Color Vision Didn’t Kill Smell

The discovery of the olfactory receptor genes in 1991 was a scientific game changer. It made it possible (as I explained a while back) to study the genetic architecture of olfaction in any species.

Of the hundreds of different mammalian olfactory receptors (ORs), we have yet to match more than a handful to the odor molecules that activate them. That hasn’t stopped scientists from trying to draw inferences about smell function from patterns in the genetic databases.

Among the first to do so were Yoav Gilad and Doron Lancet. In 2004 they compared 100 randomly selected OR genes from 19 different primate species, including Old World monkeys (e.g., baboons and macaques), New World monkeys (e.g., marmosets and spider monkeys), and hominoids (humans and apes). They analyzed the proportion of OR pseudogenes, i.e., OR genes with mutations that prevent the formation of a functional odor receptor. 

Here’s what Gilad et al. concluded:

We find that apes, Old World monkeys and one New World monkey, the howler monkey, have a significantly higher proportion of OR pseudogenes than do other New World monkeys or the lemur (a prosimian). Strikingly, the howler monkey is also the only New World monkey to possess full trichromatic vision, along with Old World monkeys and apes. Our findings suggest that the deterioration of the olfactory repertoire occurred concomitant with the acquisition of full trichromatic color vision in primates.

The idea that the ancient sense of smell deteriorates following the evolutionary emergence of color vision has come to be known as the “color vision priority hypothesis.” At core it’s a restatement in genetic terms of the dismal Greco-Freudian view that for modern man smell is a degraded and unimportant sense.

Science marches on, however, and higher resolution gene sequences are now available for many primate species. Recently genome researcher Yoshihito Niimura at Tokyo University, along with colleagues Atsushi Matsui and Yasuhiro Go at Kyoto University, revisited the relationship of the OR genome to color vision using more complete gene sequence data. They examined five primate species, including humans, chimpanzees, orangutans, macaques and marmosets. The results, according to Niimura et al.,

show that the color vision priority hypothesis is not supported.

Specifically, the five species have similar numbers of functional OR genes (320 to 400) but vary widely in the proportion of pseudogenes. By mathematically reconstructing the evolutionary lineages of these primates, Niimura et al. show that loss of OR genes is a gradual process that does not correspond to the predicted gap between hominoids/Old World Monkeys and the New World Monkeys.

[From Degeneration of olfactory gene repertoire in primates: No direct link to full chromatic vision, by Matsui, Go & Niimura, 2010; follow link for figure caption.]

In other words, there is no simple inverse relationship between color vision and smell. Based on their analysis of the new OR gene sequence data, the authors conclude:

Our sense of smell may not be inferior to other primate species.

I prefer this more upbeat view for two reasons. First, it takes account of both functional and nonfunctional OR genes. Second, it’s supported by behavioral data from smell experiments, which show that human olfactory detection ability is on a par with that of various monkeys. It had a good six-year run, but the color vision priority hypothesis has begun to show signs of, shall we say, deterioration.

[Hat tip to commenter Kaylin.}