Losing Our Senses
Liman tracks the disappearance of a chemical sense
By Usha Sutliff, USC News Service
Are humans and other animals equally equipped to sense pheromones—powerful chemical signals that govern mating, aggression and other behaviors?
USC College neuroscientist Emily Liman thinks not. Her studies suggest that evolution eliminated the ability in humans long ago.
In fact, Liman’s latest findings support the idea that the last ancestors of humans able to detect pheromones have been dead and gone for roughly 40 million years. Liman, an assistant professor of biological sciences, speculates that over the course of evolution, better color vision might have compensated for, and eventually replaced, humans’ fading pheromone-detection system.
Liman’s research adds more fuel to the fire of a long-standing debate over whether people can sense and secrete pheromones, which influence sexual and other behaviors throughout the animal kingdom. In animals ranging from single-celled organisms to orangutans, pheromones do everything from triggering mating to establishing boundaries. In humans, the best proof that people release pheromones can be seen in menstrual synchrony—when women living in a college dorm, for example, all menstruate around the same time.
At the center of this debate lies a pair of tiny pits in the nasal septum—the cartilage that divides the nostrils—called the vomeronasal organ or VNO. In mice, the VNO is the main sense organ for picking up on pheromonal cues. “There has been a lot of controversy about whether humans have a vomeronasal organ,” says Liman, adding that she believes the human VNO is a vestigial organ, similar to the appendix.
In a study published this spring, Liman and her USC colleague, Hideki Innan, a postdoctoral researcher in molecular and computational biology, studied the DNA of a large number of primate species. They looked specifically for mutations in TRPC2—a gene Liman discovered in mice in the late 1990s while at Harvard Medical School. The TRPC2 gene is essential for VNO function in mice and other mammals.
The TRPC2 gene encodes an ion channel, which is a gateway in the cells of the vomeronasal organ that lets positively and negatively charged atoms called ions pass through the organ to the brain. Via electrical impulses, those ions carry information about pheromones.
In humans, TRPC2 is what’s called a pseudo-gene. “A pseudo-gene is a gene that was once functional, but there’s something the matter with the genetic sequence so it can no longer make a functional protein,” says Liman.
“We found that mutations that might affect the functioning of the protein started to occur in the last common ancestor of Old World monkeys, apes and humans about 40 million years ago,” she says. “In our study, we were trying to understand where in evolution humans lost this gene, as a way of telling us where we stopped using the vomeronasal organ.”
Forty million years ago also happens to mark a point in primate evolution when color vision was becoming better developed—an occurrence that may explain why primates started to rely more on their eyes than their noses for important behavioral cues from members of their own species. “I’ve hypothesized that visual cues have largely taken the place of pheromonal cues in these species,” says Liman.
“The fact that we can see where this happened, and that it fits in with all of the anatomical evidence, makes this study different,” she adds. “There’s been a lot of speculation, but no one has taken a serious look at what was going on in humans and other primates. Our method was particularly powerful for looking at that.”
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