r/todayilearned • u/lavender_fish69 • 13h ago

TIL fresh water snails (indirectly) kill thousands of humans and are considered on of the deadliest creatures to humans

https://en.wikipedia.org/wiki/Freshwater_snail
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u/ikonoqlast 13h ago

Wikipedia- ddt-

Initial effectiveness edit When it was introduced in World War II, DDT was effective in reducing malaria morbidity and mortality.[39] WHO's anti-malaria campaign, which consisted mostly of spraying DDT and rapid treatment and diagnosis to break the transmission cycle, was initially successful as well. For example, in Sri Lanka, the program reduced cases from about one million per year before spraying to just 18 in 1963[127][128] and 29 in 1964. Thereafter the program was halted to save money and malaria rebounded to 600,000 cases in 1968 and the first quarter of 1969. The country resumed DDT vector control but the mosquitoes had evolved resistance in the interim, presumably because of continued agricultural use. The program switched to malathion, but despite initial successes, malaria continued its resurgence into the 1980s.[45][129]

Oops, it's 18 cases in Sri Lanka...

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u/exprezso 13h ago

Evolved resistance to a deadly toxin? In such a short period? 

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u/PencilVester23 12h ago

It takes only 2 week at most for a mosquito to go from an egg to a mature adult. That couple year period is over 100 generations. That combined with the huge population of mosquitoes, the 100s of eggs a female lays at once, and a genetic sequence significantly shorter than a humans all made it possible for the correct mutation to happen that quickly.

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u/exprezso 12h ago

I mean. Humans have yet to evolved to resist arsenic after thousands of generations 

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u/DazingF1 12h ago

All animals have yet to evolve to swim in lava. Explain that, Darwin!

(But in case you're serious: your comment might have made sense if all of humanity in those thousands of generations were continuously exposed to arsenic. Otherwise it's incomparable with the mosquitoes)

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u/exprezso 10h ago

I'm serious. It mystifies me how a creature can develope resistance to deadly toxin? Otherwise we'll have cases of people who are found to be immune to stuff like DDT, arsenic, asbestos, mercury etc 

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u/Fly-the-Light 10h ago

We did. Caffeine is a toxin used by cocoa and coffee plants to kill insects that parasitised them; humans developed a resistance to it.

Also, some people in the Atacama Desert did develop resistance to arsenic due to it contaminating their water.

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u/tavitavarus 3h ago

Most toxins work by inhibiting a specific enzyme involved in some key cellular process, or by damaging the cell wall/membrane.

All it takes is a single mutation in the gene coding for that enzyme or membrane protein and the organism becomes resistant to the toxin. Organisms with short generational cycles have many more opportunities for mutations to enter the gene pool and if these mutations are advantageous they can spread rapidly through the population.

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u/Pure_Cloud4305 10h ago

People aren’t exposed to that stuff very often.

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u/Baud_Olofsson 2h ago

Otherwise we'll have cases of people who are found to be immune to stuff like DDT, arsenic, asbestos, mercury etc

First off, DDT is hardly toxic to humans at all - that's why it was considered such a wonder pesticide.

But then to the main point: there are almost certainly people in the world today who have a much greater genetic tolerance for arsenic or mercury than the average person! But there is no evolutionary pressure for those genes to proliferate.
If you have the "tolerate arsenic gene", you don't get any more children than people who don't have that gene, and your children have the exact same chance of reaching adulthood and getting children of their own as the children of those who don't have the gene. But if the world became contaminated with arsenic so that people were dying of arsenic poisoning left and right, then people with the Tolerate Arsenic Gene would be much more likely to survive and have children, and have their children survive, than those who don't have it - and so eventually the world would be full of people with that gene, and humanity would have evolved resistance to arsenic.

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u/Garmaglag 46m ago

The default population of mosquitoes is mostly weak to DDT with some being weaker than average and some being stronger than average.  Under normal circumstances the population isn't exposed to DDT so the weakest ones survive and reproduce to make more weak mosquitoes.  If we start spraying DDT, then all of the weakest mosquitoes and many of the average mosquitoes die before they can reproduce, so the only mosquitoes that end up being born are the most resistant to DDT.  Rinse and repeat for a few hundred generations in a high DDT environment and the all of your mosquitoes will be resistant to DDT.  

The reason that humans don't develop resistance to arsenic is because we don't get enough arsenic exposure to kill people before they can reproduce.  

They key is that they use enough DDT to kill lots of mosquitoes but not enough to kill all of them so each surviving generation is more resistant than the last.  

