Submitted by yeetussonofretardes t3_zljf66 in askscience
I am wondering because it seems plausible to me that due to illnesses spreading more rapidly in human societies as opposed to hunter gatherers, it could be that the immune system advantage of producing Vitamin C could outperform whatever advantage the loss of Vitamin C production had.
The comparison to the hunter/gatherer stage is not really fitting as our live circumstances have vastly changed. We live in cities with thousands or millons of other individuals, even if we live in a rural area, going to the supermarket or something like that brings us in more contact than hunter/gatherer groups might have had in months. Thr advantage might be simple energy and resource costs to produce vitamin c. At some point, our nutrition covered the necessary daily intake enough so that there probably was no point anymore to spend bodily resources in producing it. Other mammals can still produce it, so it might have been intense, kind of knowledgable foraging (of course I know they didn't know about vitamin c, but they must have found certain plants are better than others) or agriculture, that brought us this way.
And it's not just that the gene can spread. New broken versions of the gene will keep popping up everywhere all the time. Without a mechanism to sieve those out, they will eventually drown out the working versions by sheer numbers.
You could also say: the advantage is that your offspring doesn't have to die if the gene has mutated.
The best explanation I've heard is that all it takes is a population living with environmental access to vitamin C and enough time for mutations which interfere with synthesis of it to...not do anything, and get baked into the genome, to grossly oversimplify.
You don't need any advantage for a given mutation to continue, just survival. Advantageous ones lead to new body plans, behavior, and different ecologic niches, but 'neutral' or 'negative' ones that don't get the carrier killed before reproducing are stuck with your population. If the environment makes a negative mutation irrelevant, all the more reason it won't self-regulate out of the gene pool. If most of the species ends up with it, this way, then it becomes a problem to either adapt or become extinct over when external conditions change.
Losing endogenous production of vitamin C was not a strictly negative mutation as you're making it out to be.
https://academic.oup.com/emph/article/2019/1/221/5556105
This paper, for example, outlines that the total amount of required vitamin C is much lower, because scavenging and recycling machinery related to electron transport became more efficient as a result of the mutation.
More simply, it's like the difference between cooking for yourself and ordering out. Does cooking for yourself make you more independent? In certain contexts, yes. Is ordering out VASTLY easier and more expedient if you don't care about the money? Sure. And then you can use the saved time to do other things you'd like to do.
Is one better than the other? Depends on context. In our context, it's generally pretty easy to hit vitamin c requirements: you spend like a minute eating an orange and you're good. If you're on a boat for months and months without access to vitamin C, then that context will suck to rely on exogenous vitamin C.
To answer a part nobody else has answered yet, there aren't humans that can produce their own vitamin C. The mutation that disabled C production is millions of years old and shared not only by all humans, but all monkeys and apes. So there aren't any populations of humans hanging around that still have the ability.
It be dope if I could have it turned on one day so I don’t have to supplement vitamin c anymore
If something is enough of an advantage, it can be strongly retained as mutations which deactivate it don't survive well.
If something used to be an advantage but now isn't one, it gets really complicated.
If something is a disadvantage, it is almost always lost as mutations which deactivate it spread much much faster (just another type of advantage).
The neutral one, often called "relaxed selection" is when something WAS an advantage (like synthesizing vitamin C) but isn't any longer. What happens after is super complicated. Sometimes it is retained, sometimes it is seen in a stable percentage of the population, sometimes it disappears completely. We are like....really really bad at understanding and identifying that case, doubly so when the subject isn't something big and easily studied (like losing eyesight in lightless caves) vs something so seemingly small with a lot of reliant processes and interactions (the ability to synthesize vitamin C through the GULO enzyme). Some people think the useless genes will stick around until there is a strong selection against them, some think that mutations will slowly be eroded in the population until it becomes so horrible it can't reactivate. I dunno. The interaction and transformation of genes into and by pseudogenes is a leading edge, debated subject.
So realistically, not only have we not yet found a definitive clear advantage to not producing vitamin C, we also don't even really understand how to predict what happens to things that were an advantage but aren't any longer, and we keep finding that they actually have some selective reason behind being lost after all (blindness in a cave is now thought to be probably advantageously selected for).
There are really cool studies on bats (most lost the ability to synthesize vitamin C, but some still have it, some seem to be in the process of losing it, and some lost it REALLY recently, like in the last couple million years).
Other cool studies are with Mexican tetra, which have both blind and sighted version that can interact with each other (the cave systems connect with outside water systems).
That's the correct answer. And that's also why they have put Deadpool into that torturing machine, if you know what I mean. Hmm... In that sense, it could be said that life is a torturing machine in hope of randomly developing wisdom in individuals, which makes sense because it seems to be happening as rarely as the good mutations, right? Right? RIGHT?? :P :D
What surprises me about that is that I read before that Vitamin C to most animals tastes really bad, but not to primates because we need it to survive. So if losing endogenous Vitamin C production was just a case of "it doesn't hurt", it would mean that primates consumed enough Vitamin C through their diet before losing the ability that it didn't matter, otherwise it would have been a disadvantage, right? But at that point they would have probably disliked the taste.
Very interesting! But if I understand the paper correctly, the process that is explained there is not an advantage of not synthezising Vitamin C, but it is a separate process that helps with reducing the amount of Vitamin C required, so primates can live on solely dietary Vitamin C in the first place, right?
The evolutionary advantage of primates losing endogenous Vitamin C production is thought to be related to the availability of Vitamin C in the diet. Primates that lived in tropical environments, where fruits and vegetables were abundant, no longer needed to produce their own Vitamin C and so lost the ability to do so over time. There are no known humans today who are able to produce their own Vitamin C.
Could have been any type of food-issue going around with the primates that did not dislike vitamin-c finding more food sources than those that didn't want to eat it.
The advantage they cite is protection from seasonal scarcity due to needing less vitamin C. So during resource deprivation there is a window where the mutant only suffers from conventional malnutrition, whereas the non-mutant's mechanisms for creating endogenous vitamin C are also impacted. So the non-mutant suffers from both malnutrition AND scurvy, which presumably kills them faster. Eventually, the mutant would also suffer from scurvy.
That's mostly speculative though, there would need to be some kind of starvation study done between species to figure out which one dies first. That would be unethical :(
liquid_at t1_j069i6a wrote
Evolution does not necessarily have to be an advantage, it can be enough that it is not a disadvantage.
If food supplements the Vitamin C intake, there are no negative consequences of no longer producing it.
If those with the gene variation that no longer produces Vitamin C do not have any disadvantages because of it, the gene can spread. Which is likely what happened.