Looking Back in Evolutionary Time

Researchers in Japan are studying how one monkey manages to see in the dark. Their work might change our entire perception of how the primate family tree evolved.

Most primate researchers agree that the common ancestor of today’s monkeys was nocturnal. Strangely, most modern primates are active during the day and have poor night vision. Azara’s owl monkey, which is active at night and has good night vision, is the notable exception.

This exception has led to a controversy  in the field. Primate researchers cannot seem to agree if all monkeys lost the ability to see at night before the ancestors of the owl monkey gained it back. On the other hand, it is possible that the ancestors of the owl monkey were just lucky; maybe they never lost the ability to see at night.

Enter Akihiko Koga and his team at Kyoto University in Japan. The Koga group is a team of geneticists who think that the owl monkey is in the process of gaining back the night vision that was lost by the early ancestors of today’s monkeys.

But peering back into time is hard to do. So Koga and his team needed a way to study if the owl monkeys are evolving better night vision or if they are already optimized for night sight.

They relied on a finding from another group that showed that the low light-sensing cells (rods) of the eyes of nocturnal animals package their DNA in a different way than most cells. Most cells keep gene-rich DNA in the center of the nucleus. This sets up a little hotspot for cellular machinery to get in and turn on genes. But the rods of nocturnal animals pack their gene-rich DNA around the edges of the nucleus. It seems like this DNA packaging pattern makes it easier for light to pass through the rod cell in nocturnal animals.

When Koga and his group looked at the rods of the owl monkey eye, they found that the DNA is packed in a manner that is halfway between that of nocturnal and daytime animals. This suggests that owl monkeys have decent night vision, but they probably are not the best at seeing in the dark.

The team still wanted to know if the owl monkey was on the evolutionary path to regaining night vision or if it had been stuck that way since the early nocturnal ancestor of primates. They found that a piece of repetitive DNA that is usually packed into the center of rod nuclei in nocturnal animals has been expanding like a virus into new locations throughout the owl monkey genome. This could explain how the DNA packing has been changing over evolutionary time in the owl monkey nuclei. Most importantly, it could mean that the monkeys are regaining the night vision that was lost by their ancestors. Maybe they are still evolving toward better night vision.

We are all just monkeys with bad night vision. It is interesting to think that we have some DNA lurking in us–because we share most of our DNA with our primate cousins–that has been co-opted in the owl monkey to bring back night vision. More importantly, this is a new way that biologists have been able to go against the grain of time and peer backward in evolution.

Image: Aotus Azarae by Rich Hoyer, Flickr


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