From what I've heard the reason that "Mama" or something similar is so common around the world is because the movements of the mouth necessary to pronounce it are similar to the act of sucking on a mother's breast.

And whereas it is one of the first words many children say, it seems likely that some variant of this was the first word ever said and is the oldest word still in use without changing meaning.

Of course it's impossible to know, but it certainly seems like a contender.