New genetic evidence confirms a long-held hypothesis that our earliest mammalian ancestors indeed had powerful night-time vision.
The findings published in the journal Scientific Reports suggests that adapting to life in the dark helped the early mammals find food and avoid reptilian predators that hunted by day.
The research team 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," said lead researcher Liz Hadly, Professor of Biology at Stanford University in the US.
"It's a very powerful way of corroborating a story that has been, up to now, only hypothesized," Hadly said.
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.
Therefore, experts have hypothesised that early mammals were also nocturnal.
This study offers direct, genetic evidence for that hypothesis.
To trace the evolution of nocturnality, the researchers studied genes that the researchers had previously found associated with night vision in certain birds, such as owls.
The team members examined those 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, 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, the study said.
The researchers said thatr 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.
These animals include cheetahs, camels, elephants, and, of course, humans.