Science explainer | November 4, 2026
A rotten apple or a box of moldy oranges can make someone recoil before they have time to think. Scientists have found that unpleasant odors can set off very rapid brain signals and that one chemical associated with decomposition can influence avoidance behavior. But the claim that revulsion toward decay is the oldest survival system in the mammalian brain goes beyond what the evidence establishes.
Two peer-reviewed studies—one on the chemical putrescine and another on human olfactory-brain activity—help explain what researchers can and cannot say.
What does decay smell tell us?
Decomposition produces a mixture of odor-producing compounds. Among them is putrescine, a compound associated with the breakdown of biological material. It is not the only chemical responsible for the smell of decay, and unpleasant smells are not reliable tests for whether something is safe to eat or touch.
In a 2015 study in Frontiers in Psychology, psychologists Arnaud Wisman and Ilan Shrira conducted four experiments examining human reactions to putrescine. Compared with control conditions, exposure to the compound was associated with faster responses on a vigilance task, faster walking away from the experimental location, and increased accessibility of threat-related ideas.
The findings are evidence that people may process this decay-associated chemical as a warning cue. They do not demonstrate that everyone responds in the same way, that putrescine always signals a hazard, or that present-day experiments can precisely reconstruct a prehistoric response.
The brain can respond before conscious deliberation
A 2021 study in Proceedings of the National Academy of Sciences explored the human olfactory bulb, an early brain-processing region for smell. Researchers at Karolinska Institutet and collaborating institutions measured neural responses to pleasant and unpleasant odors.
They reported that negative odors generated rapid activity associated with motor avoidance. As Karolinska Institutet explained, the olfactory bulb sent signals toward the motor cortex within about 300 milliseconds, and participants tended to lean away from unpleasant odor sources.
This study examined unpleasant odors broadly; it did not prove that every smell of decay produces an identical neural response. Its central finding is that certain odor-avoidance signals can begin quickly and without deliberate evaluation.
What happens inside the nose?
According to the National Institute on Deafness and Other Communication Disorders (NIDCD), odor molecules stimulate specialized sensory neurons high in the nose. Those cells relay signals to the brain, where different patterns are interpreted as particular smells. The sense of smell is also closely linked to flavor and to warning signs such as spoiled food or smoke.
The NIDCD offers a short official educational video explaining the pathway from airborne odor molecules to the brain:
Watch: Journey of Smell to the Brain — NIH/NIDCD (official YouTube video).
What does evolution have to do with it?
Detecting potentially hazardous food and environments could have helped animals survive. Researchers therefore examine disgust and avoidance in the context of disease protection and the long evolutionary history of olfaction.
Yet a behavioral advantage is not a timestamp. Modern human experiments cannot establish exactly when ancestors developed avoidance of decomposition odors—or show that such avoidance represents the single oldest defense circuit in the mammalian brain.
Biology and experience both shape smell responses. Concentration, context, familiarity, individual differences and learning can all influence whether an odor is perceived as unpleasant and how someone reacts.
Seeing decay does not mean smelling the same chemical
Visible mold or rot can accompany chemical changes, but a photograph cannot reveal which odor compounds are present or establish that putrescine caused an observer’s reaction. The images in this article document food decomposition, not the scientists’ experiments.
What researchers still do not know
Scientists continue to investigate how unpleasant smells are processed, why individuals differ in sensitivity and behavior, and what happens when olfaction is disrupted by aging, illness or neurological problems.
The evidence supports a careful conclusion: smell is an evolutionarily old sensory ability, and certain unpleasant odors can trigger remarkably fast human avoidance responses. It does not substantiate a precise prehistoric origin date or an absolute ranking as the mammalian brain’s oldest survival system.
Sources
- Wisman, A. & Shrira, I. (2015). The smell of death: evidence that putrescine elicits threat management mechanisms. Frontiers in Psychology.
- Iravani, B. et al. (2021). The human olfactory bulb processes odor valence representation and cues motor avoidance behavior. PNAS.
- Karolinska Institutet: Sense of smell is our most rapid warning system (October 2021).
- NIDCD: Smell Disorders and Journey of Smell to the Brain.
- Photo sources and rights: WikiBCS, CC0 1.0 (intended featured photograph); Downtowngal, public domain (inline photograph).
