Similarly and linked in the same article, the human nose is sensitive to ozone in as little as 0.1 ppm, and it seems an unlikely coincidence that it starts to become hazardous to respiratory tissue above this concentration.
That seems like a stretch to me. Sources of hazardous concentrations of ozone are generally technogenic in nature -- the concentrations produced by lightning in the troposphere are far too low to pose any real danger. As such, there's no reason that humans (or any organisms) would have evolved to specifically detect that.
Probably not directly, but the mechanisms we evolve are generalized to some degree, i.e they often end up being useful for things our ancestors never encountered.
Perhaps the method by which we smell give us a similar sense for some similarly hazardous molecules. According to the wiki O3 smells much like Chlorine, they are both highly oxidizing which is why they can be hazardous.
Wet, fertile soil indicates two beneficial things for certain: water, and fertility. The presence of these strongly implies other things we like: a food web and a source of energy and shelter.
That is the most likely reason in my opinion. Usually, when something in our natural environment is pleasant to our senses, that's because it's a good thing from an evolutionary standpoint. Vice-versa with unpleasant things.
Farming developed so recently that its difficult to imagine a scenario where the ability to be attracted to an area that is good for sowing crops was such a strong life or death driver (as opposed to just being able to poke the soil to measure moisture) that we became that sensitive to petrichor that quickly. There were some other hypotheses in the thread about the scent indicating a potential abundance of plant growth and food, which could make sense. Especially if some of our closer relatives like apes have the same ability to sense petrichor- it could have been selected for over millions of years
Not clear: both the lactase gene and blonde gene are pretty recent (last 2-20 ky -- an ice age ago or less). If agriculture, especially pre-fixed-location agriculture, is a significant evolutionary advantage (which I think most people think it is) then it's possible the evolutionary pressure for this mutation would be quite strong and its carriers would be overrepresented in the world's population.
Lactose is just keeping a juvenile gene active and is a matter of life and death - the blonde gene is a single mutation that can take root from a genetic bottleneck. AMD even lactose isn't universal. Just being able to smell that soil is moist (instead of looking at it, or touching it...) is not as powerful a survival advantage as lactase or sickle cell or some of those life and death mutations for food or malaria resistance.
I think there is more information being transmitted than just that the soil is moist. The soil is lightly moist + highly porous + has high levels of microbiological activity which are the optimal conditions for seed germination. Consider that inedible seeds could act as a preserved food that could be germinated and edible in only a few days with no energy input by hunter gatherers who were known to gather and eat seeds - giving more like 70-80k years for Petrichor to be interpreted by humans as an attractive smell.
The plant growth/food hypothesis doesn't make sense to me because this signal is timing specific and the timing isn't related fruiting periods. Also growing plants have their own smells so why would an indirect smell be preferred?
Also, consider the evolutionary path of associating a smell doesn't require any protein production or chemical changes - only neurological ones. This shows that mice can inherit the ability fear certain smells from their parents. It's possible that it could happen epigenetically in humans as well considering the significant changes in taste and smell sense experienced in ones lifetime.
FWIW the lactase gene is not a matter of life or death — people lived at high latitude for millennia before it evolved. When it happened though it clearly was selected for. Most genes don’t select Individually on a life-or-death basis, especially in long-living species as its time scale is wrong (slight exception: pinch points like pandemics). It merely adds a bit of evolutionary advantage aggregated with all others. Thus, for example, your sickle cell example: it’s pervasive but hardly universal among its carrier population even though that population was until very recently all exposed to the same (malarial) risk.
In any case my point was that mutations certainly do evolve and stabilize within the span of human civilization (however one might define that). Evolution is a big, parallel, path-dependent process indifferent to which particular kinds of selection (physical or social, endemic or peculiar) happen to favor reproduction.
The jury is still out on how recently agriculture was developed by human cultures. We are still discovering our history - its not entirely clear that it is as recent as Western academia is capable of demonstrating, empirically ..
Take, for example, the as-yet under-explored evidence in Australia that agriculture extends tens of thousands of years further into our past than we currently acknowledge.
