As for their origin, the Gospels, on close comparison, point us back to (i.) an «oldest» written gospel (to euaggelion) which unfortunately does not exist for us except in so far as we can recover any traces of it preserved in later recensions. Perhaps it began somewhat as follows: «In the fifteenth year of the reign of Tiberias Caesar, Pontius Pilate being governor of Judaea... in the high-priesthood of Annas and Caiphas, ... there came down to Capernaum, a city of Galilee; cp Luke 3:1-2; 4:31), Jesus Christ the Son of God»; and then proceeded to sketch, somewhat in the following order, his appearance at Capernaum, his casting out of devils, the proclamation of the kingdom of God, the transfiguration, the final journey to Jerusalem, his passion, death and resurrection. Nothing was said as yet of his origin, birth, early life, meeting with John, baptism in the Jordan, temptation in the wilderness, nor much of consequence regarding his mission as a religious teacher and preacher in Galilee.
This work, presumably written in Greek, may be conjectured to have arisen in the post-apostolic age in circles which sought to combine their more developed Christology (a free speculation of what would then have been called the «left wing») with (ii.) the still older apostolic tradition - not yet reduced to writing - partly historical, partly not, regarding Jesus of Nazareth as the Messiah who had once appeared and whose return was to be expected. As over against the friends of this older tradition, who were able to point to it, those whom we have described (i.) as belonging to the left wing felt the need of a clear setting forth of what had been done and suffered by the Son of God in his manifestation in the world.
(A Wave of Hypercriticism, The English Writings of W.C. van Manen
, edited by Robert M. Price, p. 55-56, my bold)
Note that it is more the Gospel of Marcion meant as proto-Luke (with his emphasis on the proclamation of a new unknown God) than the Gospel Mark (with his emphasis on Messianic secrecy) to be the best candidate for a Gospel who 'felt the need of a clear setting forth'
of who was Jesus.
Mark doesn't like clearity.