As per the textbook definition, we have lost the capability to synthesize vitamin C. However, studies on animals indicate that this loss may have actually provided a survival advantage by helping their ancestors combat parasitic infections.

Most animals produce vitamin C using an enzyme known as Gulo. Yet, in our primate ancestors, the Gulo gene underwent mutation approximately 60 to 70 million years ago, leading to the loss of this capability. Similar losses are observed in various other animal groups, including certain bats and rodents like guinea pigs.

The conventional theory posits that if an animal consumes adequate vitamin C in its diet, mutations affecting the Gulo enzyme do not pose a disadvantage, which is why natural selection hasn’t preserved the enzyme. This mutation is considered neutral.

Mikaris Agato Creos began contemplating this in 2017 after his team at UT Southwestern Medical Center in Dallas, Texas discovered that Vitamin C plays a crucial role in blood-forming stem cells. He questioned why so many animals still retain functional enzymes if the loss of Gulo is indeed neutral.

It appears that there is an additional advantage. For animals with functional enzymes, blood levels of vitamin C stay stable, while in humans, these levels may drop significantly, especially if individuals go without food for several days.

If producing vitamin C carries benefits, why do some animals lose this ability? The common evolutionary explanation is that such losses could enhance protection against diseases and parasites.

A colleague of Agathocleous at UT Southwestern Medical Center later found that the parasite flatworm known as schistosoma produces more eggs when provided with extra vitamin C.

These freshwater parasites can penetrate the skin and develop within the host. Many symptoms associated with schistosomiasis stem from the immune response to the eggs laid by adult worms, causing the resulting disease.

To investigate whether vitamin C deficiency could bolster protection against parasites, Agathocleous and his colleagues deleted specific Gulo genes in mice.

When these mice were fed a low vitamin C diet, they did not exhibit the typical fecal symptoms or excrete feces after being infected with schistosoma. In contrast, mice with functional Gulo enzymes released numerous eggs, most of which did not survive.

“What we demonstrated provides evidence of a potential benefit,” Agato Creos states. Although it remains unproven that the loss of Gulo in our ancestors was a conscious evolutionary choice to fend off disease, these findings suggest that such a scenario is plausible.

“Many textbooks illustrate this as a ‘use or lose it’ case concerning a gene. Many scientists, including myself, contend that there is enough evidence to affirm this evolutionary advantage related to gene loss.” Deborah Good at Virginia Tech, who did not participate in the study, remarked, “Parasite protection could indeed be a factor in this.”