To early mammals, the world looked something like the beginning of The Wizard of Oz: colorless. But at some point, a group of primates began seeing reds, greens, and blues—and many shades of pink, yellow, and aquamarine in between. Scientists have long thought that color vision—called trichromacy—developed in primates sometime after they stopped hunting at night and began waking up with the sun. By the light of day, the thinking went, these pioneering primates were better able to identify food, communicate, and spot predators in the brush. They prospered in the sunlight and continued evolving into some pretty impressive characters: monkeys, apes, and humans.
Tarsier eyeballs are sometimes larger than their brains.
But now the tiny, colorblind tarsier is giving an opposable thumbs down to that theory and raising new questions about just when our Technicolor-perceiving abilities emerged. The questions arose after a recent study that compares the photopigment genes of two tarsier species living today with their oldest common ancestor, a tarsier that went extinct millions of years ago. That ancient relative, the researchers found, was trichromatic, which challenges the notion that color vision came to primates only after they began burning daylight hours. Tarsiers, after all, are nocturnal.
“Tarsiers represent a really key position in primate evolution. They are kind of their own weird creature,” says Amanda D. Melin, an evolutionary ecologist at Dartmouth University. Tarsiers, which are found in Southeast Asia, can communicate via ultrasound, swivel their heads like owls, and climb like long-legged tree frogs. (The Horsfield’s tarsier can leap up to 18 feet in a single bound.) Their eyeballs are sometimes larger than their brains, and they are the only primates that eat meat exclusively. So it’s little wonder scientists have had the darndest time classifying these guys. Taxonomists first clumped tarsiers in with prosimians—which means “before monkeys”—but they didn’t quite fit in with that group of lorises and lemurs. Upon closer inspection at the molecular level, scientists now consider tarsiers to be most closely related to the anthropoids—those sun-loving, more color-savvy monkeys, apes, and humans.
But modern tarsiers prefer the nightlife and, like all nocturnal mammals, they are dichromatic, meaning they only have two kinds of cones in their eyes, instead of the three that allow for color vision. In fact, seeing a fuller spectrum would hinder their ability to detect subtle changes in brightness levels in the dark. Might this mean that the tarsier’s color-seeing ancestor had been active during the day? Melin doesn’t think so. Fossilized skulls of the prehistoric primate feature huge orbits that would have likely held big, round eyeballs, much like those of the modern tarsier. Dusk and moonlight, the researchers conclude, were bright enough that seeing color would come in handy.
“It probably was useful then under dim light, and trichromatic vision is really good for discriminating between greenish and reddish objects,” says Melin. “A lot of cats and civets are kind of a reddish or yellowish and against a green, leafy background, they would stand out.”—A good thing to notice for a snack-size primate.
So how much farther back does color vision go? What can it tell us about the evolution of our own eyesight? Like so many things in science, mark this one as: more study required.
But to do that, scientists will need tarsiers to stick around for further eye exams, and right now their future is in serious doubt. Their ancestors lived in forests in Asia, Europe, and North America, but today’s tarsiers inhabit only a handful of islands. The Convention of International Trade in Endangered Species (CITES) lists 10 of the 18 tarsier species and sub-species as vulnerable or endangered (two critically so). Along with deforestation, these primates are also threatened by the pet trade, which removes them from their native habitat. And while undeniably adorable, the shy, picky-eating tarsier makes for a terrible pet—zoos even have trouble keeping them in captivity. Worse yet, the animals have a tendency to “commit suicide” when overstressed. Apparently they’d really rather just be left alone, in the dark, where they belong.
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