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> It is done by less than one trillion neurons:
> 
> https://phys.org/news/2019-01-genes-nose-scents.html
> 
> "The human nose can distinguish one trillion different scents—an extraordinary feat that requires 10 million specialized nerve cells, or neurons, in the nose, and a family of more than 400 dedicated genes. But precisely how these genes and neurons work in concert to pick out a particular scent has long puzzled scientists.
> 
> ***
> 
> "But now, a Columbia study in mice has uncovered a striking resourcefulness: by rearranging itself in three-dimensional space, the genome coordinates the regulation of these genes in each neuron, thereby generating the biological diversity needed to detect the scents we experience. 
> 
> ***
> 
> "Smell, also known as olfaction, is mind-bogglingly complex. The receptors in our noses must not only identify a scent, but also gauge how strong it is, scan our memories to determine whether it has been encountered before, and determine if it is pleasing or toxic.
> 
> "Olfactory receptor neurons, specialized nerve cells that snake from the nose to the brain, make all this possible. And though each neuron contains the full suite of the 400 dedicated olfactory receptor genes, only one of these genes is active in each neuron. Adding to the confusion: the gene that is active appears randomly chosen, and differs from neuron to neuron.
> 
> "This unusual pattern of gene activity is known as the "one gene per neuron" rule, and has long been a focus of study by scientists such as Dr. Lomvardas. Indeed, deciphering how each olfactory receptor neuron manages to activate only one of these genes—and how this process results in such a finely tuned sense of smell—remained mysterious for decades.
> 
> ***
> 
> "Enhancers are not themselves genes but regulate the activity of genes.
> 
> "'We previously discovered a group of enhancers, we named the Greek Islands, located near the various olfactory receptor genes," said Dr. Horta. "This work showed that these enhancers create hotspots of activity to regulate the "chosen" olfactory receptor gene.
> 
> "The team also found that the protein Ldb1 plays a key role in this process. It holds the Greek Islands together, allowing them to switch on a specific olfactory receptor gene that then—as a team—interpret the particular scent at hand.
> 
> "'These teams of genes endow the olfactory system with the ability to respond in diverse ways," said Dr. Monahan. "The flexibility of this process could help to explain how we easily learn and remember new smells.'"
> 
> Comment: This type of gene alteration in cooperative design allows for the enormous number of scents we learn to recognize. Note memory has to be involved. This has to be a designed system.