Let me explain this in evolutionary analogy.
We have those 20K NT manuscripts (and citations in lectionaries, apologetic literature, etc). Unlike the fossil record, each manuscript still has its “DNA” (the words), and so it’s possible to trace the phylogeny of each by textual criticism. We find the vast majority of changes are near-neutral point mutations. The relatively few “adaptive mutations” are attempts to tidy up difficult sentences, or occasionally to correct a locally perceived “doctrinal error.” New stories are almost unknown, the exception being the woman taken in adultery, which varies in its presence and position - suggesting an independently preserved tradition.
Nevertheless, over all 20K maunsuscripts, from 2nd century onwards, it remains clear that they all represent the same four species of gospel - there has not been the kind of divergence into legend (or anything else) so often claimed.
That means any “prehistory” must come not from the strong manuscript evidence, but from the single texts themselves - and there must be a prehistory, simply because the gospels are products in history. And also, there are clear literary similarities between the texts, which could reflect common events, and/or common traditions, and/or common documents.
In the nineteenth century, the aim was to trace lost documentary sources - hence the documentary hypothesis. There is little doubt that the “synoptics” (Matthew, Mark & Luke) form a very imperfect nested hierarchy - but the upshot is that, after 150 years of intense scholarship, consensus on the nature of that hierachy no longer exists. Mark may be first and used by the other two, but plenty of scholars maintain Matthean priority, and other variations. Even John shows evidence of more distant common ancestry.
Attempts to replacesource documents with “traditions” and “schools” are equally speculative, and likely to remain so. What we can say is:
(a) That the 4 existing “species” were almost certainly in their current form within 1 generation of the events they describe.
(b) That their divergences show they were not produced by a central authority creating an official mythos, whilst their mutual dependency shows their four authors substantially agreed on the history and were in mutual contact.
(b) They all show signs of familiarity with the time and place of those events, and no others. (Note that this differs from all the apocryphal gospels).
© The supernatural elements, such as the Resurrection in particular, show evidence of belonging to the earliest traceable layers of their composition, when they could most easily have been refuted.
(d) The manuscript and citation evidence shows that these four gospels, and no others, rapidly attained widespread acceptance in the Church.
