There is endless debate about that just like there is about everything else. The three schools of thought are thus:
1. One timeline branching into two when Nero goes through the black hole.
2. Two timelines always existed separately, Nero just went from one to the other
3. One timeline branched into two
complete timelines when Nero goes through the black hole. In this theory, there is some weird quantum retroactive effect echoing back to the beginning of time that makes this timeline wholly different than the prime, although technically it didn't start until Nero came through the black hole. Sort of a paradox like the anomaly in 'All Good Things.'
Take yer pick!