Tarsier Eyes Reveal How Human Vision Evolved: Early Ancestors Stopped Being Nocturnal

The ancient, primate ancestors of humans were all once nocturnal. In fact it took them quite some time to develop the highly acute, three-color vision that allowed them to shift to daytime living. Now, a new study challenges the prevailing view that this type of vision only developed after primates started rising with the sun. Instead, it suggests that primates possessed this type of vision beforehand.

In order to better understand how primate vision may have evolved, the researchers examined the eyes of tarsiers, the tiny primates that branched off early on from monkeys, apes and humans. With rat-like tails and wide, orange eyes, these small primates can communicate with pure ultrasound and are largely nocturnal.

The researchers analyzed the tarsier genes that encode photopigments in the primate's eyes. They found that, surprisingly, the last common ancestor of living tarsiers had highly acute, three-color vision--the same type that can be found in living monkeys and apes. Although this type of vision would normally denote a daytime lifestyle, the tarsier fossil record showed that the primates continued to have large eyes, which seemed to point to the fact that they were nocturnal.

The evidence seemed completely contradictory. Why would tarsiers have this type of vision if they were completely nocturnal? The researchers came up with a reason to explain the inconsistency. It was possible that early tarsiers were actually adapted to dim light levels such as twilight or bright moonlight. This type of lighting would be dark enough to favor the large eyes, but still be bright enough to support the three-color vision.

As this common primate ancestor evolved and different species branched off, each would have carved out its own niche. It's possible that tarsiers slowly turned to nighttime hours while monkeys and primates evolved to take advantage of the daytime. This means that the three-color vision developed before primates became diurnal, rising with the sun. The findings give researchers a better understanding of how vision evolved in the earliest of our ancestors.

The findings are published in the journal Proceedings of the Royal Society B: Biological Sciences.