It's H.G. Wells' birthday today, a fitting time to think about the origins of the speculative fiction genre. I've heard different opinions on which "science fiction" story is the oldest: Gilgamesh, Beowulf, Frankenstein, Gulliver's Travels, etc. Of course, it depends on how you define science fiction. If you pull out fantasy, then the oldest myth-based stories don't count. As far as novels go, Gulliver's Travels is hard for me to classify as science fiction, as it was entirely made up from Swift's mind as a parody of his own culture and times. I consider that to be science fiction, a work has to say, in effect, "What if this happens (or had happened)?" I would give Frankenstein the edge as the earliest science fiction novel on that basis, as I consider Mary Shelley was indeed asking, "What if humanity learns how to create life?"
Certainly, her story has taken on a life of its own. It's been made into plays and movies, parodied endless times, and will doubtless live on in the form of Halloween costumes for the foreseeable future. Considering the length of time that writing science fiction novels was the almost exclusive domain of male writers, it's ironic that it all started with a poet's wife.