"Parsley is native to the Mediterranean region of Southern Europe. While it has been cultivated for more than 2,000 years, parsley was used medicinally prior to being consumed as a food. The ancient Greeks held parsley to be sacred, using it to not only adorn victors of athletic contests, but also for decorating the tombs of the deceased. The practice of using parsley as a garnish actually has a long history that can be traced back to the civilization of the ancient Romans."
I found this nice blog on food history.