Back in the 1940s, scientist Gordon Walls wrote a book called “The Vertebrate Eye and Its Adaptive Radiation.” He compared the eyes of mammals with those of reptiles and birds, and noticed something strange.

Even the eyes of mammals that were diurnal—active only during the day—had certain characteristics of nocturnal eyes, like those of creatures who are active at night.

Walls noted that many diurnal mammals had large corneas, fewer types of photoreceptor cells to see colors, and a tapetum lucidum—a reflective surface that increases light absorption by the retina.

All of this suggested that the mammals had retained traits for eyes that were good for seeing in the dark.

Mammals also have excellent hearing and a keen sense of smell, more evidence that they adapted to conditions where eyesight might’ve been slightly less important for survival.

Walls suggested that all these mammals were holding onto traits that first evolved during the time of the dinosaurs, when their ancient ancestors had to survive by hiding from the much larger dinos.

For those mammals, being nocturnal was probably a good way to stay alive.

This hypothesis became known as the “nocturnal bottleneck” and it had a huge impact on the field of paleontology.

For decades, scientists believed dinosaurs were diurnal and the tiny mammals were nocturnal.

It made a certain amount of sense, because they considered dinosaurs to be cold-blooded, like modern reptiles.

If that was the case, the dinos would need the warmth of daytime sunlight to be fully active.

But as researchers have uncovered more mammalian fossils and studied the biology of different dinosaur species, they’ve found some surprising results.

As it turns out, the difference between mammal and dinosaur behavior during the Mesozoic Era wasn’t quite so clear cut as day and night.

The closest extinct relatives of mammals appear in the fossil record around 230 million years ago, when all the continents were in one huge landmass called Pangaea and dinosaurs were starting to roam the earth.

Genetic evidence suggests that these early mammal relatives were probably nocturnal, and a lot of the fossil evidence shows that they were small, like shrew- or rat-sized.

They most likely ate insects, and had to exploit different niches than the dinosaurs evolving alongside them.

Paleontologists have also spent decades looking into the lifestyles and behaviors of dinosaurs.

At the time that Walls wrote his book, many hypothesized that dinosaurs were cold-blooded, or ectothermal.

This would mean they needed a certain amount of warmth from sunlight to get their bodies to function well, like modern reptiles.

So if dinosaurs needed daylight to really get going, it might come as no surprise that the tiny mammals would’ve adapted to a nighttime niche.

However, more recent research suggests that dinosaurs were endothermic, having the ability to warm themselves with their metabolism, which made them less reliant on the sun.

Yet, the nocturnal bottleneck hypothesis still seems to be confirmed by genetic evidence, fossil evidence, and the morphology of modern vertebrates, which looks at the relationship between body traits of different species.

Let’s start with genetics.

As recently as 2017, a study used statistical modeling to look at the evolutionary rates of nearly 1200 genes within 89 modern mammals.

The researchers suggested that the ancestors of placental mammals that lived with the dinosaurs were small, insectivorous, and nocturnal.

Another paper from 2018 looked at the genes that code for opsin in 154 modern mammals.

Opsins are light-sensitive proteins that help translate light into an electrochemical signal the brain can interpret.

These researchers found that all mammals have lost a number of genes that code for certain opsin proteins, indicating that mammal eyes are adapted to low-light conditions.

As for the fossil evidence, it’s clear that most of the ancient mammals and their early relatives were very small.

And that small body size matters a lot, according to a 2019 paper.

In it, one researcher hypothesized that the small body size of mammals in the Mesozoic was directly related to their nocturnality.

The climate during the Mesozoic was much different than what we see today.

There seemed to be far less variation in temperature across the globe, and it was generally much warmer.

Because daytime temperatures were so high, ancient mammals wouldn’t have been able to cool themselves enough without drinking an impossibly large amount of water.

And modern researchers have also looked at morphology, doing the same kinds of comparisons that Walls did almost 100 years ago.

One 2012 study analyzed the eye shape of 266 species of mammals and found little difference in the eyes regardless of whether the animals were nocturnal or diurnal.

In fact, almost all of the mammal eyes were most similar to the eyes of nocturnal birds and lizards.

In other words, since modern mammals have eyes that are adapted to low light no matter when they’re active, this suggests that they came from ancestors that were nocturnal.

But there are also little pieces of evidence that have popped up over the years that complicate the story.

Take a discovery published in 2005 for example.

Researchers in China found a surprisingly complete Repenomamus robustus skeleton.

Not only was the animal about the size of a modern-day opossum with the pointy teeth of a carnivore, it had the dismembered remains of a baby dinosaur called Psittacosaurus in its stomach!

