One attempt to peer into the 150 year darkness between the composition of the Gospels and our earlier extant manuscripts asserts that the authors of Matthew and Luke viewed themselves as merely ordinary scribes reproducing their Marcan examplar, rather than redactors producing new gospels. This, of course, assumes the Synoptic theory that Mark was written first, and that Matthew and Luke "copied" Mark.
This attempt to peer into the darkness also assumes the earliest Christian scribes typically would not have reproduced their exemplars in a careful, exact manner, but rather would have felt free to elaborate and otherwise modify their texts to create something more befitting their own needs and circumstances. This extremely loose approach to the scribal task, according to this theory, has given us the two new Gospels of Matthew and Luke, not to mention the few gospel manuscripts which contain non-canonical stories and sayings.
Proponents of this view probably would attach many caveats to their theory and state things more cautiously. Perhaps they would suggest that the processes involved in producing Matthew and Luke are merely analogous to the freedom with which the earliest scribes perceived in reproducing the earliest Christian writings.
All of this stands opposed to the view that early scribes were very faithful to their exemplars and intended to reproduce them with exacting accuracy, although scribes also felt obliged to correct some readings against one or more other copies at their disposal. It also stands opposed to the notion that the authors of Matthew and Luke were self-consciously producing new and differing accounts of the Jesus story, each having its own unique theological emphases quite distinct from Mark's Gospel.
This issue, then, is yet another issue to be addressed in the text and canon debate.