Tonsure is famous haircut for medieval monks, how far back does it go, when was the first known appearance of the Tonsure haircut and does anyone know if that very particular haircut predate Christianity ?
This is only a partial answer, but we know it goes back at least to the 7th century AD, since Bede talks about it extensively in his 'History'. In fact, rather than just one tonsure, he mentions at least three different styles of tonsure.
The eastern tonsure: practiced by what would become the Orthodox churches of the East, this seems to have been a total shaving of the head, with no hair left. Theodore of Tarsus was tonsured in this way, and had to let his hair grow back for 4 months before he could receive the Roman tonsure, prior to becoming Archbishop of Canterbury in 668. [Bede, History of the English, IV. 1]
The Roman tonsure: this is the one we're familiar with, the circle of hair left around the outside of the head. Supposedly the tonsure that St Peter received, in imitation of Christ's crown of thorns.
The Celtic tonsure: this is the main reason Bede talks about the tonsure, since there was a lot of controversy in 7th century Northern Britain over the right kind of tonsure. The Celtic tonsure seems to have been popular with Irish and Irish-influenced clerics and Abbot Adomnán of Iona supposedly was tonsured in this way. We're not quite sure what it looks like: the closest we have to a description is from a letter copied by Bede, supposedly written by Abbot Ceolfrid of Wearmouth and Jarrow [Bede, V. 21]:
On the forehead it has indeed a superficial resemblance to a crown; but when you look at the neck, you will find the apparent crown cut short, so that you may fairly regard this fashion as characteristic of simoniacs and not of Christians.
[from Leo Sherley-Price's Penguin Classics translation, p. 325]
The best guess on what that actually means is that a kind of stripe was shaved from ear to ear, possibly curving forward towards the forehead at the front, but it's all speculation really. In any case, the Roman tonsure won out, and that's what we're familiar with today.
I know it's not a full answer to your questions, but hopefully someone with a better knowledge of early Christianity can fill in the blanks on the origins of the practice.