You know a story's a good one when it's been around forever. That's absolutely the case with the tale of Io—people were telling this story back before people were even writing stories down. Like most other myths, Io's got its start orally. But eventually someone was like, "Man, that's a good one. Why don't I try out this fancy new written language thing and record the tale for later generations to ooh and ahh over." Io's story starts out in Argos, where she was sometimes worshipped, so a lot of folks think these word of mouth legends sprang up there.
You can find hints and fragments of the story of Io in the writings of super ancient writers like Homer, Hesiod, and Herodotus. But some of the oldest full versions come from the playwright, Aeschylus, the granddaddy of all Greek tragedians.
In Aeschylus' play, The Suppliants, Io's descendant, Danaus, shows up in Argos with his fifty daughters, begging sanctuary. See, they peaced out of Egypt because the pharaoh was trying to force Danaus' daughters to marry the pharaoh's sons. To try to convince the folks in Argos to help, Danaus reminds them that his granny Io came from Argos. He and his daughters retell Io's whole sad story—yep, the story at hand—and win the hearts of the Argives. (Yay for family reunions.)
Io also has an extended cameo in Aeschylus' play, Prometheus Bound, where the gadfly-tortured heifer happens upon Prometheus who is, well, bound. (He's been chained to a rock for all eternity by Zeus for giving man fire.) Prometheus is all tied up, so he's got nothing better to do than listen to Io dish out a super long monologue, telling her whole sad story. Everybody wins here. Prometheus gets some entertainment. Io gets to vent. And we get to hear Io's story straight from the horse's—or, um, cow's mouth.
Io's tale stayed super popular for a long time and tons of later writers took a stab at retelling it. And check this out, they all had kind of funny names: Ovid, Hyginus, Nonnus, Diodorus Siculus, and Valerius Flaccus.
Out of the bunch, the Roman poet Ovid gets the award most famous version of Io's story. In his massive collection of mythological poems, The Metamorphoses, Ovid goes off for a long time on the tale of Io. It's really no wonder that Ovid decided to tackles Io's tale in his epic work. When you've got story about a nymph who gets transformed into a cow and back again, you kind of have to include it in a book about metamorphoses, right? Right.
Don't go thinking that Io has totally retired from the public eye. She had a guest spot of Xena: Warrior Princess (Hey, that's cool to some people.) And most recently, she made a high profile Hollywood debut in the big-budget remake of Clash of the Titans.
In the Clash of the Titans version, she doesn't say anything about having ever been a cow. Instead, she's been made immortal for refusing Zeus, and she helps Perseus on his quest for Medusa. Yeah, so this doesn't have a whole lot to do with the original myth, but hey, it's cool she show's up. Is it time for Io to have a movie all to herself? Absolutely. Did you hear that, Hollywood? Shmoop hath spoken.