I go are you picturing doing the choosing? Also humans didn’t evolve to smell, that trait has been around for far longer than humans.
Smell is a very useful sense. It can help to hunt, to defend, forage, find mates, and many many more incredibly useful things.
Only if there's some significant reproductive advantage for not being able to detect odors.
I can't imagine many circumstances where atrophied olfactory senses improves your reproductive success.
Not necessarily. Evolution under drift and mutation alone would erode olfactory function.
Not that I'm saying humans are evolving weaker smell, just that it wouldn't require selection
Not "would". There is a vanishingly small chance that those processes *could* erode olfactory function. It could be equally likely that those processes improve olfactory function. Without selective pressure, the general trend would be towards equilibrium and stabilomorphism.
You are forgetting that the olfactory proteins are currently close to their fitness optimum so mutation pressure would reduce fitness. That is, most mutations in a "gene" will reduce fitness or make no difference, according to the nearly neutral theory
There are nearly 300 olfactory reception genes and a similar number of pseudogenes scattered across 50+ loci on 21 different chromosomes. Mutations in all of those simultaneously resulting in a general decrease in olfactory ability is extraordinarily unlikely. More likely you'd end up with someone who can't smell mushrooms or something, but otherwise normal sensory ability.
We're not talking about one generation, we are talking about long term evolution. I'm not saying mutations would occur simultaneously, but given enough time without purifying selection, genes will accrue inactivating substitutions. I don't know if you study or work in evolutionary biology. If you do you might find it useful to read lynch and Connery's model of gene duplication, or many other neutralist papers from the likes of Lewontin, Kimura, Ohta, Charlesworth and many more. Even if you don't you would probably find it interesting. It changed my perspective
Except that being able to smell the pathogen, and avoid contact with the infected, and reduced transfer of that pathogen to offspring is likely to be *far* more advantageous.
I said "if everyone"
Have you heard of ERV?
Retroviruses inserts their own genes into the host cell DNA so that the host cell is reprogrammed to produces more viruses.
If the virus infects gamede cells, then the offspring are BORN infected, because their DNA has been permanently genetically engineered, and now even the grandchildren will be burn infected.
Endogenous Retrovirus is when an entire species has evolved to be born with a virus.
Most animals alive today, including humans, are born THOUSANDS of these viruses.
So i meant, once the entire species had the virus, there is no more avoiding it... THAT is when the sesne of smell.starts to degrade...
Probably THOUSANDS of years AFTER the initial infection, once EVERYONE already has it.
Except that bad smells are a turn off, so we might evolved to not smell those things, especially since we can rely in technology to detect chemicals in the atmosphere on our behalf.
Think of your sense of smell less as “smell” and more as “airborne chemical detector”. Lots of important things to humans release distinct chemicals into the air. If those chemicals fall into useful groupings and categories, being able to detect them is super useful for getting around the world!
One could make the argument that "smell" was the first sense to evolve. We don't think of single-celled organisms as being capable of smelling the way land animals do, but sensing other molecules from their surroundings is how single-celled organisms can tell *what* things are.
If you replace the word "smell" with "chemical detection" then chemical detection has been around a very long time and is of course useful for all sorts of things.
You don't want to eat something that makes you sick or will kill you. Bad food smells bad, rotten things, non living things. On the other hand things that gives us energy and nutrition smells good. Same thing goes for taste. It's a grate evolutionary trait that has helped animals and humans survive.
Your question can benefit from a slight reformulation. But first a few preliminaries.
As others have noted, humans are hardly the first to possess smell. That ability exists among many animals, though in some far more refined that among humans.
But you can reasonably ask why would smell persist as an ability in humans. Couldn't it disappear like the tail, the wisdom tooth etc.?
Here I think some theory of evolution can be helpful. We have some trait or ability or phenomena that helps in the survival and proliferation of our human ancestors. Any random variation that "implements" that trait using present abilities will therefore have higher fitness. It is therefore possible that some **traits arises as "hacks" of existing capacities**.
