How humans lost their scents

In at least one type of endeavor, humans can’t even begin to compete with their best friends. Dogs can be trained to sniff out drugs and explosives or to track down a crime suspect by smell. Why can’t we do the same? Scientists from the Weizmann Institute of Science and the Max Planck Institute for Evolutionary Anthropology propose an explanation for this ancient quandary.

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 probably stemmed from the development of the brain in the human direction – a direction that entailed increased emphasis on vision, development of the ability to distinguish colors and the capacity to identify other members of the species by facial appearance rather than by smell.

The research team included Yoav Gilad, a Ph.D. student at the Weizmann Institute’s Feinberg Graduate School who conducted collaborative research at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, his adviser Prof. Doron Lancet, Weizmann graduate student Orna Man, and Prof. Svante Pöäbo, head of the Max Planck Institute in Leipzig.

Prof. Doron Lancet’s research is supported by the Crown Human Genome Center, Alfried Krupp von Bohlen und Halbach Foundation, the Avraham and Yehudit (Judy) Goldwasser Fund, Ms. Emilia Mosseri, London, Mr. James Klutznick, Chicago, IL, Kalman & Ida Wolens Foundation and the Jean-Jacques Brunschwig Memorial Fund.