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When something dies, certain chemicals are produced as the proteins decompose. They're broken down into it's smaller molecules, amino acids, lysine, and ornithine. They break down further into two compounds: putrescine and cadaverine. These two molecules are what we associated with "death"; when they hit our noses, we react with revulsion. Humans aren't the only animals with an instinctual aversion to this smell. Proceeds of the National Academy of Sciences, for example, published a study of what happens in the brain of the Zebrafish when put in contact with cadaverine: the fish instantly swam away from just a tiny drop of it. The researchers found that a certain receptor in their brains, TAAR13c, fit exactly the compound for cadaverine. It's kind of like a lock-and-key system: the receptor only binds to that compound. While that particular receptor is only found in bony fishes, humans have similar receptors in our brains.
We probably evolved this specific receptor to help us avoid death: Putrescine and cadaverine warn us when something is wrong. Eating meat that has gone bad could potentially poison us, and when something dead is nearby it usually means there's danger afoot. The smell of dead humans is reportedly different, however, and a study published in the journal PLOS one measured tried to measure the smell of human death. Belgian researchers isolated specific compounds from decomposing human remains. They put human, pig, mouse, mole, rabbit, turtle, frog, and bird remains in separate jars and let them decompose over the course of six months. Researchers periodically took samples of the gases and found 452 organic compounds. Only eight compounds distinguished pig and human remains from those of other animals. Which makes sense they'd look for ones exclusive to pigs and humans since human and pig flesh are pretty similar. But the researchers also found five fragrant ester compounds that separated pigs from humans. These esters could be the key to the smell of human death, but more research is needed.
Have you ever experienced the smell of death? How would you explain it? Let us know down in the comments.
High-affinity Olfactory Receptor for the Death-associated Odor Cadaverine (PNAS)
"Cadaverine and putrescine, two diamines emanating from decaying flesh, are strongly repulsive odors to humans but serve as innate attractive or social cues in other species. Here we show that zebrafish, a vertebrate model system, exhibit powerful and innate avoidance behavior to both diamines, and identify a high-affinity olfactory receptor for cadaverine."
The Smell of Death (The New Yorker)
"The physics governing sound and light is much better understood and easier to manipulate than the physics governing volatile aromatic compounds, which disperse and persist in ways that are difficult to predict."