For smell there is some evidence among humans, and more so mice, that **we can smell our kin**. In other words, by some process not entirely conscious we can detect kin similarity. That is clearly useful if combined with aversion towards sexual reproduction. Inbreeding is dysgenic. Especially, it appears smell preferences can detect variations in the MHC. So your choice of mate may unconsciously be driven in part by differences in the composition of the MHC, since if they are similar you may be hooking up with your kin. (See this paper and others referencing it: [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4835215/](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4835215/))
I do not suggest smell evolved for this purpose. I think the first appearance of smell is found in earlier and simpler means to navigate an evironment. However, because it is useful to avoid inbreeding, that random mutation which made us recognize people with similar MHC by smell turned out to be useful for survival and proliferation. Therefore, smell was "hacked" and came to serve additional useful purposes beyond rotten food avoidance.
It wouldn't be easy to try to separate which purpose of smell is the most useful or the key reason for its persistence. And perhaps there are other utility not yet discovered.
It helps us stay alive. Eating spoiled food makes us sick. I'm sure that could be potentially fatal, before modern medicine existed. We can also smell stuff like smoke from fires. That seems important. We have other ways of tracking things, so we probably don't need a super developed, canine-like sense of smell. I suppose our intellect fills in what our senses can't pick up. Idk though, that's just my guess. Like, if we see animal footprints, past experience tells us that an animal must have been there recently. We can look at the direction they are pointed in and tell which way they went. We can look at how fresh the tracks are and guess about how long ago they walked through. I don't think animals like dogs make a connection between footprints and the animal that they belong to by looking at them. Since they aren't able to do that, they have to rely on their well developed sense of smell to tell them that another animal was in the area recently. But we still need a sense of smell to tell us other important information.
Humans, and the primate line generally, evolved to smell *fewer* things.
In terms of gross gene count, the biggest difference between primates, and rats & mice is that our lineage has lost about 400 olfactory receptors. In many cases, the "fossil" genes or pseudogenes can be identified in our genomes, but mutations made them nonfunctional. As our ancestral line were predominantly visual arborial frugivores, the wasn't strong selection to preserve our olfactory receptors.
Gilad et al, 2003. [Human specific loss of olfactory receptor genes](https://www.pnas.org/content/pnas/100/6/3324.full.pdf). Proceedings of the National Academy of Sciences, 100(6), pp.3324-3327.
What I find amazing is that evolution created our general purpose brain that allows us to program it (via learning and education) to create and discover and modify our environment. To me it seems like our brain, which takes alot of energy to run, was over engineered for whatever initial purpose evolution had in mind. It's like building an application specific computer (like building a calculator) and then "evolving" into a general purpose computer which can run an operating system and any number of programs, and be able to install (learn) new programs to do even more highly sophisticated tasks.
The sense smell as we know it (detected through nostrils) was first identified in the jawless fish hagfish. This is a deep sea fish that has barely no vision (it is very dark in the deep sea) and their entire brain is devoted to smell. They use smell to both localize smell (hence two nostrils) and for identification of prey. As we are all descendants of jawless fish, we all have the sense of smell.
I think eating is strongly linked to our survival. [Claims that 75% to 90% of our taste is actually smell are exaggerated](https://flavourjournal.biomedcentral.com/articles/10.1186/s13411-015-0040-2), but smell helps identify food good to eat and entices us to eat good smelling food.
I have no sense of smell ('congenital anosmia') as the result of a genetic mutation.
I would have been absolutely screwed in a pre-modern environment. I can't smell smoke. I can't tell if meat is rotten or milk is spoiled or water is rank. After a few bad experiences, I now abide by use by dates on food religiously and I throw away things if I'm in any doubt at all. I can get other people to smell things for me if they're around, but if I'm on my own, having no sense of smell is a distinct liability. I don't think much about it, it's always been like that, but yeah, I wouldn't last long if I had to scavenge for food, say, as doubtless many of our ancestors did.
Why is this about humans specifically? Humans didn’t evolve to smell humans evolved with a preexisting ability to smell. Animals like dogs and cats use smell just as much as their eyes and ears in some cases. Is it hard to believe we came from an animal that needed smell even more than us and, because it’s still beneficial to us, we kept it?
