Our sense of smell is “devolving”

ResearchBlogging.orgHumans have a really poor sense of smell, as anyone who hasn’t been living under a rock surely knows. It just takes a brief glance at the many animals we keep as pets to realise that our olfactory senses are pretty shabby. The police rely on sniffer dogs to identify illicit substances, not specially trained police officers. Whilst few smell receptors is pretty common amongst primates (who instead have better vision, which is more useful for swinging through trees), humans have few receptors even by monkey standards. Now new research by an international team of scientists have identified that this isn’t the result of chance but evolution is actually decreasing our sense of smell. What’s more it shows no signs of slowing down and it would seem we’ll continue to get worse at smelling into the foreseeable future.

A blind cavefish along with their close relatives from lighter waters who have retained eyes

Natural selection is, after all, a two way street. The “fittest” organisms are more successful and reproduce more so subsequent generations will have a higher proportion of these fitter individuals. Eventually the vast majority of the population will possess the fitter traits, at which point they are said to be “fixed.” But fittest doesn’t always mean better. Indeed, creating an organism costs time and effort and if corners can be cut without sacrificing fitness then doing so will be the best tactic. That’s why many cave dwelling animals have lost their eyesight; it simply isn’t useful in the darkness and so the energy it takes to grow eyes would be better spent elsewhere.

This loss of function occurs via negative selection, which can happen through two mechanisms. First, if a trait is particularly detrimental then those who possess it will die or not be as successful at reproduction, gradually removing it from the gene pool. This process not only eliminates harmful traits but also harmful mutations. If an organism suffers from a genetic defect which stops a crucial body part developing then they are unlikely to survive so this mutation will not be passed on. As a result of this negative selection not only removes harmful traits but also keep the gene pool pure. Which brings us on to the second process. If this genetic defect damages a part of the body which is useless then the organism can still survive to pass on this trait, allowing it to spread through the population and sometimes rise to fixation by pure chance. Alternatively, if not developing the trait is beneficial because it reduces the cost of building and maintaining the organism then positive selection could also become involved.

When natural selection is relaxed it will allow deleterious mutations to survive

This second process is known as a “relaxation” of negative selection (since it simply doesn’t bother removing bad traits, which is quite laid back) and appears to be the process by which our sense of smell is getting poorer. An international team of scientists studied the genomes of 1,301 individuals and found that genes associated with our sense of smell had more deleterious mutations than natural selection should allow. These mutations would severely compromise our olfactory ability, rendering many of our smell receptors useless. With no evidence of positive selection for these detrimental mutations the team was forced to conclude that this is a result of the relaxation of negative selection. Damaging our olfactory ability does not harm our chances of survival so they survive to be passed on.

Further, the researchers noted that the relaxed negative selection shows no signs of “tightening” and that we have not reached the point where these changes start to eat into our ability to survive. In other words, our sense of smell can continue to get worse and negative selection will still do nothing about it. Our sense of smell will continue to deteriorate. The researchers postulate that natural selection tolerates this “devolution” because our sense of smell isn’t as useful as it used to be. As I already mentioned, apes have focused more on their visual ability to better travel through trees. On top of that, we’re now bipedal and so our nose is a lot further from the ground than it used to be. We aren’t going to be smelling much up here.

The olfactory system has more damaging mutations than other senses

But it’s not all doom and gloom, other researchers disagree with this conclusion. Our decrease in smell receptors has been known about for a long time but some scientists believe that this is compensated for by improvements in our brain. After all, our brain – including the bits responsible for smell – has more than tripled in size since our lineage diverged from the rest of the apes and this could well have enabled us to continue to smell quite well. Unfortunately there’s little evidence that this is the case and it remains a hypothesis. As far as we know our sense of smell has gotten worse and it would appear that it will continue to do so.

My one qualm with their study is that their conclusion about whether a particular mutation is damaging is based off various computer predictions. With my limited understanding of genetics I can’t really say if this is a good or a bad thing, but it does open up the possibility that they may be wrong. However, from what I can gather this does seem to be a fairly common and reliable practice. So it would seem this one concern does not overturn their results. Our sense of smell is indeed devolving and will likely continue to do so.

Pierron D, Cortés NG, Letellier T, & Grossman LI (2012). Current relaxation of selection on the human genome: Tolerance of deleterious mutations on olfactory receptors. Molecular phylogenetics and evolution PMID: 22906809

13 thoughts on “Our sense of smell is “devolving”

  1. Devolution is an interesting topic. In most cases things have wound downward not upward over time. For example the first Dinosaur (latin for “terrible lizard”) was calle Iguanidon as it was noted to be identical only more than 10x larger than todays Iguana. It is also this initial discovery that has caused the common belief that these terrible lizards had leather skin with scales. But the Iguanadon has seeemingly devolved into todays Iguana. This is also the case for sharks which have devolved in size and plant species which have much smaller leaves than they did 6000 years ago. There is a scorpian that was found in an underground cavern system which appears to have lost its eyes over time. Wholly mammoths have lost their hair and are much smaller today (now called elephants). Humans that used to live up to 1000 years are now living only 80 years on average. The Neandrethal is an example of ancient humans that lived a long life. We know this because of their lengthened noses and their extra bone along the eyebrows. (It has been observed that noses never stop growing all of our lives and that extra bone accumulates above the eyebrows all of our lives.) Today no one lives long enough to develope the skull features of the Neandrethal. Opposite to what Darwin believed it appears that things are increasingly becomming reduced rather than improved over time.

