Just after I posted my last note. I noticed this from Carl Zimmer's blog. He is discussing research into how some of the key genes in placental development are actually co-opted from a virus.
By the way if you are interested in science, Carl's blog is probably one of the best to follow. By training he is a journalist, but he is very careful to get the science correct. But since he is a writer, he is able to present these complicated topics very clearly.