen hearing and many carnivores today rely on sound and smell to hunt. Whiskers, or vibrissae, on mammalian snouts are functionally guidance systems, allowing mammals to navigate their environments in the dark.

The evolution of fur and brown adipose tissue, a type of fat tissue that is used for rapid heat generation, in mammals can also be attributed to a nocturnal lifestyle, though as an adaptation against the cold rather than lack of light.

There are several other factors that appear to support this hypothesis as well, such as lack of ocular shielding in mammalian eyes against ultraviolet radiation and the lack of production of several light sensitive enzymes.

While Wells’ nocturnal bottleneck hypothesis is not universally accepted, with scientists pointing to other environmental factors that could influence the evolution and adaptation of some of the features now seen in mammals, it seems to be the one that answers most questions about how mammals got the characteristics that define them today.