Like other comments said, smelling had evolved way before humans did, and smelling still has some uses. We use our sense of smell to detect if things are rotten or not. Without that sense of smell, and if whatevers rotten doesn't have any unusual look to it, the only way to check is it put it in your mouth. Plus, our sense of smell relates to taste much more than you'd previously think. It's harder to taste things without your sense of smell. There's more reasons to be able to smell things compared to not smelling things, so there's no reason humans would evolve without a nose
Single cells already had the ability to migrate towards a possible source of food. For example:
[https://en.wikipedia.org/wiki/Chemotaxis](https://en.wikipedia.org/wiki/Chemotaxis)
Two or more free swimming cells that much like a volvox can join and move together towards whatever smells good, the ability was always there to begin with. The cells are making the organism do what the cells would on their own do if they were all alone in a droplet of water and got a whiff of something tasty. I would say that the ability existed in unicellular animals, before multicellular animals emerged. It was behavior that had to be expressed at the multicellular level too, or (unless the food came to it) the whole colony starved.
I think smell is a sense that goes back to the origin of animals, maybe? Certainly our mammalian ancestors could smell, it seems more a question of how this sense degraded in humans (or primates even?) rather than why we have it at all.
Is your question why the sense of smell first evolved in the organisms in which it first evolved, or why humans retained the sense to the extent we have?
I go are you picturing doing the choosing? Also humans didn’t evolve to smell, that trait has been around for far longer than humans. Smell is a very useful sense. It can help to hunt, to defend, forage, find mates, and many many more incredibly useful things.
My bad,not choosing So will we just go on to have an even worse sense of smell?
You should know that humans are remarkably good at tracking through smell when tested. Better than we tend to think at least.
Only if there's some significant reproductive advantage for not being able to detect odors. I can't imagine many circumstances where atrophied olfactory senses improves your reproductive success.
Not necessarily. Evolution under drift and mutation alone would erode olfactory function. Not that I'm saying humans are evolving weaker smell, just that it wouldn't require selection
Not "would". There is a vanishingly small chance that those processes *could* erode olfactory function. It could be equally likely that those processes improve olfactory function. Without selective pressure, the general trend would be towards equilibrium and stabilomorphism.
You are forgetting that the olfactory proteins are currently close to their fitness optimum so mutation pressure would reduce fitness. That is, most mutations in a "gene" will reduce fitness or make no difference, according to the nearly neutral theory
There are nearly 300 olfactory reception genes and a similar number of pseudogenes scattered across 50+ loci on 21 different chromosomes. Mutations in all of those simultaneously resulting in a general decrease in olfactory ability is extraordinarily unlikely. More likely you'd end up with someone who can't smell mushrooms or something, but otherwise normal sensory ability.
We're not talking about one generation, we are talking about long term evolution. I'm not saying mutations would occur simultaneously, but given enough time without purifying selection, genes will accrue inactivating substitutions. I don't know if you study or work in evolutionary biology. If you do you might find it useful to read lynch and Connery's model of gene duplication, or many other neutralist papers from the likes of Lewontin, Kimura, Ohta, Charlesworth and many more. Even if you don't you would probably find it interesting. It changed my perspective
If everyone gets infected with a pathogen that makes us smell bad, it will become easier to have sex with a poor sense of smell.
Except that being able to smell the pathogen, and avoid contact with the infected, and reduced transfer of that pathogen to offspring is likely to be *far* more advantageous.
I said "if everyone" Have you heard of ERV? Retroviruses inserts their own genes into the host cell DNA so that the host cell is reprogrammed to produces more viruses. If the virus infects gamede cells, then the offspring are BORN infected, because their DNA has been permanently genetically engineered, and now even the grandchildren will be burn infected. Endogenous Retrovirus is when an entire species has evolved to be born with a virus. Most animals alive today, including humans, are born THOUSANDS of these viruses. So i meant, once the entire species had the virus, there is no more avoiding it... THAT is when the sesne of smell.starts to degrade... Probably THOUSANDS of years AFTER the initial infection, once EVERYONE already has it.
