The sharp color vision that allows us to appreciate red sunsets and purple mountains' majesties may have cost our primate ancestors their sense of smell, according to new research.
In general, animals with great vision have so-so smell, and vice-versa. In humans, for example, about 60% of the 1000 genes for olfactory receptors are so-called pseudogenes that have been decommissioned over time. But in mice and dogs, animals with a keener sense of smell, only about 20% of olfactory receptors are pseudogenes. Yet mice and dogs are colorblind. Many researchers have speculated that once vision became sharper, smell became less critical for finding food and mates.
To investigate the situation in primates, evolutionary geneticist Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and colleagues studied sight and smell genes from humans, apes, and Old and New World monkeys. The team sequenced a sample of 100 olfactory receptor genes and determined, based on those DNA sequences, whether they functioned. They also sequenced photopigment genes to determine if the animals' eyes had two or three kinds--the difference between dichromatic and trichromatic vision.
The general pattern held: Better vision meant a less sensitive nose. About 30% of olfactory receptor genes are defunct in apes and in Old World monkeys, such as rhesus monkeys and baboons--leaving them with twice as many sniffer genes as humans have. The New World monkeys--even more distant relatives of humans--almost all appeared to be slightly better sniffers, with only about 20% pseudogenes. But the New World monkeys lacked the genes for full-color vision. One intriguing exception was the howler monkey, a New World monkey that had both the color vision gene and a paltry selection of working olfactory receptors. Together, the results suggest that full-color eyesight arose independently in the Old and New Worlds and coincided with loss of functioning olfactory receptors, the authors report in the 20 January Public Library of Science, Biology.
The work "very clearly" shows that there was a tradeoff between full-color vision and smell, says biological anthropologist E. Christopher Kirk of the University of Texas, Austin. Evolutionary ecologist Nathaniel Dominy of the University of Chicago says a tradeoff between the two was likely necessary due to brain constraints. "The visual and olfactory cortices take up a lot of brain," he says, "so the question was: What did we have to lose to get trichromatic vision?"