As the game evolved .
Alternate formations came in and out of fashion , halfbacks ( reduced to typically just one rather than two ) emerged as the offensive back most likely to run the ball , although , again , any eligible player may do so .
As the game evolved and alternate formations came in and out of fashion , halfbacks ( reduced to typically just one rather than two ) emerged as the offensive back most likely to run the ball , although , again , any eligible player may do so .