"Tow the line" comes from an old boxing rule where each boxer would put the tow of the left boot on a line drawn on the ground then start fighting anyone heard a different version.
It's "toe", not "tow". It may have sprung from the starting of a race, but the origin does seem to be from the sporting world, regardless.
In one suggested origin the Line actually exists and is still found in the House of Commons. It was put there to mark the sword distance between Government and Opposition front benches. Members were told to toe the line if, in the eyes of the Speaker, they became too excited.
Strange as it may seem, the British do not carry swords as a matter of course. Ceremonial swords are still used by the great and the good for ceremonial purposes, but even gentlemen refrain from going armed these days.
The earlier poster was nearest the mark - to "toe the line" came out of the history of boxing. It was some time back, but there used to be a type of boxing where the pugilists went "toe to toe" and both had to keep a toe on a line on the ground. That kept them within striking reach of each other and led to bloodier encounters than these days. {aside: even in those days Iron Mike would have been disqualified for taking a snack during the about - he would not have had any kind of points win - he would have had to stand there until Lennox Lewis pulped him}.
The House of Commons tale may also be true, but the pugilist version is fairly common coinage.
http://www.phrases.org.uk/bulletin_b...sages/437.html
"Toe the line" first appeared in the early 18th century, and there are two possible "lines" to which the phrase might originally have referred. One would be the starting line of a foot race, the mark upon which each runner places his or her foot in preparation for the starting gun. The other possibility, which I find more likely, is a line drawn on a ship's deck or a parade ground which new recruits must "toe" as they assemble in formation. I find this source more persuasive because it echoes the connotations of "order" and "obedience" that "toe the line" retains today. It is to a runner's advantage, after all, to "toe" the starting line, but a recruit being ordered to "toe the line" is very quickly learning who is in charge.
From "The Word Detective" (October 30, 2001)
http://www.phrases.org.uk/bulletin_b...sages/426.html