Ooh, there's lots you could go into: they suck their thumb in utero, the 'rooting reflex' is a big part of it (if you touch a newborn's cheek it'll turn its head and open its mouth).
Their 'suck' when breastfeeding isn't quite like ours - it's more of a squeezing motion, the baby literally milks the mother by squeezing the areola (not the nipple) between its tongue and the roof of its mouth. A different type of suck is required for the teat of a bottle, which is why it would be difficult to switch back to breast after bottles (though not impossible).
I know none of that is medical, or to do with pathways, but may be food for thought?
