Errant thought

This occurs to me on a semi-regular basis: basically every time I watch or think about the Terminator movies. The question is simple: who was John Conner's original father? The movie has an inherent paradox in Reese being his father from the future. In the very first timeline, whenever it happened to be, before any time travel was invented, how did John Conner come to be? Did he have a different father the first time that got overwritten by Reese being sent back? This gnaws at my brain some days.