In part 4 we saw that roughly half of the 800 genes in the human genome that code for olfactory receptors are broken remnants of our evolutionary history. They were vital to our distant ancestors but in humans, as in our primate cousins they have been allowed to fall into disuse as our eyes became more important to our survival than our nose. The damaging mutations in these genes beautifully confirm the evolutionary history that is known from multiple other lines of evidence.
This post is going to look at another excellent example of a pseudogene - one of around 19,000 the human genome.
Vitamin C - ascorbic acid - is essential to life. Without it we are unable to repair connective tissues resulting in the disease known as scurvy that claimed the lives of as many as 2 million sailors from the 16th to the 18th century. It was discovered that citrus fruits provided an almost miraculous cure for the disease - men who were very sick fully recovered within less than a fortnight of receiving fruit in their diet - but the chemical responsible was not isolated until the 1920s.
Not until far more recently was it discovered that humans possess the genetic machinery to produce their own vitamin C.
The majority of animals produce vitamin C in a multi-stage process - this is why you don't need to buy dog food enriched with vitamin C. Humans have the genes for all but the final stage. Actually we do have the final gene - it's known as GULO - but it's broken.
The significant thing for evolution is the fact that the same gene is broken in all our closest relatives.
The diagram below is a cladogram showing evolutionary relationships based on multiple independent sources of evidence. The clades shaded in red are all unable to produce vitamin C due to the same mutation at the same place in their genomes.
There is no rational explanation for this fact other than the conclusion that humans evolved from a common ancestor of other primates.