Thursday, March 22, 2012

Scurvy Dogs, Landlubbers, and the Hi-C Mystery

If you’ve ever been on an expedition to circumnavigate the globe, you may have noticed, perhaps near San Pablo Island or the Cape of Good Hope, that you were lethargic, jaundiced, and losing a few teeth here and there.  Haven’t you always wondered why?  Well, you had scurvy, of course – a deadly condition brought on by severe lack of vitamin C!

If you had any livestock on the ship with you, which you undoubtedly did, you may have found yourself wondering why they seemed to be getting along just fine, even as you suffered from general edema, neuropathy, and violent convulsions.  Were they simply deteriorating at a different rate?  Were they making their own vitamin C in secret?  Was some sort of sinister “Rime of the Ancient Mariner” thing going on?  Actually, it’s the second one.  Almost all animals are capable of synthesizing vitamin C on their own.  Humans, most other primates, some passerine birds, and guinea pigs are among the few that cannot.

But wait, you might say.  Aren’t humans supposed to be at the top of the evolutionary tree?  We’re clearly the best at this whole “survival of the fittest” thing, right?  Are you telling me a lowly vole or a sea cucumber can go forever without supplementary vitamin C and I can’t?

First of all, there is no “top” of the tree.  The tree is the history of life on Earth, and the branch tips are all the currently existing species.  Each one is suited to its own particular niche, and although we are more intelligent than sea cucumbers, we are not “better” than they are.  We can’t survive in their environment and they can’t survive in ours.  We are each adapted to our own surroundings.

Adapted.  That word always arises in these discussions of evolution.  We associate it with novel traits that give various animals advantages in their environments: giraffes’ necks are adapted for reaching lofty leaves, and zebras’ stripes are adapted for savannah camouflage.  Adaptations are genetic changes that allow organisms to compete better and reproduce more than their cohorts.  By the same token, if a mutation in a gene makes an organism less fit, it should be rooted out by natural selection.  Similar organisms without that negative mutation will survive longer, find mates more effectively, produce more offspring, or otherwise pass on their genes better than the one with the unfortunate disadvantage.

If a trait is universal in a species (if it has reached “fixation,” that is), or across several species, it is likely that trait got to where it is by being advantageous.  A moose’s antlers, then, although they are cumbersome and ridiculous looking, must provide an advantage, or they wouldn’t be on every single male moose!  It is possible for minimally disadvantageous traits to reach fixation despite natural selection acting against them, but this possibility rapidly subsides to nothing as population size increases into substantial numbers.

Alright, then.  We are nearing the big resolution.  But first, I must add another twist: Primates and a few other haphazardly chosen species cannot make vitamin C, sure, but that’s not the end of it.  You see, our ancestors could make it.  We still have the genes, albeit non-functional versions of them, that code for vitamin C synthesis.  So do guinea pigs and all the others.  This can only mean that on several occasions throughout evolutionary history individual organisms lost the ability to provide themselves with a substance essential to life and somehow not only survived, but performed better than their competitors to the point where inability to produce that substance reached fixation!  No way, right?  There must be an argument for Creationism in here somewhere!

Hold your horses.  Seriously.  I’m not going to leave you hanging.

Long ago, when the ancestors of guinea pigs and men and Saffron-breasted Whitestarts were all living comfortably on balmy, equatorial shores, vitamin C flowed like sunshine.  Every day was a Sunny-D commercial.  Citrus fruits were everywhere, and of course all the monkeys were constantly gorging themselves.  But back then, they didn’t even need to - they could still synthesize all the C they would ever need.  That was all about to change.  One day, a rebel monkey was born.  We’ll call him “Cheeto.”  Cheeto, unlike his parents, couldn’t make any vitamin C because of a few misplaced nucleotides in his vitamin C-making gene.  The world held its breath.  Cheeto ate a banana.  Then he had some berries.  Years passed.  Cheeto was getting along just fine, it seemed.  He was getting more than enough C from all the fruit, and all the excess he consumed was just excreted in his urine, so his condition passed unnoticed.  In fact, since he wasn’t wasting energy making a substance he didn’t need, and since all the others were still monkeying around with unnecessary metabolic pathways, Cheeto and his offspring thrived.  The same thing happened with Randolf the guinea pig and Clarence the Yellow-rumped Eremomela. 

Eventually, Cheeto’s and the others’ genes achieved fixation in their respective species.  And they’d have gotten away with it too, but humans had to move to areas without ample fruit and attempt crazy voyages with inadequate lime stores.  Nowadays, if you live on a latitude greater than 25 degrees or so, your inability to make vitamin C is no longer an advantage.  Now, we must make sure to eat from all the food groups and get 4-6 servings of fruit or something per day.  If we follow the recommendations of the dieticians, we only get enough vitamin C to avoid scurvy.  Some schools of thought recommend much more.

So if we could lose our "C" genes, can we get them back?  Unfortunately it's much easier to "break" a gene than to "fix" it.  If it makes you feel any better though, there are attempts being made to artificially restore the gene.  -In mice that have previously been genetically manipulated, that is.  Don't expect this any time soon:

1 comment:

  1. Interesting things happen when traits that are an advantage for reproduction are actually a disadvantage for survival...

    A situation of "runaway sexual selection" may have contributed to the extinction of the Irish Elks.