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You know it the moment it hits your nose. Whether you're opening a package of expired lunchmeat or passing a recent road kill, the odor is unmistakable. The smell of rot, of putrefaction. The stench of death.
Whether we're repulsed by such odors—as humans consistently are—or attracted to them as flies, rats, and other scavengers tend to be, the evolutionary roots of our responses run deep. Now, a recent study of zebrafish published in last week's issue of the Proceedings of the National Academy of Sciences has uncovered the type of neurological trigger that may be at the heart of our powerful innate reactions to the slightest whiff of rot.
Although we encounter thousands of odiferous chemicals every day, many foul-smelling in their own right, two compounds—the aptly named putrescine and cadaverine—are largely responsible for the smell of rotting flesh. First discovered in the late 19th Century, these compounds result from the bacterial breakdown of amino acids, the building blocks of proteins.
In humans and many other animals, the close association between these chemicals and rotting flesh has evolved to become a powerful signal—a warning of the presence of pathogens and the diseases they carry. Ignoring these signals could lead to the death of some individuals. This gives those who heed the warning an advantage, reinforcing the signal's importance. As a result, we tend to steer clear of things that smell bad. It's a basic but powerful instinct, and only occasionally overridden by the culinary indulgence in such foods as Camembert, kimchi, and durian. But what goes on in our brains to signal a "bad smell" vs. a "good smell"?
Scientists at the University of Cologne conducting this recent study found that zebrafish also have a negative reaction to putrescine and cadaverine, swimming away at even the slightest whiff. And because the fish have olfactory systems surprisingly similar to ours, they're good models for how humans process these smells.
Neurons in both the human and zebrafish olfactory systems are studded with millions of different proteins, each receptive to a particular chemical structure. It's a lock-and-key type mechanism and when a chemical key fits, a signal is transmitted along the neuron to the brain where it can be processed and interpreted more fully as a smell.
Now for the first time in any species, the researchers have identified in zebrafish a receptor—known as TAAR13c—into which cadaverine fits. While TAAR13c is unique to bony fishes, humans and other mammals have similar receptors that might serve in the detection of cadaverine. The discovery of this trigger is an important step toward understanding the molecules and neural networks involved in processing these chemical signals and driving our responses of disgust or attraction whenever we encounter them.
Steven Bedard is editor of the Academy website. A recent Bay Area transplant, he now understands what all the fuss is about.
Image: Larry Lamsa/Flickr