    • You’re saying that the Iguanadon turned into the Iguana, and it’s “identical only more than 10x larger than todays Iguana”? You appear to be quite stunningly misinformed on that, and all your other examples.

    • I too find devolution an interesting topic, mostly because it is often overlooked but also because – when it is discussed – it is often surrounded by misconceptions. You raise one of the more common ones: that “devolving” does not result in improvement.

      What people fail to realise is that building and maintaining an organism requires an extraordinary amount of energy; energy which cannot be spent on other useful things like reproduction, hunting etc. Any unnecessary trait quickly becomes a liability and “devolving” it actually results in a better, more efficient organism. It’s a lot like how a racing enthusiast might pull out all the extra seats from their car to lighten it. Granted you’ve lost the ability to ferry passengers around but that loss of weight might give you the edge you need to win the race. Overall you’re better for it.

      Unfortunately your mistakes don’t end there. As EOICR pointed out, the comparison between iguanas and the Iguanodon was based off limited data. The vast majority of their anatomy differs significantly. For example, not the difference between a classic “lizard hip” and the “bird hip” of the Iguanodon. These are fundamental differences between the two animals which cannot be resolved by simply adding a thousand year life span to the equation.

      Whilst I can’t comment on your references to plants and scorpions since you don’t provide specifics I am familiar with the classic “neanderthals were just old humans line.” This came from, iirc, the book “Buried Alive” in which the author analysed the dimensions of neanderthal skulls and applied human rates of growth to them. For example, a mature adult neanderthal’s skull is 16mm longer than a sub-adult neanderthal’s. The human skull lengthens during this period of the life-span by ~0.06mm per year, making the mature adult 287 years older than the other! However, if one performs the same analysis for skull height, rather than length, the difference is 137 years. Each part of the skull you compare seems to have aged a different rate.

      Clearly the conclusion is not “neanderthals were really old” but “applying human rates of growth to a non-human species doesn’t work.” If the former were true then one would expect some consistency between measurements, but there isn’t. A more plausible explanation is that neanderthals matured at a different rate to modern humans.

  2. Pingback: Devolution « Truth Is What Matters

  3. An interesting question is whether there even will be a point where natural selection against deletorous smell-related mutations finally kicks in. How much do you really need smell, even when its relation to taste is considered?

    Computer models is probably the best they can do, as you can’t really do an expirement and monitor all the variables here. That’s not to say it’s unreliable – you saw, I presume, that miRNA thing on the SC the other day? In that paper they did some (computer based) predictions about the effect the new miRNA would have on existing genes that by chance would respond. These were confirmed by their testing, so I’d say that computer predictions probably work at least some of the time.

    • I do believe some people lack a sense of smell entirely and seem to get through life fine. However, there may be some events – such as smelling smoke – where smelling is beneficial which aren’t detectable when looking at anecdotal data. If 1/100 is saved by their sense of smell then it may be preserved in some form, but examining it on a case by case basis might not reveal this fact.

      I’m just weary of computer programs whose basic principles I can’t understand. And boy, do I not get genetics. Nonetheless, it seems to be a common and reliable practice so I’m going to bury my gut instinct.

  4. One interesting thing about some cave fish is that the eyes disappear faster than would be expected. The reason for this was that some gene in the eye developmental process has a negative impact on smell development. This means that the fishes’ eyes are strongly maladaptive in the dark, rather than being a relatively minor waste of physiological resources. (Sorry can’t find my ref on this.)

    • Neanderthals had developed larger eyes and visual cortices than humans, most likely to deal with the lower light levels at northern latitudes. Although we don’t have to go hunting in dark winters anymore, vision is still very important given the prevalence of reading and modern things. Might this link between smell and sight also be present in humans, and might it be working in reverse with us trading smell for vision?

  5. Loss of function is very interesting as is selection (positive, negative, relaxed, etc)! The blind cave fish are interesting–one of my colleagues studies them and says that, in his populations in Mexico, they have taste receptors on their faces! There’s a bigger story about human olfactory evolution than presented in the article you used for the blog though. There are studies that show positive selection for olfactory receptor expansion after a population crash out of Africa and, if you look at comparative functional genomics studies, our number of olfactory receptors is on par (and better than) many other primates. Our olfactory tissues are relatively not well enervated though and they take up less square footage in the nose than many of our mammal relatives–this reduces our threshold capacity (concentration at which we can detect an odor)–but we can track odors in nature like a dog (the famous Stanford study with students tracking chocolate in the grass!) and our OR subgenome is as varied as the immune system with a tendency to heterozygosity! Of course, I am biased in defending our olfactory power and complexity since I research this 🙂 Nice blog!

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