In
1602, A Spanish fleet began their expedition to the New
World. During the long voyage across the Atlantic Ocean, the
entire crew of the ship developed symptoms of bleeding gums
and spotting of the skin, with many cases leading to death.
Nearly two decades later, a British military surgeon John Woodall
suggested the consumption of lemons, limes or oranges as a cure
for this mysterious disease. However, for more than three centuries,
the cause of this fatal disease was unknown. It was eventually
discovered to be a disease known as scurvy, which is caused by
a vitamin C deficiency. Vitamin C is abundantly present in citric
fruits. We, as humans, are unable to synthesise our own vitamin
C. Consumption of vitamin C is thus necessary for human survival.
During the prolonged voyage, the fleet did not have access to
fresh foods containing essential vitamins and thus resulted in such
tragedy.
To attempt to answer the question of why we are unable
to synthesise vitamin C, we must examine the mechanism of
Vitamin C synthesis in organisms that are able to do this. Vitamin
C synthesis in vertebrates involves a complex pathway. Specific
protein factors are required, and the absence of any single
protein will result in the failure of the entire pathway. In the
human genome, a mutation at the coding
known as L-gulonolactone oxidase (GULO) renders this pathway
dysfunctional. Thus, humans are unable to synthesise vitamin C on
their own. Interestingly, similar defects were found in guinea pigs,
gorillas, chimpanzees and other primates, none of which are able
to self-synthesise vitamin C.
It has been shown that ancestral mammals, living some 100
million years ago, were actually able to synthesise vitamin C.
Therefore, the hypothesis is that humans have lost the ability to
produce vitamin C for evolutionary reasons. Yet, natural selection
should highlight the traits that positively contribute to survivability
and individuals possessing the gene mutation should have a lower
chance of surviving and reproducing. Why then, would natural
selection continue to include the gene mutation, blocking such a
vital biosynthesis capability?
One school of thought suggests that the reason may be
attributed to the fact that humans have included vitamin C-rich
Why
Humans Are
Unable
to Synthesise
Vitamin C
