The first use of the straight left is credited to Gentleman Jackson. Jackson was an 18th Century boxer who "was the first to stand with the left hand left foot extended. Before that the men faced each other squarely and held their arms like wrestlers. Jackson was not only the first to use the present method of standing, but he also was the first to use the straight left, which has come to be the primary blow in scientific boxing." Tacoma News Tribune (Tacoma, WA, USA) Jan. 1, 1924.
It has also been stated that the straight left differs slightly from the left jab. With the jab, the elbow should be pointing at the floor at full extension. A straight left turns the elbow upward. (While the hook has the elbow parallel with floor, with the arm bent.)