Our earliest mammalian ancestors likely had powerful night-time vision, allowing them to skulk through the dark to find food and avoid reptilian predators that hunted by day, according to a new Stanford study.
Researchers used genetic data to support existing fossil evidence suggesting that our distant relatives may have adapted to life in the dark.
The team, led by Liz Hadly from Stanford University in the US, examined genes involved in night vision in animals throughout the evolutionary tree, looking for places where those genes became enhanced.
"This method is like using the genome as a fossil record, and with it we've shown when genes involved in night vision appear," Hadly said.
"It's a very powerful way of corroborating a story that has been, up to now, only hypothesized," she said.
Mammals and reptiles share a common ancestor, with the earliest mammal-like animals appearing in the Late Triassic period - 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.
Therefore, experts hypothesised that early mammals were also nocturnal. This study offers direct, genetic evidence for that hypothesis.
To trace the evolution of nocturnality, researchers studied genes associated with night vision in certain birds, such as owls.
Researchers examined those genes in many mammals and reptiles, including snakes, alligators, mice, platypuses and humans.
Using what they know about how those animals are related, they figured out when in their evolutionary histories, if ever, the function of these genes was enhanced.
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," said Yonghua Wu, visiting scholar at Stanford.
"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," Wu said.
Not all mammals are still nocturnal. Some groups of mammals have reoccupied the day, adapting in various ways to daylight activity. These animals include cheetahs, pikas, camels, elephants and humans.
"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.
The study was published in the journal Scientific Reports.