It is believed that horses were first controlled by means of a rope
around the lower jaw, a primitive hackamore or even a nose-ring as used
in some early cultures (for example the Numidians) the horse was controlled entirely by the riders' legs, with no bridle at all.
There is evidence of bit wear on horses' teeth found in the Ukraine from 4000BC (see the Institute for Ancient Equestrian Studies ).
Antler cheekpieces used as toggles for rope, hide or sinew mouthpieces have been found at sites on the Black Sea.
Metal bits are thought to have originated in the Near East around 1500
Plain and jointed mouthpieces appeared at the same time, often with
highly ornamented cheekpieces .
Examples of such figurative horse bits from 8th Century Luristan can be seen in many museums.
Early mouthpieces could be quite severe, for control of chariots or horses ridden without saddles or stirrups.
The curb bit originated around the 4th Century BC (Edwards). The mediaeval warhorse or destrier was often ridden in a curb bit with an extremely high port which could put pressure on the roof of the mouth, and long shanks which increased the leverage on the curb and the pressure on the horse's poll, some bits also put pressure on the nose, much in the manner of the modern hackamore.
Theoretically such bits were used with very little pressure on the reins
By the nineteenth Century there were an enormous number of bit designs. In Benjamin Latchford's introduction to The Loriner published in 1883 he wrote "the horse's mouth and temper may be compared to a lock, so made that only one key will fit it...". The same publication also includes Don Juan Segundo's treatise "A New Method of Bitting Horses"
Over time bits developed into several familes : the snaffle, curb bits (including Weymouths in double bridles), and gags. Each bit works on a different part of the horses mouth and has a different action.
References: The Country Life Book of Saddlery - edited by Elwyn Hartley
Tack Explained - Carol Green (1979 - Concorde Books)
Bit by Bit - Diana R.Tuke (1965 - J.A.Allen & Co. Ltd)
The Loriner - Benjamin Latchford (1883)
Bitting in Theory and Practice - Elwyn Hartley Edwards (1990 - J.A. Allen & Co. Ltd)