Freshly brewed coffee, rancid meat, the scent of a mother, an alarming waft of smoke—good or bad, smell is a powerful sense, capable of rousing remote memories, guiding behaviors and influencing moods. Scientists have long suspected that the immense diversity in human olfactory experience results at least partly from heredity, and now a research team has shown that the perception of specific odors can indeed be traced to one’s genes. Beyond connecting the dots between genes and odors, these findings raise intriguing questions about human evolution. Our repertoire of functional olfactory genes has been shrinking over evolutionary time—and no one knows why.
Scientists at the Rockefeller University and Duke University have demonstrated that chemicals secreted in male sweat can smell like stale urine to one person, sweet flowers or vanilla to the next, or nothing at all to another, depending in part on which variant of an odorant receptor they have in their noses. The sweat chemicals investigated, androstenone and androstadienone, are particularly sexy odors because they are degradation products of the hormone testosterone.
“These chemicals are, in part, interesting because there is a link to potential human pheromones,” says Hiroaki Matsunami, the molecular geneticist who led the Duke team in identifying and characterizing the receptors. Androstenone is a pheromone secreted in the saliva of male pigs that induces females to assume the mating stance, and androstadienone can drive fluctuations in stress hormones among women sniffing it.
For humans, detecting these compounds could be useful “if you’re shopping around for the man with the highest level of testosterone,” conjectures team member Leslie Vosshall of Rockefeller. But without more data, Vosshall says, it is impossible to know what role—if any—these chemicals play in human society.
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Researchers suspect that the ability to smell these compounds—thanks in part to a receptor called OR7D4—served a more critical function in the past. The gene for the receptor exists in different forms: among the study’s 391 human participants, the majority had two functional copies of the OR7D4 gene, but a substantial minority had at least one nonfunctional copy—meaning that a significant number of inoperative OR7D4 variants swim through the gene pool. The presence of these apparently useless genes suggests that OR7D4’s importance has diminished, if not disappeared, among humans. When a gene’s product provides no evolutionary advantage, mutations accumulate, and eventually that gene becomes inactivated. Natural selection may have relaxed its grip on OR7D4, allowing it to drift through human populations in both its intact and broken forms.
If this is the case, OR7D4 is not alone.
“We have been losing functional olfactory receptors for a long time,” remarks Barbara Trask, director of human biology at the Fred Hutchinson Cancer Research Center in Seattle. And some of these losses began before humans diverged from other primates. By analyzing genetic sequences of different species, Trask and others have found that compared with mice, rats, dogs and other primates, people have a greater percentage of pseudogenes—defunct genes that arise through mutation—littering our olfactory genome. No one knows why human olfaction has been circling the evolutionary drain; most likely, somewhere along the line, smell became less relevant to our survival and reproduction.
One hypothesis is that improvements in eyesight shifted reliance away from our noses and onto our eyes. Yoav Gilad of the University of Chicago and his colleagues have shown that the gradual deterioration of functional olfactory receptors paralleled the acquisition of trichromatic (full-color) vision in humans and other primates. If sight replaced smell, it might have happened in many aspects of living, including foraging, courting, and detecting predators, Gilad speculates.
Other theories also try to explain the loss of olfaction, points out Andreas Keller, the Rockefeller geneticist working with Vosshall. For example, the evolution of upright walking brought our heads up, reducing the amount of time spent sniffing around on the ground for food, Keller says. Better verbal communication or improved intelligence may have also reduced the reliance on olfaction, Trask speculates.
Perfume makers may be disappointed to know that the gradual diminution of serviceable olfactory receptors appears to be continuing, according to genetic analyses. Still, the average human nose can detect some 10,000 odors. Not bad.
