A new study offers direct, genetic evidence for the hypothesis that early mammals were nocturnal.
The study, to be published Friday in Scientific Reports, is based on examinations of night-vision genes in many mammals and reptiles, including snakes, alligators, mice, platypuses and humans.
Using what they know about how those animals are related, the researchers led by Liz Hadly, professor of biology at Stanford University and senior author on the paper, figured out when in their evolutionary histories the function of these genes was enhanced.
Mammals and reptiles share a common ancestor, with the earliest mammal-like animals appearing in the Late Triassic about 200 million years ago.
Fossil evidence suggests that early mammals had excellent hearing and sense of smell and were likely also warm-blooded. All of these features are common in their descendants, the living mammals, most of whom are nocturnal.
To trace the evolution of nocturnality, the researchers studied genes that the lead author, visiting scholar Yonghua Wu, had previously found associated with night vision in certain birds, such as owls.
From this, they deduced that the earliest common ancestor did not have good night vision and was instead active during the day.
However, soon after the split, mammals began enhancing their night vision genes, allowing them to begin to roam at night, thus avoiding the reptiles that hunted during the day.
"Early mammals coexisted with early reptiles in the Age of the Dinosaurs and somehow escaped extinction," Wu was quoted as saying in a news release. "This research further supports the hypothesis that diurnal reptiles, such as lizards, snakes and their relatives, competed with mammals and may have led them to better adapt to dim light conditions."
In the millions of years that have elapsed since mammals and reptiles diverged, natural selection and evolution haven't stopped. Not all mammals are still nocturnal.
Some groups of mammals have reoccupied the day, adapting in various ways to daylight activity. Besides humans, these animals include cheetahs, pikas, camels and elephants.
"Understanding the constant pressure to get better at seeing the world at night for over 100 million years is a beautiful way of thinking about evolution," Hadly said. "We think of it as something simple -- seeing in the light or the dark -- but these genes are being constantly refined and altered by natural selection."
The methods used by these researchers could be applied to different areas of the animal evolutionary tree to learn more about the evolution of vision, including how humans made the switch to bright-light vision. "It's a very powerful way of corroborating a story that has been, up to now, only hypothesized," Hadly said.