Indeed, but it comes really early in development. A 2 months old baby understands that if you smile at other people they will interact with you. And they'll try

Indeed, but it comes really early in development. A 2 months old baby understands that if you smile at other people they will interact with you. And they'll try to use the to get more interaction / playing (in a very simple way) which also makes sense evolutionary because more attention from adults means higher chance of survival.