Just what makes an odor skunky or sweet is mysterious, though. Here’s some of what we know about the act of smelling. A molecule floating through the air gets sucked through our nostrils and travels north about seven centimeters until it reaches a ridged tissue called the olfactory epithelium. In that membrane, twenty million olfactory receptors serve as the landing pad for odors. Roughly speaking, an odor molecule locks into a receptor, triggering the nerve to send a signal to the brain.
The most widespread theory for how we detect various odors (and humans are thought to be able to smell about ten thousand different smells) is that receptors have different shapes, and various odor molecules therefore lock into different receptors. Rachel Herz, a psychologist specializing in smell at Brown University, wrote, “Different scents activate different arrays of olfactory receptors in the olfactory epithelia, producing specific firing patterns of neurons in the olfactory bulb. The specific pattern of electrical activity in the olfactory bulb then determines the scent we perceive. The scent of a mango elicits a different pattern of neural impulses than the smell of a skunk.” That’s a description from her book The Scent of Desire.{20} Then our brains translate these patterns into “skunk spray, yuck” or “mango, yum.”
That’s a broad-stroke picture of scent perception, but what drives our preferences for certain odors is perhaps more contentious. When it comes to our widespread dislike of skunk spray, Wood and Block theorize that our aversion to these molecules may be protective, something skunks have taken advantage of.
“Thiols and certain nitrogen compounds are intimately associated with the decay of living materials, of protein material,” says Block. “When food decays, it’s usually because there are bacteria present, and those bacteria can use toxins. Generally, higher animals can detect when food is bad and avoid it. And they detect it with their noses.” Maybe this sensitivity to rancid food was favored by evolution. “Animals with the best sense of smell toward molecules associated with decay survive better than those that have an impaired sense of smell,” says Block. It’s not only the ability to pick up the scent—if Block’s theory is correct, the organisms that found the smell unpleasant would survive better.
Wood has a similar theory, although his is based on the relationship of thiols to hydrogen sulfide—a compound we are also sensitive to in low doses. Hydrogen sulfide is often found in places that have no oxygen. “Animals that breathe oxygen want to stay away from areas that don’t have oxygen,” Wood says. “So our receptors for finding hydrogen sulfide are very highly tuned.” Skunks might have evolved to take advantage of this sensitivity by producing a spray with a chemical that has a similar base structure, says Wood. Both theories suggest that skunks capitalize on our evolved sensitivity to odors that signify something harmful. This is the genius of skunk spray: it’s not particularly harmful; it reminds us of something that is. This is very annoying.
There’s usually a reason that things are annoying, even if the reason isn’t immediately obvious. Consider the events that occurred in the lab of Nikolaas Tinbergen, a Dutch scientist who studied sticklebacks, a small common fish. In 1973, he and Konrad Lorenz and Karl Von Frisch won the Nobel Prize for their work on animal behavior. The story goes like this: Tinbergen kept sticklebacks in tanks in his lab. Every day, around eleven in the morning, the sticklebacks acted very agitated. Under normal circumstances, the fish would glide placidly through the vegetation in the tanks, but at eleven they darted around the tank as if something was bothering them. After pondering this strange behavior for a time, Tinbergen realized that the timing of the fish agitation coincided with the daily arrival of the mail truck. The fish that exhibited the strange behavior were in a tank by the window in sight of the truck’s arrival, and the truck was bright red.
A stickleback with a reddish tinge is a stickleback spoiling for a fight. Evidently, this red truck was enough to get the sticklebacks in the tank annoyed and ready for a fight. “We’ve tried to eliminate the color red from most things in the stickleback room so that we don’t redo the mail truck experiment,” says David Kingsley, a geneticist at Stanford University. There’s nothing dangerous to a fish about a mail truck, but the general stimulus brings a very specific response, and that’s really interesting.
Kingsley is trying track down the genetic changes that gave us our big brains, our ability to walk upright, and our largely hairless bodies. To understand human evolution, Kingsley has turned to fish. Here is Kingsley’s thinking. New environments allow new forms to appear. That’s part of what Charles Darwin concluded when he studied the beaks of finches on the Galapagos Islands off the coast of Ecuador. Each island has a slightly different ecology, and on each island, the finch beak was uniquely adapted to the ecology of that island. So Kingsley wanted to find a species that had recently—recently, in evolutionary terms—been forced to adapt to new environments. It turned out that the stickleback was ideal.
Sticklebacks are about two or three inches long. They normally live in the ocean but migrate to coastal areas to breed each spring. Fifteen thousand years ago, ocean sticklebacks all looked pretty much the same. Then came the end of the ice age, and glaciers started to recede. That created a number of new streams, lakes, and coastal estuaries, all potential new homes for sticklebacks. Each of these new environments presented challenges. Different colored water and vegetation required different coloration to make it possible for the sticklebacks to avoid detection by predators. The various predators prompted the stickleback to evolve different kinds of body defenses, such as changes in skeletal armor that make the sticklebacks harder to catch. In some places, merely being a larger size was adequate to allow the sticklebacks to thrive.
Kingsley wanted to find the genes that were responsible for all of these changes, because he hoped that they would allow him to track down the kinds of genes that also changed when humans made a similar migration from one fairly homogenous environment to a variety of new environments with new challenges. That migration took place about a hundred thousand years ago, when our ancestors left Africa. As we moved away from the intense sun near the equator, we lost some of our melanin, which had protected our skin from all of that sunlight. Colder climates also resulted in thicker hair and stockier builds. Even our diets changed, requiring new sets of enzymes to help us digest our food.
Kingsley has collected sticklebacks from all over the world, in a variety of shapes and colors. Just as Gregor Mendel crossed pea plants to find genes, Kingsley crossbreeds sticklebacks and tracks the genes he’s interested in through successive generations. He breeds the fish in dozens of thirty-gallon tanks in the basement of the Stanford Medical School.
Nikolaas Tinbergen’s mail truck story sounds like one of those apocryphal tales that’s simply too good to be true. According to Alun Anderson, however, it is true.
Anderson is a science journalist and author, but before he began his journalism career, he earned a Ph.D. in ethology, the study of animal behavior. “I worked in the same laboratory at Oxford as Niko from 1972 to 1976,” says Anderson. “Niko told me that growing up in the Hague, he had loved to bring home sticklebacks from local streams in a jam jar and watch them.”