Broken genes

"Some of the most striking examples of these broken genes come from our noses. All mammals carry several hundred genes for producing receptors on nerve endings in the nose. These genes evolved through accidental duplications. When a single gene became two, both genes at first encoded the same receptor. But then a mutation struck one of them, changing the receptor’s ability to catch odors. If the receptor did a worse job with the mutation, natural selection tended to delete the gene. But in some cases the mutation caused the receptor to catch a new odor molecule, expanding the smells the mammal could detect . Over millions of years, this process gave rise to a huge family of odor receptor genes.
 
In mice, dogs, and other mammals that depend heavily on their sense of smell , almost all the copies of these genes work properly. But in chimpanzees and humans, the majority of odor receptor genes are defective. They can’t make a receptor at all . Scientists generally agree that these mutant genes must have accumulated in our genomes because ancient apes were evolving to rely less on their noses and more on their eyes. As a result , chimpanzees and humans share a strange legacy of our common ancestry: broken genes."