Faldage-- I sent email to a friend of mine who has a PhD in linguistics from UCLA (on extrinsic tongue muscles); he studied with Ladefoged in the late '60s. He taught for a decade at UCSD and elsewhere. I asked:

"What's your verdict on bilbabials are easier to pronounce because your mouth is in the default position? Also, how about those babies who learn bilabials first, along with their words for mama and papa? "

He replied:

"Don't know about that. I can't see, from a physiological/anatomical POV, why bilabials should be any easier to make than dental/alveolar stops or even velars. For example, from the cine-X-ray films I've seen (including the one of me), there is mandible movement involved in all stop+vowel sequences, including bilabials. You could even make a somewhat sophistic case that bilabials involve the coordination of more separate muscles than does a simple tongue touch to the alveolar ridge. (I wouldn't make that case.) In thinking about the muscles involved, I can't see how there's much of a difference. Of course bilabials might be easier for little Elmo to imitate from watching the parents."

I've forwarded the PDF link, and will get to you if he has anything interesting to say.