Labiodental sounds, such as F and V, have been known to be rarely met in hunter-gatherer languages. To understand how this has occurred, the authors undertook a massive statistical inquiry. 2,400 languages were analyzed, and a biomechanical model of mouth and lip movements was created.
The following explanation was put forth - when transition from hunter-gatherer society to agriculture and animal husbandry happens, diets also undergo major changes; specifically, foods become softer. This leads to changes in bite configuration and thus facilitates easier pronunciation of labiodentals.