Commonly, martial arts today are traced back to a lesser level, thus masking the commonality of lineage. This is not without reason, though. For, there exist many problems in tracing back the lineage of even one art. The main problem lies in the communication of arts. Word of mouth has been and will most likely continue to be how arts are taught.
Word of mouth is the easiest, most effective, and time-honored method.
However, it leaves open the possibility for human error in memory and loss of information through death. Countless arts have undoubtedly been lost because their practitioners have simply died out. Before modern times, lineage was commonly held as secondary to the art itself. If the art worked, that was good enough for its practitioners. Thus, early martial artists cared much less about lineage than we do today - either that, or they cared much less about writing it down than we do today.