The Curious Case of Vitamin C in Humans

Textbooks have long taught us that humans lost the ability to produce vitamin C because our diets regularly supplied enough of this vital nutrient. However, recent studies suggest that this evolutionary change might have served a more cunning purpose: helping our ancestors fend off parasites!

The Evolutionary Game of Survival

Most animals, including many primates, generate vitamin C thanks to an enzyme known as GULO. About 60 to 70 million years ago, a mutation disabled this gene in our primate lineage. Interestingly, this same mutation is seen in various mammals, such as bats and guinea pigs.

A Non-Malicious Mutation?

Typically, biologists conclude that if an animal takes in enough vitamin C from food, the inability to produce it internally doesn't hurt its survival odds. Consequently, because the GULO mutation doesn’t seem detrimental, it remains unprotected by natural selection.

Vitamin C: A Key to Blood Health?

In a game-changing study led by Michalis Agathocleous from the University of Texas Southwestern Medical Center in 2017, researchers uncovered that vitamin C is crucial for the health of blood-forming stem cells. This revelation raises a vital question: if dietary vitamin C is enough, why do so many species with abundant sources continue to produce it?

The Hidden Advantage of GULO

One explanation might be that animals with a fully functioning GULO enzyme maintain steady vitamin C levels in their bloodstream, while humans experience significant drops during times of food scarcity. Could there be more to the story regarding disease resistance?

Parasitic Protection Uncovered!

In their pursuit of answers, researchers found that schistosome flatworms, responsible for schistosomiasis, produced more eggs when fed extra vitamin C. To explore this parasitic protection hypothesis, they manipulated the GULO gene in mice. Those without the gene on a low-vitamin C diet showed resistance to infection and shedding eggs, while those with the enzyme suffered immensely.

A New Perspective on Gene Loss

Recent theories propose that losing certain metabolic abilities, like internal vitamin C production, may be the result of evolutionary trade-offs. When food sources are reliable, natural selection might favor the loss of costly metabolic pathways that could also alter immune responses and improve defenses against specific pathogens.

The Balancing Act of Evolution

Interestingly, this gene loss could minimize oxidative stress from excessive antioxidant production, potentially avoiding disruptions in normal immune functions. This subtle shift could turn out to be more beneficial than the drawbacks of dietary dependency, especially when vitamin-rich foods were prevalent.

The Evolutionary Upside of Vitamin C Loss

In the case of our primate ancestors, the trade-off might have been quite logical, offering enhanced resistance against harmful parasites. As Deborah Good from Virginia Tech asserts, while some textbooks may see this as a 'use or lose it' scenario for the GULO gene, ample evidence points to significant evolutionary advantages stemming from this gene loss.

A New Era of Understanding Vitamin C