[talideon]

I think you're misunderstanding things a bit: it's the jaw misalignment that leads to an overbite (or underbite) the allows the pronunciation of these sounds. Whichever you have, you're still producing labiodentals. What changed is that huge swathes of the population went over time from having aligned jaws to misaligned jaws, which is what lead to the sounds becoming common. The fact that people can have underbites and produce the sound is immaterial: the number of people with underbites has never been particularly large except in groups unless your surname was Habsburg.