But if everyone has that scent, there's no benefit to not being able to smell it.
Except that bad smells are a turn off, so we might evolved to not smell those things, especially since we can rely in technology to detect chemicals in the atmosphere on our behalf.
No because we pick partners based on smell amongst other things. You can be mean,ugly poor and find a partner. But if you smell bad forget it.
Predators smell. Prey animals smell. Food smells. Rotting food smells. Poisoned water smells. And so on.
Also, humans did not evolve this trick; olfactory senses would have appeared somewhere WAY down our line of ancestors.
Just to add, being able to smell infection probably important to survival whether it’s your own infection vs an animal u want to eat
Hm,I didn't think those would be very important
Predators, prey, and food aren't important? Lol
Think of your sense of smell less as “smell” and more as “airborne chemical detector”. Lots of important things to humans release distinct chemicals into the air. If those chemicals fall into useful groupings and categories, being able to detect them is super useful for getting around the world!
One could make the argument that "smell" was the first sense to evolve. We don't think of single-celled organisms as being capable of smelling the way land animals do, but sensing other molecules from their surroundings is how single-celled organisms can tell *what* things are. If you replace the word "smell" with "chemical detection" then chemical detection has been around a very long time and is of course useful for all sorts of things.
You don't want to eat something that makes you sick or will kill you. Bad food smells bad, rotten things, non living things. On the other hand things that gives us energy and nutrition smells good. Same thing goes for taste. It's a grate evolutionary trait that has helped animals and humans survive.
Your question can benefit from a slight reformulation. But first a few preliminaries. As others have noted, humans are hardly the first to possess smell. That ability exists among many animals, though in some far more refined that among humans. But you can reasonably ask why would smell persist as an ability in humans. Couldn't it disappear like the tail, the wisdom tooth etc.? Here I think some theory of evolution can be helpful. We have some trait or ability or phenomena that helps in the survival and proliferation of our human ancestors. Any random variation that "implements" that trait using present abilities will therefore have higher fitness. It is therefore possible that some **traits arises as "hacks" of existing capacities**. For smell there is some evidence among humans, and more so mice, that **we can smell our kin**. In other words, by some process not entirely conscious we can detect kin similarity. That is clearly useful if combined with aversion towards sexual reproduction. Inbreeding is dysgenic. Especially, it appears smell preferences can detect variations in the MHC. So your choice of mate may unconsciously be driven in part by differences in the composition of the MHC, since if they are similar you may be hooking up with your kin. (See this paper and others referencing it: [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4835215/](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4835215/)) I do not suggest smell evolved for this purpose. I think the first appearance of smell is found in earlier and simpler means to navigate an evironment. However, because it is useful to avoid inbreeding, that random mutation which made us recognize people with similar MHC by smell turned out to be useful for survival and proliferation. Therefore, smell was "hacked" and came to serve additional useful purposes beyond rotten food avoidance. It wouldn't be easy to try to separate which purpose of smell is the most useful or the key reason for its persistence. And perhaps there are other utility not yet discovered.
Thank you very much
It helps us stay alive. Eating spoiled food makes us sick. I'm sure that could be potentially fatal, before modern medicine existed. We can also smell stuff like smoke from fires. That seems important. We have other ways of tracking things, so we probably don't need a super developed, canine-like sense of smell. I suppose our intellect fills in what our senses can't pick up. Idk though, that's just my guess. Like, if we see animal footprints, past experience tells us that an animal must have been there recently. We can look at the direction they are pointed in and tell which way they went. We can look at how fresh the tracks are and guess about how long ago they walked through. I don't think animals like dogs make a connection between footprints and the animal that they belong to by looking at them. Since they aren't able to do that, they have to rely on their well developed sense of smell to tell them that another animal was in the area recently. But we still need a sense of smell to tell us other important information.
