Well, much as I like Kilt Day and all, their facts are a bit off. Even in the time of Caesar, the Celts were wearing pants. In fact, their usage of bifurcated garments was one of the things that caused the Romans to call them barbarians. So in the 1st centuryd BC, at least, we have bifurcated garments in common use. I believe that Aristophanes makes reference to the 'barbaroi' wearing pants in a couple of his comedies (late 5th/early 4th century BC), but I'd have to double check that, since I'm a Romano-Celtic archaeologist, not a Hellenist.

In any case, off the top of my head, I can't think of an example predating the Celts that there is literary/archaeological evidence, but I would happily defer to someone who can find that evidence.