I'm not entirely sure how to introduce this article. Carl Zimmer is usually a good read, this article is no exception. It's one of those things that crop up in science every so often that I just find ... cool.
So we know about viruses that write themselves into our genomes, there's a bunch of them, the most famous at the moment being retroviruses. HIV is one. Hepatitis B another. Some of these stick around long enough that they insert themselves stably into germ cells, eggs and sperm. the eventually become Endogenous Retro Viruses, which is to say, they pretty much become part of us. If you go trawling through the human genome, there's bucket loads of sequences that look like they have come from virus in years gone by. This by itself is pretty cool.
Zimmer is talking, briefly, about an ERV that has actually become useful to us (mammals). Syncytin, a gene derived from a virus is an essential participant in the formation of the syncytiotrophoblast, which is a layer of cells fused together that sit around an foetus in the womb, protecting the foetus from the mothers immune system. If you shut down production of syncytin A in mice, the cells can't fuse together, no syncytiotrophoblast is formed and the offspring is lost at the embryonic stage.
So somewhere back in aeons past a virus infected our ancestors wrote itself into our genomes and eventually we found a use for it, a use that is now essential to our development. As I said, very, very cool.