It’s impossible to tell whether the Repenomamus got its meal through scavenging or hunting, but it upends the idea that Mesozoic mammals were completely out of the dinosaurs’ league.

And Repenomamus robustus wasn’t the only carnivorous mammal large enough to potentially compete with some of the smaller dinos.

An even larger species of Repenomamus was also noted in the 2005 paper, this one named Repenomamus giganticus, with a skull 50 percent larger than that of the smaller species.

This larger species would’ve been about the size of a Tasmanian devil and weighed 12 to 14 kilograms - and it’s possible that it could get that big because no big dinos have been found in the same deposit.

Whether it was a scavenger or hunter, it seems clear that it was carnivorous.

Even if researchers can’t say if either Repenomamus was nocturnal or diurnal, they still don’t fit the picture of meek mammals hiding away from the hungry dinos.

And, to throw in a small twist from genetic evidence, there was a study from 2012 that looked at the evolution of photopigments in the eye that detect color.

The researchers concluded that early mammals may have had the ability to see in twilight—the lower light conditions that happen at dawn and dusk.

So maybe the Mesozoic lifestyle wasn’t quite so strictly split between dinos active during the day and mammals active during the night.

Meanwhile, other studies have offered a new perspective on the dinosaurs and even older vertebrates.

In 2010, a group of researchers looked at a sample of eye bones, called scleral rings.

These are round bones within the eye socket that are present today in modern birds and many reptiles.

By comparing the size ratio between the scleral ring and the orbit of the eye - also known as the eye socket - researchers found a direct relationship between the shape of these two bones and the daytime or nighttime behavior of modern creatures.

Now, some dinosaurs also had scleral rings, and when they applied that same approach to 23 dinosaur fossils in 2011, the researchers found that plenty of them were most likely active at night—including hunters like Velociraptor.

Other researchers have even described it as a possible “arms race” between the dinosaurs and the mammals for the nocturnal niche.

Maybe mammals started exploiting the nighttime in order to stay alive, but then carnivorous hunters evolved to follow their prey.

In other words, if you’re a mammal trying to avoid being eaten, nighttime may have been a good option at first but later it became a little more dangerous.

And in a 2014 paper, when the researchers looked even farther back in the fossil record, they found that nocturnality evolved in vertebrates before the earliest mammals appeared, within a lineage known as synapsids.

The researchers found that nocturnal behavior evolved as long as 300 million years ago, in carnivores like Sphenacodon ferox and herbivores like Tritylodon longaevus.

And along with the scleral rings, researchers have unearthed more fossil evidence that dinosaurs weren’t entirely limited to daytime hours.

They’ve found indications of proto-feathers that would’ve provided insulation, and maybe kept dinosaurs warm in the colder nights.

Whatever the case, the story isn’t quite as simple as it originally seemed.

The assumptions about the nocturnal bottleneck have led a lot of scientists to look specifically for evidence supporting this hypothesis—which can definitely lead to oversimplification.

As more mammalian fossils are discovered, they bring up interesting new questions about the complicated evolution of our ancient ancestors and their relationship to dinosaurs.

But the picture is still too muddy for us to completely overturn the nocturnal bottleneck—especially when we consider what happened next.

When paleontologists and molecular geneticists looked further ahead in the story of mammals, a few interesting things happened.

After an asteroid smacked into the planet 66 million years ago, the changes to the climate that resulted led to the extinction of most life, including the non-avian dinosaurs.

But mammals and some smaller vertebrates still managed to eke out a living, and as the Earth rebounded, there were suddenly a lot more niches to fill.

In a 2017 study, researchers looked at more than 2400 species of mammals today and created algorithms to study the genetic transitions from nocturnality to diurnality.

To their surprise it seemed that some species of mammals started spending more time out during the day within 200,000 years after the meteorite impact—a really fast evolutionary transition, if the mammals were largely nocturnal before.

And a cache of fossils from Colorado Springs that scientists described in 2019 gave another piece of exciting evidence.

Within the first 700,000 years after the extinction event, mammalian body mass was 100 times bigger than mammals living immediately after the mass extinction or even during the time of the dinosaurs, reaching an estimated 47 kg.

The researchers who discovered this site don’t think the demise of the dinosaurs was entirely to blame for the remarkable size of some of these mammals.

There were other big changes happening to Earth’s climate and plant life.

Among the mammal remains were also fossils of early legumes, or bean pods.

These nutrient-dense plants may have contributed to the large size of some of the mammals.

While many factors allowed mammals to take over the world, it’s clear that the disappearance of dinosaurs was a big help.

But as for the behavior of mammals living alongside the dinosaurs, it’s clear there’s still a lot to uncover.

Whether we want to thank or blame the dinos for so many modern mammals still being adapted