I was assuming that each skill line goes up independently based on how much you use said skill line but was not sure if it was based on something else so hence the question.
That's a really good question. I hope we get an answer.
Although judging from the racial passives that have been released, it sounds like we will be at good ole TES and using the specific skill to level up.
however I don't recall any confirmation for the way the XP system works, so any light that could be shed would be fan.