Setting aside for a moment the possibility that Smurfs are entirely fictional, it's worth pointing out that assuming they do exist, they're biologically unique. The previously cited Terminally Incoherent article leaves a tantalizing trail of clues here. First, the removal of one's hat appears to be taboo for these creatures. Why? Then there's the unusual fact that a gifted (albeit widely-shunned) wizard repeatedly failed to breed Smurfs in captivity for research purposes. How can this be?
One interesting theory posited in Terminally Incoherent's seminal foray into Smurf morphology is that Smurfs are, in fact, mushroom symbiotes. The implications are dizzying. Perhaps the hat isn't a hat at all, but the cap of a psychoactive mushroom with a direct biological pathway leading directly to the Smurf's frontal cortex. If so, removal of the cap wouldn't merely cause social embarrassment. The act would be tantamount to performing a complete and likely irreversible lobotomy.
In one fell swoop, this theory covers off the mystery of Smurf reproduction and perhaps offers an intriguing explanation for why Smurfs seem to have an endless supply of mushrooms for habitation, ingenious evasion mechanisms, and so forth. Perhaps the entire Smurf community is some kind of meta-organism, an intelligent fungus unknown to (again, admittedly fictional) science?
It all begs the question: Is Smurf/human reproduction viable? While shrouded in mystery, history offers tantalizing clues that Homo Smurfeus may indeed once have roamed this planet. There's so much we've yet to learn.