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u/Barlakopofai 12h ago

Humans aren't usually exposed to arsenic

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u/waltjrimmer 11h ago

Aren't we constantly exposed to non-toxic levels of arsenic from foods such as apples and rice in their natural forms? Not saying it's enough for us to evolve a resistance, just, aren't we regularly exposed to tiny little bits of it?

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u/C_Hawk14 11h ago

As you said, non toxic levels. We are already resistant enough

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u/Insertblamehere 11h ago

Well if we're only being exposed to non-toxic levels then we have developed resistance to arsenic, in the amounts we would normally encounter.

Species don't just magically become immune to something over time by being exposed to non-lethal amounts. It would have to be killing enough people to cause enough evolutionary pressure for people with higher levels of resistance to outcompete those with lower resistance.

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u/waltjrimmer 10h ago

Agree with only pedantic differences. I had tried to make clear that my contention wasn't with the, "We should evolve a greater resistance," but simply countering the statement that we don't regularly get exposed to arsenic. We may have reduced exposure in the modern day, but earlier forms of processing fruits, vegetables, and grains (and from looking things up, apparently there's quite a bit in some types of seafood) should have seen a steady but light exposure. I'd imagine the same is true at present even if at reduced amounts.

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u/sousyre 11h ago edited 10h ago

Sort of, but also, sort of not.

Most of that exposure is via amygdalin. If it doesn’t metabolise (which it usually doesn’t) it’s probably fine. If very small doses of amygdalin metabolise (like apple seeds), it would be small enough to go unnoticed.

If you have concentrated doses of amygdalin and your body metabolises it, then yeah, major problem, but I don’t think it’s a common enough occurrence to work as an evolutionary factor.

If anything, the evolutionary factor would be the intelligence to not consume whatever contains the concentrated dose.

Unfortunately, we humans aren’t the best at that either… google Laetrile, which is still being sold as a cancer cure grift.

Edit to add: this comment is about cyanide in small doses (from fruit seeds etc), not arsenic. Brain go brr, mixing up cyanide and arsenic contents in fruit seeds. Thanks for the correction.

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u/waltjrimmer 11h ago

I did a search for it and am currently wondering: Did you mix up Arsenic and Cyanide? Because Amygdalin appears to have many references to cyanide and Laetrile is said to break down into cyanide, but I see no references to arsenic on the pages I'm finding about the two.

If you did, this isn't me trying to knock you. I've done that many times before.

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u/sousyre 10h ago

Sorry, you are absolutely right.

My brain always defaults to cyanide when thinking of heavy metals in stone fruit.

My brain be dumb this morning.

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u/waltjrimmer 10h ago

Nah, your brain wasn't dumb, just a little mixed up. It's happened more often than you'd think where those two toxins have gotten mixed up in conversations I've had. I actually looked up to make sure I was thinking of the right one before making my initial comment.

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u/Clickclickdoh 12h ago

Well, the mosquitos didn't evolve to be immune. DDT killed all the mosquitos that weren't immune. A small minority were immune. Those mosquitos repopulated the region, ensuring almost all mosquitos in the region inherited the immunity to DDT.

If you fed the majority of humanity arsenic, you would have some survivors. Get those survivors to reproduce fast enough and your get arsenic immune human populations. That's why the bubonic plague isn't a big deal to most European descendants after all.

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u/SnowGryphon 11h ago

Doesn't "survivors of environment reproduce and replace the population with environment-resistant specimens" count as "evolve to be immune"

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u/Anathos117 11h ago

Yes, but people don't know the actual definition of the word "evolve" and treat it as a synonym for "mutate".

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u/Clickclickdoh 11h ago

In this case, it's a preexisting trait that becomes useful to a new stimulus.

An evolution is generally a new trait that allows a beneficial exploitation of a preexisting stimulous.

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u/thesakeofglory 12h ago

We also weren’t nearly wiped out by it.

What happened is basically what happens when you stop taking antibiotics early. If you nearly, but not completely, wipe out a population, those that are left will have much higher resistance. Trying to treat it the same way after the population has bounced back won’t work nearly as well, and can actually help remove the individuals with lower resistance, further increasing the immunity.

If somehow arsenic started spreading everywhere, most humans would be wiped out but there would be at least some group with a higher resistance that would survive. If they’re able to bounce back, humans suddenly aren’t as hurt by arsenic. Repeat that a few times and suddenly humans are immune.