Its entirely feasible we developed a sense of soil as a consequence of our technological use of it.
Right ... we're still talking on the scale of tens of thousands of years. That's a blink of an eye evolutionarily. Evolving complex smell and behavior responses to something like being able to tell what soil is good for planting seeds (when, remember, we can just look at and feel the soil! Evolution isn't some force that invents new features in life for every minuscule need- natural selection is the survival of the good enough) in just a few tens or hundreds of thousands of years across the entire population is not feasible. Consider also that farming didn't just begin once, and presumably hunter gatherers can also smell petrichor. If it was tied to farming (and somehow was a life or death situation) we'd see that trait with a distribution similar to lactose tolerance, not as universal as the ability to smell petrichor.
>tens of thousands of years. That's a blink of an eye evolutionarily.
I dunno about that. It seems we can gain major advances in evolution over just a few generations. Look, for example, at the rapid changes that have occurred in the West over the last century, particularly with regards to sexual maturation of healthier classes. This is clear evidence that evolution can happen a lot faster than we might imagine, even so.
I don't find it difficult at all to imagine that the human genome alters itself over a period of, even a thousand years, in order to respond to the changing environment. We see examples of these shorter orders of magnitude throughout our history ..
That would make far more sense than being tied to farming. Ability to detect where to move to to forage for food, select for that evolutionary pressure over millions of years beginning from our ancestor species, etc
Evolution doesn't have to switch off things of no use, but no direct burden. There is no inherent upside statistically in the mating game to "not" being able to smell soil, so it would be inherently "wasteful" in gene terms to turn this off. We might have carried this up from simpler forms who benefited from it in positive gene-sense. We won't neccessariy loose it, unless there is a statistically measurable upside benefit, or something like energy consequences.
This seems very unlikely given that most of the various structures and pathways involved in smell in humans don't exist in our earliest (in the bacteria sense) ancestors.
It seems far more likely to be a positive associating due to a higher likelihood of finding growing food.
ok. so at some point we acquired it. it must be either very old, or recent. in either case, the fact it confers at best marginal evolutionary benefit, suggests that it is not strongly linked to what we are as humans, and its net cost to us persisting in the genome confers no disadvantage or, is coupled in gene terms to other attributes (sense of smell in general) which are sufficiently beneficial, it will not disappear
Because of mutations, anything that isn't used will tend to stop working. On the other hand the "tech debt" that comes with everything accidentally relying on everything else working a certain way, is a pressure to conserve past behavior.
Agriculture is unlikely to be the reason it evolved, as agriculture is only a few tens of thousands of years old. However fertile soil is also a good sign if you are a hunter/gatherer; it implies rich plant life, and therefore things to gather and maybe also things to hunt.
Adult lactose tolerance isn't much of a change, because humans were already lactose tolerant as infants. The evolutionary complexity to keep producing enzymes into adulthood is minor compared to the complexity of evolving those enzymes to begin with.
So soil bacteria smell good to invertebrates for the same reason that fertile/virile humans smell good to their sexual complements: it helps to distribute their genes.
I am a little confused, am I reading this correctly?
- the smells are coming from streptomyces
- springtails are attracted to the smells
- springtails feed on the streptomyces
therefore the conclusion is not that bacteria are attracting invertebrates with the smell, but rather the invertebrates evolved to smell something they feed on?
It's hard to tell which is which and it's probably a bit of both. I don't really understand why whenever the living nature is being explained, it's done in a way that attributes to some design what really belongs to an accident of evolution. Species develop traits continuously and in a chaos, every individual is a mutant, then environmental pressure determines combinations that are most suited to reoccur (through reproduction).
In this case two species found each other. Maybe a few springtails had the genes to detect the smell. Of those, a few found it pleasant, it could be that others didn't. Maybe a few streptomyces had the genes to produce the smell. A happy accident that allowed both these genetic traits to endure environmental pressure.
Producing that smell is 'costly' and doesn't seem to have any other use, so if it wouldn't benefit the bacteria then they would be expected to evolve to not manufacture these chemicals.
Similarly, what're the secrets to the amazing fresh spring/summer rain smell...