Vitamin C is essential for humans although we can’t make it. It is, for example, essential for the production of collagen without which human beings get scurvy. Interestingly, the inability to produce vitamin C is the exception and not the rule for living organisms with the large majority of vertebrate and invertebrate species being able to make their own. The diagram below shows the organisms that can make vitamin C in black and those that cannot in grey. The ancestors of mammals were able to make vitamin C but this ability has been lost in bats, guinea pigs and anthropoid primates. By contrast, some species lost the ability and later regained it. Anthropoid primates and guinea pigs still contain the genes required for the manufacture of vitamin C but one gene is in effect broken namely the GLO gene. This gene has lost seven of the twelve exons found in functional vertebrate GLO genes, whereas the guinea pig has lost its first and fifth exon as well as part of its sixth exon. Comparing the working and broken genes indicates that anthropoid primates lost the ability to produce vitamin C 61 MYA and guinea pigs lost it 14 MYA. Vitamin C is essential for the species that cannot produce it so it seems strange that species that have lost the ability to make it have survived. Interestingly, all the errors are mutations in a single gene which codes for a single enzyme which is only used in making vitamin C. If any of the genes that drive the rest of the process were damaged then the production of many other molecules would be affected. All the species that have lost the ability to make vitamin C have a vitamin C-rich diet. Anthropoid primates consume much more vitamin C than they need for their daily use; for humans this is 1 mg/kg/day. For example, gorillas (Gorilla gorilla) consume 20-30 mg/kg/day, howler monkeys (Alouatta palliata) consume 88 mg/kg/day, and spider monkeys (Ateles geoffroyi) consume 106 mg/kg/day. No species that does not have a functional GLO gene has ever been found to have a diet poor in vitamin C. The current evidence suggests that gains and losses in the ability to synthesize vitamin C are random, as would be expected for a neutral trait. The neutrality of vitamin C loss is a function of the environment in which species lives. Individuals from a species which have lost the ability to make their own vitamin C will not be disadvantaged while they have a diet high in vitamin C. Vitamin C loss is therefore only neutral when sufficient quantities of vitamin C are part of the diet. There is no purpose for the broken GLO is humans and the development of species matches the dates determined by the corruption of this gene. There is no coherent explanation for the gain and loss of vitamin C other than natural selection.
Posted on: Tue, 03 Sep 2013 11:41:27 +0000
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