All mammals, including humans, have about 1,000 genes encoding smell-detecting proteins, or olfactory receptors. These receptors, located in the mucous lining of the nose, identify scents by binding to molecules of odorous substances. However, not all olfactory receptor genes are functioning in all species. It is the percentage of the working olfactory genes that determines the sharpness of smell in animals and humans.
In previous studies, the team of Prof. Doron Lancet of the Weizmann Institute's Molecular Genetics Department discovered that more than half of these genes in humans contain a mutation that prevents them from working properly. In a new study, published in the March 18, 2003 Proceedings of the National Academy of Sciences (PNAS), the scientists tackled the next question: is the genetic "loss" a relatively old phenomenon affecting all primates, or did it occurr only in humans?
To resolve this issue, the researchers compared the DNA sequences of 50 olfactory receptor genes that are common to humans and different species of apes and monkeys. They found that 54 percent of the genes were impaired in humans, as opposed to only 28 to 36 percent in the other species. This research has made it possible to reconstruct this sense's deterioration over the course of evolution: apparently, its decline took place within an "evolutionary moment" only 3 to 5 million years and occurred four times faster in the branch leading to humans compared to other primates.
The scientists conclude that the drop in the sharpness of smell is a purely Homo sapiens feature. It probab
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American Committee for the Weizmann Institute of Science