It comes from the 1700's when a fighter had to put his toe on the (scratch) line to show he was capable of fighting another round. As well as toeing the line the phrase coming up to scratch also was created because of this.
I understand that toeing the line came from within the houses of parliment. Two lines are etched on the floor of the main chamber with just enough distance in between to prevent any contact between two outstretched arms and swords. The phrase 'to toe the line' meant to 'step up to the plate' i suppose is the modern equivolent, or to basically enter the arena and 'fight your corner' (another boxing phrase that has crossed over). I definatley read somewhere that that was the origin of the phrase, but I wouldn't bet on it as I can't remember the source.
You may be right, the originality of some phrases are very often very hard to pin point down, and your point makes sense.
Just looked at Wickipedia and both our stories are present with no conclusive proof either is correct. We have a split decision!!:good