Are there evolutionary reasons for reproduction?

Because the only other possibility is that a retrovirus inserted itself into one little mammal's genome, where it became involved in placental formation; and then coincientally the same sort of retrovirus inserted itself into a different mammal's genome, in the same place, and became involved in placental formation as well. The same event happening twice, in such specific detail, at about the same time, is far less intrinsically likely than it happening once. Unless there's some special reason to opt for such an unlikely event, it makes more sense to assume it happened just the once.

Sorry for the big delay in replying - I've been away. I'm not really sure what you mean. Are you asking for a tree of placental mammals? Or a tree based specifically on this one gene. If the latter, then I don't know, but trying to work out relationships on one gene is a bad way of doing things. Odd little things could happen to an individual gene over the course of the millenia that give strange answers. if you want a robust tree of relationships within a group, you need to use lots of genes together, to avoid getting thrown off by exceptions.Think about if you were trying to work out how tall people are. If you solved this question by looking at one individual then you might wind up with a hopelessly wrong answer. This person could be a midget, or a towering giant, and your estimate of the average would be miles off.