Green leaf lettuce is thought to have originated in Egypt a couple of thousand years ago beginning with the cultivation of a wild plant that is related to the sunflower and the Aster. As a weed, it was grown for its seeds to produce oil for lamps and cooking, and gradually was bred for its leaves, which eventually become popular with the Greeks and Romans.
Green leaf lettuce was a boon to plant breeders in 18th century Europe, where many of the types of lettuce we see today are still grown in many gardens. In the 19th century, green leaf lettuce morphed into other varieties like red leaf lettuce once it arrived in the United States.
Green leaf lettuce grows best in cool temperatures, which average 20 to 30 degrees above freezing. Excessive heat will stop leaf growth and produce stalks that will quickly flower and turn green leaf lettuce leaves bitter.