One can't know for certain, but given the geographical spread of the various ancient manuscripts, it's most likely that the common ancestor is the original autograph.
The problem is that we don't have nearly enough early manuscripts (nor are the ones we have old enough) to really find THE common ancestor.
For example:
Let's say that we have so far only found copies of the OT from 700 C.E. and later, like the Masoretic text. But let's say we have a lot of them, and compare them all, and find out that Goliath is 6 cubits 6 span tall in all (or at least a consensus) of the manuscripts. It's safe to assume that the common ancestor for these manuscripts ALSO said Goliath was that tall.
And this is true. Most late manuscripts have him at this height.
But, now let's say we find earlier manuscripts, like the 4QSama, Septuagint, Dead Sea Scrolls, Codices Vaticanus and Alexandrinus, and the work of Josephus which say that Goliath was only 4 cubits 6 span.
Well, now we must have a new, and different, common ancestor, despite the fact that by the time the 700 C.E. texts were around, the geographical distribution is wide, and they were still able to get a consensus figure for Goliath's height which was probably wrong. It only takes the distribution of a popular, but erroneous, copy to establish this. The Latin Vulgate may have contributed to this particular event.
So...Given that the New Testament books were not even as well established as the OT was when the OT obtained a "new common ancestor," how can you know that there were not similar events between the autographs and the earliest extant manuscripts?
Keep in mind, also, that most of the earliest NT manuscripts are just fragments, and we often don't have full texts of some books until close to 300 C.E.