Genetic evidence found to suggest early mammals have good night-time vision

Source: Xinhua| 2017-04-21 08:17:23|Editor: MJ

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SAN FRANCISCO, April 20 (Xinhua) -- 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.
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