[W]e 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!
Human embryos also develop primitive gills, which get reabsorbed. Check out PZ's pics.
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