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Conventional wisdom has it that humans have a poorer sense of smell than most other animals. Sure, we can smell – most of us appreciate the aroma of our morning coffee or a delightful fragrance, and we're able to detect burning toast or a gas leak. But we have nonetheless long been thought to be relative weaklings in the animal kingdom's league of olfactory excellence, which puts dogs and rodents near the top.

But now a , , challenges this idea. It suggests the very notion that humans lag behind stems from promulgation of a 19th-century myth.

According to the , the myth can be traced to the work of French neuroanatomist . Writing in 1879, Broca divided mammals into those for whom smell is critical in day-to-day behaviour (such as dogs and rodents) and those for whom it is not (mainly primates including humans).

He based this distinction on two main pieces of evidence: the comparatively small size of the (the neural structure involved in smell) relative to total brain volume, and the observation that odour has less influence on human behaviour compared with other species. He concluded that we had effectively sacrificed our olfactory apparatus as a result of and in exchange for expansion of the higher reasoning centres in the brain's frontal lobes.

This idea that the human sense of smell is no longer needed in an evolutionary sense was subsequently embraced by other anatomists. The psychologist Sigmund Freud extended this, interpreting an interest in body smells to be a throwback of our ancestral past, even of psychiatric disorder in certain cases.

The myth continues to influence researchers. One example is how genetic analyses of olfactory gene function have been interpreted. Of around 1,000 known genes for , about 40% are non-functional "pseudogenes" – meaning they do not actually code for odour receptors in the nose. This compares to only about 18% in mice. that this indicates that natural selection in humans may have favoured senses such as vision at the expense of smell.

Surprising findings

Several lines of evidence are emerging to challenge, even overturn, this viewpoint. For example, new studies suggest that pseudogenes may in fact have some on other genes involved in smell reception – and that the proportion of functional to non-functional genes across species is neither a reliable predictor of nor of differences in among primates.

Even more interesting is that recent evidence has undermined the two foundations of Broca's original assertion. On his first point, Broca was right to some extent. Humans have relatively small olfactory bulbs when calculated as a proportion of overall brain volume. In these terms, mice have olfactory bulbs 200 times the size of humans, while those of dogs are 40 times larger.

But a that enables us to quantify the number of olfactory brain cells (neurons) in the respective bulbs of different species has yielded an extraordinary and unexpected result. Regardless of bulb size, the absolute number of neurons is remarkably similar across a range of species that otherwise vary widely in body size and apparent reliance on smell. It would appear that whether one is human, mouse, monkey or even star-nosed mole, there is a magic number of neurons – about 10m – that is necessary and sufficient to achieve olfactory function. In other words, it is the abundance of these neurons, not the size of the anatomical structures that houses them, that is important.

When it comes to how odour influences behaviour, Broca would have been intrigued to see the a wealth of new evidence showing the extent to which humans are indeed driven by smell. We now know that exposure to certain odours can induce involuntary and responses – as well as psychological outcomes involving and . Indeed, odours undoubtedly shape our actions, from our to our . For example, that when subliminally exposed to citrus-scented all-purpose cleaner, people rated cleaning as more important than they otherwise would and were more fastidious about tidying up.

Freud might have been alarmed by new knowledge showing how significant body odour is for humans – whether it be how we unconsciously smell our hands after a handshake, how it shapes our , or how we use fragrances to our own smell.

Training the nose

All this suggests we have both the capability and opportunity to put our noses to good use. So how do we actually compare with other species? Such experiments are challenging to implement, but the suggests that we can outperform even rats, monkeys and dogs in perceiving trace quantities of certain odour molecules that, presumably, are more salient in our experience.

So at least if it's chocolate, humans can follow a through a field just as dogs can track pheasants.

In such experiments it is important to note that people get significantly faster and more accurate at this tracking task after just a few trials. And, at least in my view, it is this training effect that explains how the myth about human smell has persisted for as long as it has and why it tends to ring true. The research suggests that our much-maligned nose – and all the associated neural apparatus involved in smell perception – is not really the issue. It's just that, like a muscle, we need to use it. When we do, we may be the weakling no longer.

This article was originally published on . Read the .