Before there was harmony as we know it, there was only unison (with the occasional octave, of course) plainsong. It was quite common a little later for a given tune or chant to be sung in canon, thus creating harmony from plainsong. I have never seen anything to identify a composer for this particular Dona Nobis Pacem, nor a date. However, to my ear it does not sound at all like the work of Palestrina. I am now quite curious to find out if anyone knows the origin or date.