I agree that there must be some genetic component to it, but if that were the case then by the law of large numbers we should have seen some counter examples by now too, i.e. babies that lack the instinct to suck or cry, etc. It'd be interesting to know if they show any obvious genetic changes that could explain that behavior. Casual googling hasn't turned up anything, but I'm sure someone has looked into it somewhere...
Behaviors like sucking and grasping are considered reflexes. Those reflexes are present in all mammals and are relatively primative, the more modern parts of our brain eventually learn to suppress them and they typically disappear in most humans after around 1 year old.
Interestingly, patients with brain lesions in certain places or with diseases like parkinsons sometimes lose the ability to suppress these reflexes.
