Olfactory enhancementSaturday, 24 October, 2009
Sorry I haven’t posted in a while, I’ve been busy. In the meantime, here is a draft of a post I’d written a while ago about enhancing the human sense of smell:
Compared to that of most other mammals, the human sense of smell is generally considered pitiful. It is estimated that dogs can smell up to hundreds or thousands of times better than humans. Why is this?
While smelling is a useful way to investigate the world, it’s not quite as useful as vision or even hearing. With a brain big enough to process the information, it makes more sense to dedicate resources to visual and auditory processing rather than chemosensation (olfaction – smell – and gustation – taste). Animals like dogs use smell as a means of investigating objects and for communication. Primates, such as humans, have evolved a sophisticated visual system that, in combination with our large brains and the use of hands, provides more information about objects, and language provides a better means of communication. Indeed, evidence suggests that the evolution of trichromatic vision (the ability to see the full spectrum of red, green and blue colours) was a major cause of the loss of the sense of smell (Gilad et al, 2003).
Olfaction took a backseat to vision in higher primates, but why should it then have decayed, rather than just stagnated? As alluded to above, it could be that it was more efficient to use the brain for vision rather than olfaction. There is limited space available in the skull and a brain takes up a lot of energy, so you’d want to get the best out of the brain as possible. Alternatively, evolutionary psychologists have theorised that for apes, because they usually live in groups, the ability to smell others (and thus experience the emotional reaction towards them) became a disadvantageous trait. That is, before showers were invented, living in a close-knit group with active and hairy apes would have been most unpleasant for those apes with the best sense of smell, but living in crowds would be easier for those with poor ability to smell. So evolutionarily, it was easier to lose the ability to smell, and with it the evoked emotions, and to rely on more specific means of choosing a mate or friends (like visual cues or using language).
Yet smell (and also taste) can have some significant advantages over the other senses, which is why dogs are used as sniffer dogs to detect illicit items or blood and as search dogs to track down missing people or vigilantes. Some things are invisible yet can be smelled, such as toxic gases, allowing a supersmeller to tell if food is safe to eat or if a drink has been spiked. An odour will linger, and thus can provide clues about the past where a visual inspection would not be useful. Perhaps you could use use your enhanced sense of smell to track down your friends if they desert you, or determine who used your computer without your permission. Odour molecules also can pass through opaque surfaces (like fabrics, or dirt). Imagine being able to use smell to work out which of piece of luggage has your mobile phone inside, or detect if a house is infested with termites, or locate landmines burried underground.
So, enhancement of the human olfactory system may prove to be a very simple yet effective improvement to our sensory abilities.
And it might just be simple to do too. Likely the most important factor governing how effectively odours can be recognised is the variety of olfactory receptors (ORs), as an odorant molecule in the air has to bind to a receptor in order to be detected. Humans only have 300 functional OR genes, whereas dogs have about 800 and mice have about 1000 (Niimura & Nei, 2007). Comparative genetic studies have shown that humans still have the remnant of the vast repertoire of olfactory receptors found in animals such as mice, but 70% of those genes are inactivated (Rouquier et al, 2000). These inactivated genes, known as pseudogenes, still remain in the genome, but have been mutated so severely over the course of evolution that they can no longer function. But enough of each gene remains that a good guess can be made about what the gene was, and these functional versions of genes are most likely still found in rodents or canines (in which evolution was selecting for OR integrity). There are about 800 functional and non-functional olfactory receptor genes still left in the human genome, so restoring the 70% of pseudogenes with their functional equivalents would give a human enhanced in this way the ability to detect almost three times the number of distinct odorant molecules.
Evolution, however, created these olfactory receptor genes during the distant past, explaining why we can distinguish between many types of fruit or flowers by smell but we are limited in our ability to distinguish cleaning solvents. Many odorant molecules commonly encountered in the modern world would not be recognised. Strong-smelling (or tasting) compounds are added to many toxic chemicals in order to allow us to detect and recognise them (e.g. natural methane gas and hydrogen gas are odorless, but odorants butanethiol or tetrahydrothiophene are added in very minute concentrations so that humans can detect gas leaks). A promising endeavour, therefore, might be to design (or evolve) odorant receptors for dangerous artificial compounds.
It should be stressed, however, that the number of olfactory receptors is not the only factor that governs how well we can smell. The entire olfactory pathway is important. Deletion of the a subtype of voltage-gated potassium (Kv 1.3) channel in mice produces super-smeller mice’ that are 1000-10,000 times more sensitive to odours (Kadool et al, 2004). The researchers hypothesise that this potassium channel is involved in olfactory signal transduction (the process by which the binding of an odorant molecule causes the olfactory receptor cell to fire an action potential back to the brain) and the gene for it need only be deleted, or downregulated, in the olfactory bulb to produce this olfactory enhancement. The position and size of the noses could also be important, as many mammals have noses which are closer to the ground and therefore pick up many more odorant molecules than humans, with our upright bipedal posture, do. This is perhaps not such a big problem because we can work out which objects we want to smell and use our hands to bring those objects closer for inspection. In addition, associative memory and congitive discrimination of smells plays a strong role in recognition of smells (possibly humans don’t need as many odorant receptors because we have such a good memory for smells and can use our large brains to interpret the nuances of each smell). But we’d want to enhancements memory and cognition anyway, regardless of how our sense of smell might benefit.
In summary, humans have a reduced number and diversity of olfactory receptors compared to other animals, which correlates with our poor sense of smell. Why this loss occurred is debated, but there must be some disadvantages to having a great sense of smell. We need to be cautious that we don’t uncover an unbearably stinky world. But despite all that, olfactory enhancement would fill a deficit in the sensory repertoire of humans and is therefore a very useful enhancement to be investigated.