Pharyngula: Reproductive history writ in the genome

We don’t have any fossilized placentas, but we know that there was an important transition in the mammalian lineage: we had to have shifted from producing eggs in which yolk was the primary source of embryonic nutrition to a state where the embryo acquired its nutrition from a direct interface with maternal circulation, the placenta. We modern mammals don’t need yolk at all … but could there be vestiges of yolk proteins still left buried in our genome? The answer, which you already know since I’m writing this, is yes.First, a little background. It’s not that surprising to find traces of yolk proteins in our genomes, because we also have the evidence of embryology that shows that our embryos still make a yolk sac! Below is a series of diagrams of the human embryo over the last several weeks of the first month of pregnancy, and you can see the large sac hanging from the embryo; it’s a useless fluid filled space that contains no yolk at all, but is homologous to similar structures that form in birds and reptiles.

Nothing but a bunch of meatsacks. Real organisms are made of steel. Like Superman.