Well as it turns out, B.F.ing India.
Dude, a couple hundred years ago, linguists started noticing a lot of similarities between most of the languages in Europe, the Middle East, and India. And then they noticed a lot of similarities between those languages' parent languages: Persian, Latin, Greek, Celtic, and freaking Sanskrit. Sanskrit, dude.
So these old parent languages, we'll call 'em the Titans, they all share a parent, or parents, or grandparents. We call it Indo-European, and we don't have a single printed word of it. We can guess a little what it looks like based on some words that the Titans have in common - the words for sheep and dog and willow and stuff. Which in turn tells us something about the climate - it was temperate, and the animals are Eurasian, so there's that. Dude, people were speaking an early of the version of Indo-European language in the stone age. Woah.
I find this interesting in light of the fact that we're pretty sure humanity was born in Africa. Language, however, the languages we speak, wasn't born there. I'd like to know how inter-related African languages are. I know click languages (Swahili) are confined to a tiny geographical area - if that didn't travel throughout Africa, I wonder if any language did.
But yeah, Sanskrit.
As a side note, Basque, spoken in parts of Spain and France, isn't Indo-European in origin. We've got no idea where it comes from.
Most info here is from Guy P Harrison's Race and Reality. Bill Bryson's The Mother Tongue and from my college history of the English Language class in college.