爹 and 娘

I've been wondering for a while about the origin of the Chinese words 爸 (ba, dad) and 妈 (ma, mum). Why? Well, in films set as late as the sixties the words 爹 (die, dad) and 娘 (niang, mum) are more commonly used. lzh tells me 爹 and 娘 are the older words. But what really got me intrigued is that I have heard 爹 and 娘 used by young people in Yanqing- lzh and her brother tend to use 爹 and 娘 only in specific contexts, and usually use 爸 and 妈, but I've heard some of their younger cousins use them as the default words for dad and mum.

Well, I'm online now. I suppose I could see what information I can turn up.