When people think of the Jurassic period, what mostly comes to mind is really, really big dinosaurs. During this chapter of the Mesozoic era, some of the largest land animals known to history—colossal sauropods like Diplodocus, Brachiosaurus, and Apatosaurus—reigned supreme. But the Jurassic period was also a formative time for early mammals, including the oldest known ancestor of all placentals (like humans), though many of them looked more like possums than any living primate.
Paleontologists have long theorized that the threatening presence of dinosaurs during the Jurassic period helped, not hindered, mammalian diversity. Since the ecological niches for large carnivores and plant-eaters were already taken, most mammals would have been forced to develop their own unique traits to survive. If you're tiny, outnumbered, and have significantly smaller teeth, why compete when you can cleverly adapt instead?
A prominent theory born out of this notion, commonly referred to as the "nocturnal bottleneck" hypothesis, argues that as early as 250 million years ago, placental mammals adopted an active nightlife to avoid predation by dinosaurs, which were primarily active during the day.
Some Mesozoic predators were nocturnal as well, but our tiny forebears became better evolved for seeing, hearing, hunting, and foraging in the dark. They did so by developing retinas that contained high quantities of super light-sensitive rod cells, which made it easier to detect motion and details in extremely low levels of light. These beneficial traits were selected for over millennia, and by the time non-avian dinosaurs died off at end of the Cretaceous period, many genes associated with a diurnal lifestyle had been lost. The imprints of nocturnal adaptations continue to persist in most living mammals today.
However, the biological mechanism behind this adaptive change has remained frustratingly elusive. According to DNA lineages, vertebrate ancestors possessed mainly cone-dominant retinas, which would have made them better suited for daylight. Living vertebrates, such as reptiles, amphibians, and birds, still possess high amounts of cone photoreceptors.
But according to a new study published this week in Developmental Cell, a single protein might hold the answer.
"Early mammals that were able to have more rods flourished in this new nighttime niche, and populated the diversity of mammals we know today," the study's co-author Ted Allison, an associate professor of biological sciences at the University of Alberta, told me.
The international team of biologists was able to pinpoint the origin of rods in Jurassic period mammals by examining the stages of photoreceptor development in two modern vertebrates with starkly different evolutionary paths: mice and zebrafish.
In young mice, the authors discovered that the embryonic precursors for cone cells eventually matured into rod cells, due to a transcription protein called NRL that suppressed the genes required for cone development. In zebrafish, however, cones and rods developed independently, and on a genetic level, didn't resemble each other at all. This meant that rod cells evolved at separate points in history for mammals and all other vertebrate lineages.
Some of the biggest clues that the nocturnal bottleneck theory could be correct still exist today, in modern mammals. With the exception of anthropoid primates like us, most diurnal mammals possess traits that should only be found in nocturnal species. Heightened sensory aides such as acute hearing and smell, whiskers for feeling invisible obstacles, nocturnal circadian rhythms, and poor UV protection are seemingly paradoxical vestiges of another time.
"Like many evolutionary events, once some change in the animal has occurred, it's somehow challenging to go backwards. Thus the analogy 'to bottleneck'—once the evolution went through to producing a rod-dominant retina, there seems to have been something blocking it from going back," Allison said.
One previous study, published in the Proceedings of the Royal Society B, analyzed the eye morphology of 266 extant mammal species and found that eye shapes of both nocturnal and diurnal mammals were functionally indistinguishable.
"The fact that nearly all living mammals have eye shapes that appear 'nocturnal' by comparison with other amniotes is a testament to the strong influence that evolutionary history can have on modern anatomy," said the study's co-author Chris Kirk, an associate professor of anthropology at The University of Texas at Austin. "It's a bit surprising to still see the effects of this long period of nocturnality on living mammals more than 65 million years after non-avian dinosaurs went extinct, but that's exactly what we found."
And while it's tempting to attribute the emergence of nocturnal behavior in mammals solely to the need to avoid hungry dinosaurs, Allison told me that a host of ecological pressures were likely involved. In theory, while mammalian species did become active at night to avoid predatory dinosaurs, the evolution of nocturnal vision was the quality that allowed some of our ancestors to feed, forage, and hunt in the dark, and ultimately dominate their unique new niche.
"Now that we have the mechanism of the evolution, we're now much more able to let our imaginations churn towards addressing such questions."
Correction: An earlier version of this piece mistakenly said the study examined stages of photoreceptor development in invertebrates, but in fact the research was done in vertebrates. The story has been updated to reflect this.