History of sackcloth and ashes?

I had always considered that the wearing of sackcloth and ashes was a particularly a Hebrew penance tradition. In Jonah the Ninevites, who were decidedly not Hebrew, wore sack cloth and ashes per the kings order. The King of Nineveh was not Hebrew either. So how did this decision to wear sackcloth and ashes as a symbol of penance occur so readily to the Ninevites?