When Biblical scholars do venture outside the canon, The Gospel of Thomas, sometimes called the Fifth Gospel, is the text they are most likely to include. Thomas likely predates Mark, Matthew, Luke and John as it contains no reference to later ideas such as the crucifixion or resurrection, nor does it speak of Jesus as a Messianic figure. However, several sayings from Thomas are found in Matthew and Luke. The hypothetical Q document both Matthew and Luke quote from could be a modified Gospel of Thomas.
The Gospel of Thomas was branded as heretical by Eusebius, and thus all copies of it were destroyed. However, in 1945 several gnostic documents were discovered accidentally by some farmers in Nag Hammadi, Egypt including the Gospel of Thomas. The documents are dated to around 340 AD, when the Catholic church had just been founded to unify the numerous competing brands of Christianity. Since gnosticism was now considered a heresy, the gnostics had to bury their writings in caves so they wouldn't be destroyed.
The earliest references to the Gospel of Thomas are made by Hippolytus of Rome and Origan of Alexandia around 230 AD. However, the Oxyrhyncus papyrus fragments of the Gospel of Thomas date to as early as 130 AD, much earlier than the first papyrus fragments that exist for the canonical gospels.
It is difficult to date the Gospel of Thomas since, just like the canonical gospels, sayings were added to it and taken away from it by different editors over the years. The earliest version of Thomas was probably written around 50 AD, while the version we have today was written much later. We do know the canonical Gospels were written later since they show an awareness of Thomas, but Thomas shows no awareness of them. Thomas also lacks any reference to the end of the world indicating it was written before the Roman wars with the Jews.
The Gospel of John particularly shows an awareness of Thomas. John attempts to put down Thomas (and by extension his Gospel) by portraying Thomas as someone who doubts the resurrection of Jesus. Both John and Thomas speak of a divine Light and personify the Light as Jesus. John 1:9 (That was the true Light, which lighteth every man that cometh into the world) is a reference to the idea found in Thomas 77 (Jesus says: I am the Light above them all, I am the All. All came forth from me, and all attained to me.) John and Thomas also both emphasize salvation via the logos of Christ.
Peter is portrayed as the successor to Jesus in the canonical Gospels. However, before the canonical Gospels were written, Peter was subservient to James. As Paul tells us:
When Peter came to Antioch, I opposed him to his face, because he was clearly in the wrong. Before certain men came from James, he used to eat with the Gentiles. But when they arrived, he began to draw back and separate himself from the Gentiles because he was afraid of those who belonged to the circumcision group. (Galatians 2:11-12)
This is another indication that Thomas predates the canonical Gospels. Thomas agrees with Paul that James, not Peter, was the leader of the early Christian movement:
The disciples said to Jesus, "We know that you are going to leave us. Who will be our leader?" Jesus said to them, "No matter where you are you are to go to James the Just, for whose sake heaven and earth came into being." (Thomas 12)
There is a striking similarity between 1 Corinthians 2:9 and Thomas 17 which indicates one author may have been aware of the other:
But as it is written, Eye hath not seen, nor ear heard, neither have entered into the heart of man, the things which God hath prepared for them that love him. (1 Corinthians 2:9)
I shall give you what no eye has seen, what no ear has heard and no hand has touched, and what has not come into the human heart. (Thomas 17)
Since the Gospel of Thomas was written before Mark, Matthew, Luke and John, and since Thomas, like the epistles of Paul and every other first century Christian writing, lack any historical information about Jesus, we're forced to conclude that no historical information about Jesus existed in the first century. Christians simply didn't care about the biography of Jesus until about 130-140 AD when the Gospel of Mark was written.