This entry is part of a recurring series called Use It Or Lose It that looks at those things that didn’t make the evolutionary cut and have been lost in modern species.
“…the first sign is an inflamed, swollen condition of the gums. The whitish pink tinge next the teeth is replaced by an angry red; as the disease gains ground the gums become more spongy and turn to a purplish colour, the teeth become loose and the gums sore. Spots appear on the legs, and pain is felt in old wounds and bruises; later, […] the legs, and then the arms, swell to a great size and become blackened behind the joints. After this the patient is soon incapacitated, and the last horrible stages of the disease set in, from which death is a merciful release.”
It sounds like a description of a horrible tropical disease that is about to ravage the world in an apocalyptic sci-fi novel. In reality it is an excerpt from The Worst Journey in The World, a true account of Robert F. Scott’s 1910-1913 Antarctic expedition. The disease being described is scurvy, which was caused by a lack of Vitamin C in the tinned foods expedition members were eating.
The Antarctic is an inhospitable place, and it seems unsurprising that humans would struggle to survive in a land of snow and ice that is in darkness for nearly half the year. However, the Antarctic is still rich in other animal life. Sea birds, marine mammals, and fish are plentiful in the Antarctic and none of them suffer from scurvy. In fact, humans are among a handful of species including some fish, birds, guinea pigs, fruit bats, and primates that are susceptible to scurvy because they have lost the ability to make their own Vitamin C. The Antarctic explorer’s cure for scurvy came at the dinner table in the form of either fresh meat or citrus juice and all of those species that are unable to produce Vitamin C instead absorb it from the insects, fruit and meat they eat. Plants synthesize Vitamin C in their fruits, and vertebrates like seals, penguins, whales, and fish all have the ability to synthesize Vitamin C in their livers, meaning it can be a simple matter to incorporate this nutrient into your diet.
It seems like an open and shut case of evolutionary use-it-or-lose-it. If your food provides you with more than enough Vitamin C, it is a waste of energy to produce your own and not worth the damaging cellular effects of the hydrogen peroxide that is a biproduct of its synthesis. By extension, losing this ability would be beneficial in an evolutionary sense and would have been selected for in any organism that has a diet rich in Vitamin C. However, it is simply not true! If you look more closely at the patterns of evolution it appears that the loss of Vitamin C production is what is referred to as a neutral trait. Neutral traits are not actively selected for or against during evolution and change more-or-less randomly. There are many species with high Vitamin C diets that still make their own supply, and while it is possible for lineages to regain the ability to make Vitamin C there is no trend to suggest that losing the ability again is more beneficial than not.
To find the reason behind Vitamin C’s neutral trait status you have to think small, all the way down to the level of metabolism in a single cell. The way a vertebrate species loses the ability to make Vitamin C is remarkably similar across bird, fish and mammals. All those species that can’t synthesize Vitamin C have mutations in a single gene called GLO, which codes for a protein that catalyzes the final step in Vitamin C biosynthesis. The GLO protein has no other function, so a mutation in this gene only stops vitamin C biosynthesis and doesn’t have any negative or positive effects on any other part of the organism’s metabolism. In species that have a diet rich in Vitamin C, a random mutation that inactivates GLO has little or no effect, and likewise a mutation to reactivate it doesn’t change the organism’s likelihood of survival. But what about those species with low dietary Vitamin C intake, like birds that eat only seeds? For these species, it is essential to make your own Vitamin C, and mutants lacking the ability are completely unknown. In the case of Vitamin C, ‘use it or lose it’ is more like ‘use it or maybe lose it, but you could get it back again and either way you won’t notice’. Or in the case of Scott and his crew, you won’t notice until you undertake an Antarctic expedition.
Want to read more about GLO and the loss of Vitamin C production? Check out this review article by scientists at the University of Ottawa.