The Howler Test

If you’ve ever been to a Central American forest, you’ve probably heard the hoots and wails of a howler monkey. But these creatures deserve our attention for more than their howls. They turn out to tell us a lot about the evolution of our own senses. We and some of our close primate relatives are remarkable for having powerful color vision. What triggered the evolution of this adaptation some 25 million years ago? Some researchers have proposed that as the global climate cooled, our ancestors were forced to shift from a diet of fruit to leaves. An ability to detect red and green colors would have helped these early primates detect the best leaves to munch on. The descendants of these leaf-munching primates shifted to other foods in later years, but they held onto their color vision.

Before the ancestors of today’s Old World monkeys and apes acquired color vision, primates had already spread to South America and this continent began to drift away. None of today’s New World monkeys has trichromatic color vision–except for the howler monkey. And a major part of the diet of the howler monkey is, interestingly enough, leaves.

Meanwhile, other scientists have been studying the evolution of our sense of smell. Devolution might be a better name for it. We have hundreds of genes for the receptors that snag odor molecules in our nose. But more than half are broken pseudogenes, mutated beyond any use. Most of the corresponding genes in a mouse are in good working order. Given that mice depend profoundly on their sense of smell, that makes sense, and it also suggests that we have lost much of our distant ancestors’ sense of smell. In order to test this hypothesis, a group of researchers recently did a major survey of the olfactory receptors in primates, looking for when in evolution our ancestors lost their receptors. They found that all Old World monkeys have significantly more pseudogenes than a more primitive primate, the lemur. So our noses have been on the downhill slide for some 25 million years. Could it be that a shift in diet from eating fruit–which depends on sniffing out ripe and rotten food–to eating relatively odorless leaves was the trigger for the shift? It’s a neat idea but tough to test. You have to find another case in which the same shift happened and look at the noses involved. Oh, wait–the howlers. In a wonderful bit of evolutionary elegance, it turns out that, unlike all other (fruit-eating) New World monkeys, leaf-eating howlers also have lots of pseduogenes for olfactory receptors.

Comparing our genes with howlers reveals another interesting thing. It turns out that olfactory receptor genes mutate beyond use so fast that we should theoretically have far few working genes for smelling. While smell may not be the way we make our way through the world the way it used to, it seems that it still pays to be able to tell when the mayonanise that’s been on the counter for a while isn’t quite right.