by Roger Bourke White Jr. copyright July 2005
Humans and guinea pigs have an odd trait in common: Neither can synthesize Vitamin C. All other mammals can. In the case of humans, the inability is caused by a single mutation on a gene that synthesizes one of the enzymes used in Vitamin C creation.
This is a single mutation. In most cases such a mutation -- a mutation which completely disables an enzyme which makes a chemical vital to life -- would be instantly fatal, and it would never propagate. It is a fluke of nature that Vitamin C is easy to obtain from other plants and animals, so this mutation is neutral in its effect on human survivability, rather than harmful.
A mutation which is neutral in effect should either die out quickly or slowly drift towards becoming 50% of the gene pool. The genes controlling eye color are an example of genes that are fairly neutral in effect. A person can have blue eyes or brown eyes or something in between and do well in life, so the proportions of blue-eyed and brown-eyed people in the world are fairly stable.
But there are few, if any, people in the world who can synthesize Vitamin C. What does this mean?
It means that the Vitamin C mutation is strongly linked to some other mutation that is very valuable to humans. The Vitamin C mutation is a "marker gene" for some other, very positive change. The other change is so positive that all the humans who didn't have it, did not "live long and prosper" and are now gone.
The interesting question is: What is that other change?
I don't know, but it might prove very interesting to find out.
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