I think some of you all are overly romanticizing nature. My bachelors degree is in evolutionary health (anthropology). The best we can tell, cross-nursing was extremely common. Women couldn't stop working to go bond with their baby over a good nursing session, the baby was fed by whoever was around. Oh, and while some kids might not self-wean until age 4, 5, 6, 7...that most certainly wasn't the norm. Sometime in the year between 2 and 3 was overwhelming the norm, across all areas and cultures. That's not to say there's anything wrong with letting kids wean themselves, but it's certainly not the norm in our history as a species.
This is kind of a broad statement. At what point in history are you discussing? Close to our primate roots, midway in our evolution? It's muddying an already muddy thread, in my opinion. Great topic for another thread, though!