This is the final post in this exciting guest blog series by Clairwil! ead on to find out what was found when scientists went looking for a specific vestigal gene in the human genome.
Scientists predicted, based on a vestigial organ in the human body, that the human genome should contain a broken version of a particular reptile gene. This predicted gene has now been discovered, in the hidden 'comment' lines of the human genetic code.
Part 1 - the code of computers
Part 2 - the code of life
Part 3 - why they're similar
Part 4 - and why that's significant
The Yolk Sac
When they're embryos, all bony fish, reptiles and birds have an attached yolk sac, to contain yolk for the young creature to live off once the egg has been laid and is no longer attached to or contained by the adult's body.
A similar structure is present in humans and other mammals but, although the embryo does make use of it as part of their developing circulatory system, the fluid filling it (called "vitelline") does not contain all the proteins present in yolk. The image is from the University of New South Wales.
Creationists assert that this structure should be called the "umbilical vesicle", and that it has no relationship with the organ in reptiles.
Everybody else accepts that the structure is a vestigial yolk sac, the remains of a once fully functioning one used by ancestral species.
Modern scientists are lucky enough to be in the position of the judge in the story in part 1. They don't have to just go on functionality. They can peek at the source code. We can predict that, if the scientists are right about the structure being a vestigial yolk sac, we should find in our DNA the genes that would be required to make it a fully functional yolk sac, 'commented out' and degraded, but still there and recognisable with the right tools.
There are three genes used to make the proteins in yolk, all present
and working in reptiles, named VIT1, VIT2 and VIT3. If the prediction
is correct, one or more of these should be present but broken in the
And indeed, in 2008, that is precisely what three Swiss scientists discovered.
David Brawand, Walter Wahli & Henrik Kaessmann  "Loss of Egg Yolk Genes in Mammals and the Origin of Lactation and Placentation" PLoS Biol 6(3): e63. doi:10.1371/journal.pbio.0060063
They traced the presence of these genes through multiple species (image from Pharyngula, a popular scienceblog):
And they confirmed the presence of the broken genes, hidden in the
intron 'comments' of the human genome, locating the exact chromosome and
position on that chromosome it was hiding on. (Which has now been independantly verified.)
They were even able to use bit rot to provide a cross-check on
exactly when each gene stopped being functional. Like the eyes of a
blind cave fish, producing those yolk proteins was a waste of energy for
a creature that didn't lay external eggs and so, once the gene got
'commented out' by random mutation, that had an evolutionary advantage
and the change spread through the gene pool.
It is absolutely certain that the human species descended, via
evolution, from a common ancestral species shared with modern birds and
reptiles. There is no other explanation for the presence in our genome
of broken copies of working genes. You can read much more about this
story in an excellent article by P. Z. Myers in his blog: Pharyngula. But I want to finish by quoting his conclusion:
The story is all right there in your genes. You're walking around carrying the crumbling record of hundreds of millions of years of history — all we need is the tools to extract it and read it.
And we now have those tools. Expect more exciting discoveries in the next few years!
A huge thanks to Clairwil for this special series that walked us through using human genetics to establish ancestry of our species. For more from Clairwil, or discussion on the topic of evolution, see her new group Debate Evolution vs Creationsim. It's not just for debate - by reading, you'll learn alot about evolution, genetics and what's new in our scientific understanding of where we came from.
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