Friday, January 11, 2013

Vitamin C Provides Evidence for Human Evolution

Hello everyone and happy New Year. Now that I have finished teaching my course on Science and the Christian Faith at Alberta Bible College, I actually have time to write on this blog again! My New Years' resolution is to post one blog a week (I began this resolution today, so ignore the fact that I missed the first week). I certainly have a wealth of things to share from the last semester! Today though I would like to discuss what I consider to be a fairly compelling piece of evidence for the evolution of humans from nonhuman primates, and it comes from a strange source: vitamin C.

When I was home over Christmas I visited my great uncle Dick, husband to my Grandpa Morris' sister. Growing up Uncle Dick was a huge inspiration for me getting into biology. Some of my fondest memories were visiting him on the farm, where he'd always have something interesting going on, whether it was searching for arrowheads in the field, feeding his pet Venus' flytraps, or taking a microscope down to the creek to hunt for freshwater arthropods. Today he is an accomplished sculptor, with his bronze renditions of native mythology showcased in, among other places, the Royal Ontario Museum. And the man is a great story teller. This past Christmas he began a story by saying, 'When I was on board a Canadian destroyer in the Virgin Islands...' which is always a great way of beginning a story. He went on to share all sorts of interesting anecdotes about life at sea. One that stood out to me was the value placed in lime juice. Lime juice was like gold on the ship. Sailors were required to drink it with dinner (not that they would drink gold, I meant it was valuable). The reason, of course, was to avoid scurvy.

Scurvy is the well-known enemy of pirates, sailors, and children who hate to eat fruit. Its a pretty terrible disease, resulting in bone loss (especially evident in the mouth as teeth begin to fall out - trust me, do not Google Images scurvy), feelings of overwhelming exhaustion, and neurological effects. This devastating illness is virtually unknown to us Canadians today, because it is avoided by eating apples or oranges (or lime juice), or eating those sour orange-flavoured pills my mom would feed me every time I got a cold. Scurvy is caused by a deficiency of ascorbic acid in your diet, otherwise known as vitamin C. Drink your lime juice and you will be fine.

It turns out that most mammals are incapable of getting scurvy. Why? Because they can generate their own vitamin C. They do this through a fairly complex process that involves numerous genes and the production of different precursor molecules. The last step of the pathway requires a protein with the easy-to-remember name of gulonolactone oxidase. If that's too hard, you can call it GLO for short. This protein converts a chemical with an equally easy name (I-gulono-g-lactone, or what I like to call 'Iggy') into vitamin C.

The biosynthetic pathway for the production of vitamin C in different living things.

Most mammals, as I said, can do this. You can't give a whale scurvy. Your average pangolin is just fine without oranges. So are dogs, and lemurs.

There are a few exceptions, however. Bats need to eat vitamin C, because they cannot produce it on their own. Same with guinea pigs. Same with chimpanzees, rhesus monkeys, gorillas, mandrills, and any other non-lemur monkey you can think of. And humans.

Recent work published in 2011 uncovered the reason why these mammals are incapable of producing vitamin C. It turns out that guinea pigs, bats, and non-lemur primates (humans included) have all of the genes required to make vitamin C. They even have the GLO gene. However, these mammals have mutations in their GLO gene that prevent functional GLO protein from being formed. The gene is there, but it can no longer function. We call these lazy kinds of genes pseudogenes. Poor Iggy just lounges around, never achieving its perfect transformation into vitamin C. It is a sad story.

Mammals that cannot produce vitamin C are shown in grey.

And so we all have to eat creatures that can synthesize vitamin C, and citric fruits tend to have a lot of it.

But that's not the whole story. These researchers also found that bats cannot produce vitamin C for a different reason than guinea pigs or humans. Bats have a particular deletion in their GLO gene that renders it useless. Guinea pigs have a different mutation in the same gene that has the same effect. Humans have yet another mutation in the same gene - 7 of 12 protein-coding regions of the gene are simply missing in humans. No wonder the gene can't function.

This pattern is consistent with evolution. Bats, guinea pigs and humans are unrelated. If they were to share the same trait, a trait found in no other mammal, it would be very strange to explain evolutionarily. It is much easier when we realize that their inability to produce vitamin C is due to different mutations. That is, the evolution of the loss of vitamin C production in guinea pigs occurred independently and for a different reason than the loss in bats, or the loss in humans.

However, I have yet to tell you about chimpanzees and gorillas and rhesus monkeys and the like. Does each monkey species have its own unique mutation in GLO? No. All non-lemur primates have the same mutation found in humans: a loss of the same 7 of 12 protein-coding regions.

This is compelling evidence for the evolution of humans from non-human primates. Lemurs, the most ancient of primates, do not need to eat fruits to get vitamin C. The mutation in GLO occurred after lemurs split off from the other primates. The mutation must have occurred in the common ancestor of humans and all other non-lemur primates. The reason we cannot synthesize vitamin C today is because a creature that may have looked something like a lemur was born without this capacity millions of years ago, and it has been passed on to us today.

So you can thank evolution next time you're forced to eat an apple.

You may be skeptical. You may be quick to dismiss the evidence as not really pointing towards human evolution. If so, you have to ask yourself, why would God give us the GLO gene, but prevent it from working? Why would he create us with everything else we need to produce vitamin C, and then take that ability away from us? And why would he do the same thing to all other primates but lemurs, and why use the same mutation when a different mutation was used in bats and guinea pigs? It really makes no sense. The evidence is more consistent with evolution. It is almost as if God had placed in us a stamp that said 'product of evolution.'

You might ask why this mutation would evolve in the first place, and that's a good question. For primates surrounded by vitamin C, losing the ability to make vitamin C really has no negative effect, and recent work suggests it may have had a benefit: the conversion of Iggy into vitamin C also produces a harmful by-product. Relying on vitamin C from our diet instead of our bodies removes this nasty little chemical, and we are all healthier for it. Until we get scurvy, that is.

Images from:
Drouin et al. 2011 (Current Genomics)


chrislantz said...

I'm surprised to find that I could understand this article and the technical reasoning therewithin. But I'm even more surprised at the appearance of the humble pangolin in a book not titled, "Obscure Mammals of Tropical Asia & Africa".

Matthew said...

I'm not surprised at all - you are a sharp one, Mr. Lantz. And I am on a campaign to raise the pangolin from obscurity, simply because it is a really cool looking mammal. I challenge you to use pangolin in conversation at least once daily.

Keith Shields said...

Pangolins in Cul de Sac: