The Loss of Vitamin C: One More Proof for Evolution
August 17, 2014
Irreligiosophy, tackled "5 Evidences for Evolution." Apart from the unpronounceable name, it's quite a good podcast. Its main focus is a critical discussion of religion (Mormonism in particular), but from time to time they also tackle skeptical topics, like this one. The episode is marked as "explicit" since they talk about anatomy and use strong language, so be forewarned if you wish to avoid such content.Evolution never ceases to amaze me. It's one of those scientific facts that has been proven correct over and over. I didn't know however what to expect when one of my favorite podcasts,
I was however pleasantly surprised to hear 5 cases that contained recent research, like the following one (the 4th, just before the 35-minute mark). We humans, as almost all simian species (lemurs and prosimians excluded) are not able to synthesize our own vitamin C.
This is due to a mutation in the genes encoding for gulonolactone oxidase (GULO for short), rendering the final phase in the creation of ascorbic acid (vitamin C) inoperative.
There are a couple of important points there. First of all, we still have the gene that codes for GULO, but it is no longer activated due to the mutation, making that gene a pseudogene. Secondly, it's a small mutation. Chuck, the co-host from Irreligiosophy speaks about only one mutation, but it seems that there are actually several mutations. Regardless, it's probably the first mutation that nixed the ability. As soon as the gene became a pseudogene, other random mutations could happen in that region of the chromosome without any consequence.
So why does this prove evolution? Very simple: because we (humans, apes, and monkeys) still have the entire instruction set, which we share with all mammals, it shows that all mammals have a common descent (dogs and cats for instance can synthesize it pretty well), and furthermore, that all monkeys and apes (humans included) also share a common descent. Slam-dunk, if such a thing was still needed...
But then why did we lose it? As far as I can tell we don't know, which is exactly the answer I love about science. It means there is still work to do and maybe we'll never know. And really, there is nothing wrong with that.
From what I understand, it might have been the case that there was no pressure 63 million years ago to keep it, as the simians then probably consumed enough food containing vitamin C. Which is actually another proof of evolution—if there is no or insufficient pressure on a function, then there is a chance it will be dropped.
Some researchers have even speculated that there were additional benefits of having this gene knocked out. For instance there is less antioxidant reaction if you consume vitamin C instead of fabricating it yourself, since synthesizing it in the organism leads to the by-product hydrogen peroxide. Another researcher speculated in a 1982 paper that it might have helped to survive malaria. Another hypothesis that I like (but for no rational reason) is that it might have helped to gain weight by increasing fat storage, handy when the food supply is not stable. And that's just skimming some references; multiple other hypotheses have been formulated.
All of these explanations remain speculation however. What it shows is that there was no disadvantage in losing the ability, and maybe a small advantage, otherwise our mutated simian ancestors would not have survived. The same applies for bats and guinea pigs, who have also lost the ability (but due to a different mutation).
One interesting study from 2008 tried, of course, the obvious: to reactivate this gene. They used engineered mice and human liver cells (in a petri dish) and reported success in expressing the gene. I tried to look for some more recent studies on the subject, but there hasn't been a lot going around. One study this year reported success, but only when providing gulonola (the input to the GULO reaction). It's not just one mutation that removed the GULO ability—it seems that several factors in the entire chain were disturbed. That shouldn't be surprising, given that all the genes and pseudogenes involved have had 63 million years to undergo mutations.
What it shows is that we don't have a complete answer for this phenomenon yet, and yes, more research is needed. In any case it's too late for those ship crews suffering from scurvy, but it could have been nice to have another tool to combat obesity. But now I'm speculating too ...
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