The eye evolved in much the same way, a bit at a time. It didn't just turn up one day.

As for the virus/placenta thing, I recall reading a similar thing about wasps, though I couldn't provide any citations, that the method by which a wasp stops the immune system of a host (such as a catepiller) from attacking its eggs/larvae is viral in origin, and remarkably similar in some ways to the way a mother's body doesn't attack an organism that in any other circumstances would read as a parasite.

Interesting stuff.