Some say the thong may be the oldest form of clothing, starting who knows when, as little more than a strap of leather. Bushmen of Africa wore a type of thong as did the Sumo of Japan (but perhaps these examples are more pre-historic jock straps). Generally it's pretty well agreed that cultures around the world figured out someway to string up a man's junk when wrestling, hunting and generally cruising through jungles and forests. What's not very clear is when women began to wear thongs.
Some say women's thongs are a relatively modern invention. They may have come into popular culture sometime in the early 20th Century worn by erotic dancers. It's not clear if dances such as Josephine Baker at Paris' famous Folies Bergère ever wore thongs. Supposedly New York's famous Mayor Fiorello LaGuardia ordered nude dancers to "dress more appropriately" in 1939. In many states, strippers cannot dance full-nude, and thus wear as "just enough" to stay within the law. The cover-up codes gave rise to dancers wearing "pasties" and no doubt influenced the reduction of as much underwear as possible.
Another theory is that when the French bikini made its way to sunny Brazil, the sun-loving south Americans invented the thong bikini, known as fio dental (literally, dental floss).
As a form of women's underwear, the thong seemed to make a sudden appearance. This seemed to correspond with the shift to "low-rise" jeans and exposed mid-drifts in the early 2000s. When women bent over, the thong would rise, exposing a "whale tail." This sight, of course, became very popular with boys. The Wall Street Journal reported that "Thongs are the fastest growing segment of the $2 billion-a-year women's panty business." Companies, such as Victoria's Secret, Frederick's of Hollywood, Lane Bryant and even discount stores, such as Target and Wal-Mart credited thongs for their jump in sales then.
The thong craze had faded, just as exposed mid drifts, tramp stamps, and the popularity of Brittney Spears. However, the thong, while not a much in the public eye, has become an accepted, mainstream form of underwear. In fact, recent surveys place the number of American women who wear thongs as their preferred underwear style at 28%.