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Scents and sense ability
By Adam Marcus

The number of olfactory genes in our genome has dwindled, perhaps because we are less reliant on our sense of smell than our ancestors. Human genes involved in detecting odors—olfactory receptor genes—have accumulated more disabling mutations over time than corresponding genes in monkeys and apes.

In a new study, researchers compared 50 regions of human DNA containing olfactory receptor genes with their corresponding regions in chimps, gorillas, rhesus monkeys and orangutans.

They found that humans have accumulated mutations in odor receptor genes four times faster than have the other primates. That, in turn, means humans have twice as many odor receptor genes that no longer function as other primates.

"Humans have fewer functional receptors, and this points to a lesser need of the sense of smell," says Yoav Gilad, of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and a co-author of the study.

Olfactory receptors are the largest "superfamily" of genes in mammals, with over 1,000 different genes. But in humans, more than 60 percent of these genes no longer work. These are called "pseudogenes."

The new study is the first to show that humans have more olfactory pseudogenes than our close animal relatives. "We do not know if this is limited to olfactory receptor genes, or if it's a general trait of human gene families," says Gilad, whose group reports their findings in the Proceedings of the National Academy of Sciences. "Future studies will help elucidate this."

Richard Lane, a molecular biologist at Wesleyan University in Middletown, Connecticut, says the latest study "makes evolutionary and biological sense. It's pretty clear that the human genome seems to have undergone a precipitous decline in the power of the olfactory system" since we split from primates.

The human nose can still detect a broad a range of odors but we are less able to discriminate among highly similar scents, he adds.

So does the paper suggest that as humans evolve further our noses will become obsolete? No, says Charles Wysocki, of the Monell Chemical Senses Center, in Philadelphia. "But we certainly may have a much more restricted 'view' of what the olfactory world is really like."

See related GNN article
»Scents and Pheromones

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Gilad, Y. et al. Human specific loss of olfactory receptor genes. Proc Natl Acad Sci USA. (Published online February 28, 2003).

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