Bomb-sniffing dogs at airports are living proof that the human species is olfactorily challenged. Even our fellow primates seem to have keener noses than we do. New genetic evidence shows why this is: Humans are losing so-called olfactory receptor genes at much higher rates than monkeys and apes.

Like all mammals, the human genome has about 1000 genes for proteins that detect smells, or olfactory receptors. But more than half of those genes don't work. In other mammals, such as mice, that proportion is only 20%. To check whether this is a general primate problem, geneticist Yoav Gilad and colleagues at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and the Weizmann Institute of Science in Rehovot, Israel, compared our olfactory receptor genes with those of our evolutionary brethren, the monkeys and apes.

First, they sequenced the same 50 olfactory receptor genes from two humans, two chimpanzees, two gorillas, two orangutans, and two rhesus macaques. Then, they scanned the sequences for mutations that would stop the gene from working. In many sequences, the normally continuous code for translating the DNA sequence into protein had been disrupted by a stop-codon, a signal to end translation.

As expected, 54% of the human genes had such an error. Among the other primates, this percentage only ranged from 28% to 36%, they report online this week in the Proceedings of the National Academy of Sciences. And when the researchers plotted the mutation events on the primate family tree, it became even more obvious that the molecular grim reaper had been winnowing genes from our ancestors with particular zeal: Olfactory genes have been rendered useless about four times faster in the branch leading to humans compared to other primates.

Gilad speculates that culinary habits are to blame for the genetic erosion. Humans have been cooking their food for thousands of years, he says; this could have obviated many smell receptor genes, because cooking can destroy toxic substances that other primates need to keep their noses primed for.

Not likely, says Rebecca Cann, who studies human molecular evolution at the University of Hawaii, Manoa. Most toxins in food plants are concentrated in parts that are not eaten, she says. Although she regards the study as "a very important signpost paper," Cann suspects that our social behavior may be the key to our poor sense of smell. Since agriculture appeared, people have been forced to live in crowded tents and huts, she says. "If you share a small tent with many relatives, the air can get unpleasant."

Related sites

PNAS paper

Human Olfactory Receptor Database Exploratorium (HORDE)

Yoav Gilad's home page