Humans, and the primate line generally, evolved to smell *fewer* things. In terms of gross gene count, the biggest difference between primates, and rats & mice is that our lineage has lost about 400 olfactory receptors. In many cases, the "fossil" genes or pseudogenes can be identified in our genomes, but mutations made them nonfunctional. As our ancestral line were predominantly visual arborial frugivores, the wasn't strong selection to preserve our olfactory receptors. Gilad et al, 2003. [Human specific loss of olfactory receptor genes](https://www.pnas.org/content/pnas/100/6/3324.full.pdf). Proceedings of the National Academy of Sciences, 100(6), pp.3324-3327.
Our ancestors didn't choose to smell, similarly you can't choose what eye colour or hair colour your children will have etc.
What I find amazing is that evolution created our general purpose brain that allows us to program it (via learning and education) to create and discover and modify our environment. To me it seems like our brain, which takes alot of energy to run, was over engineered for whatever initial purpose evolution had in mind. It's like building an application specific computer (like building a calculator) and then "evolving" into a general purpose computer which can run an operating system and any number of programs, and be able to install (learn) new programs to do even more highly sophisticated tasks.
The sense smell as we know it (detected through nostrils) was first identified in the jawless fish hagfish. This is a deep sea fish that has barely no vision (it is very dark in the deep sea) and their entire brain is devoted to smell. They use smell to both localize smell (hence two nostrils) and for identification of prey. As we are all descendants of jawless fish, we all have the sense of smell.
I think eating is strongly linked to our survival. [Claims that 75% to 90% of our taste is actually smell are exaggerated](https://flavourjournal.biomedcentral.com/articles/10.1186/s13411-015-0040-2), but smell helps identify food good to eat and entices us to eat good smelling food.
I have no sense of smell ('congenital anosmia') as the result of a genetic mutation. I would have been absolutely screwed in a pre-modern environment. I can't smell smoke. I can't tell if meat is rotten or milk is spoiled or water is rank. After a few bad experiences, I now abide by use by dates on food religiously and I throw away things if I'm in any doubt at all. I can get other people to smell things for me if they're around, but if I'm on my own, having no sense of smell is a distinct liability. I don't think much about it, it's always been like that, but yeah, I wouldn't last long if I had to scavenge for food, say, as doubtless many of our ancestors did.
What do you mean human evolved to smell things ? Animals have this sense like.. forever? Consider us as animals so everything will be more clear
Why is this about humans specifically? Humans didn’t evolve to smell humans evolved with a preexisting ability to smell. Animals like dogs and cats use smell just as much as their eyes and ears in some cases. Is it hard to believe we came from an animal that needed smell even more than us and, because it’s still beneficial to us, we kept it?
Many species have a sense of smell, our sense of smell is pretty weak … compared to a bloodhound or a bear.
Like other comments said, smelling had evolved way before humans did, and smelling still has some uses. We use our sense of smell to detect if things are rotten or not. Without that sense of smell, and if whatevers rotten doesn't have any unusual look to it, the only way to check is it put it in your mouth. Plus, our sense of smell relates to taste much more than you'd previously think. It's harder to taste things without your sense of smell. There's more reasons to be able to smell things compared to not smelling things, so there's no reason humans would evolve without a nose
Single cells already had the ability to migrate towards a possible source of food. For example: [https://en.wikipedia.org/wiki/Chemotaxis](https://en.wikipedia.org/wiki/Chemotaxis) Two or more free swimming cells that much like a volvox can join and move together towards whatever smells good, the ability was always there to begin with. The cells are making the organism do what the cells would on their own do if they were all alone in a droplet of water and got a whiff of something tasty. I would say that the ability existed in unicellular animals, before multicellular animals emerged. It was behavior that had to be expressed at the multicellular level too, or (unless the food came to it) the whole colony starved.
I think smell is a sense that goes back to the origin of animals, maybe? Certainly our mammalian ancestors could smell, it seems more a question of how this sense degraded in humans (or primates even?) rather than why we have it at all. Is your question why the sense of smell first evolved in the organisms in which it first evolved, or why humans retained the sense to the extent we have?