Exploring the human genome is like roaming around an historic town. Some places are pristine and obviously still in everyday use. Others have been unused for a while and starting to show signs of decay and if you look carefully there are relics of buildings that were abandoned long ago.
In this post we are going to look at a gene that we still carry in every cell in our body but which fell into disuse 10s of millions of years ago.
Humans are placental mammals. We nurture our young in the womb for a long time and then feed it with milk for a relatively short period. Marsupial mammals have much shorter pregnancies but they lactate for longer. There is a third group known as the monotremes which consists of just the duck-billed platypus and four species of echidna. The reproductive strategy of this has not changed much from their amphibian ancestors. They lay eggs filled with yolk and then feed their young with milk that is secreted onto a patch of skin.
The major nutritional ingredient in egg yolk is a protein called vitellogenin. Geneticists decided to have a look at the genes for this protein in chickens and then see if it still existed in mammals.
The first task was to locate vitellogenin in the chicken genome. They discovered chickens have three copies of this gene. Next they identified genes that were physically located either side of vitellogenin on the same chromosome. Next they located these active genes in the mammal genomes. When they examined the region that lay between these genes they found a genetic relic that was millions of years old.
In monotremes they found one active copy of the gene but in placental and marsupial mammals all three were inactive relics or pseudogenes.
When they studied the sequence of code for these broken genes in humans and in primates they found exactly what evolution predicted. Precisely the same mutations had occurred in the very same places in our closest relatives.
They were further able to show that the production of milk predated the switching off of our final vitellogenin by at least 100 million years.
The existence of the genetic machinery for yolk production is irrefutable evidence for our distant evolutionary history.
| Part 1 - Protein Functional Redundancy | ... Part 4 - Smelly Genes |
| Part 2 - DNA Functional Redundancy | ... Part 5 - Vitamin C |
| Part 3 - ERVs | ... Part 6 - Human Chromosome 2 |