Please note in this excellent discussion that DNA is most useful to discount a particular line. For example, if you have no DNA match, you are almost certainly NOT related.
However, if you do match on DNA, then you are only talking in "probabilities" of a common ancestor at some point in the past. The DNA does not "prove" who that common ancestor is or how far back it was. You still have to do that using standard genealogical techniques. So DNA is just one more tool in our toolkit for genealogical research.
Gay Family DNA Study