What we wear usually has its antecedents in sport or underwear, and sometimes, like the tee shirt, both.
The tee shirt that is, with his denim, the urban hipster's standard garb began as a gym shirt for athletic wear in the 1930s, and morphed its way under the dress shirt a few years later. Somewhat separately, knitted sport shirts with collars started the outerwear portion of the trend, with the polo being the father of the breed. Polo and undershirt combined into the current collarless outerwear incarnation, a trend so recent that Esquire's 1973 Encyclopedia of 20th Century Men's Fashions does not so much as mention them, and yet clothing industry sources report that tee shirts are currently by far the most popular clothing item sold in the West.
There are tee shirts and there are tee shirts of course. At one end of the spectrum is the white knitted version intended to be worn as underwear, and that is where it tends to stay. The outerwear tee shirt is anything but white, with black by far the most popular. And though a collared polo is considerably more appropriate for wear to a Beverly Hills cocktail lounge, characters such as Mr. Duchovny set a mass market precedent for the uncollared version on Californication, his made for Showtime series.
With the continued decline in Victorian influences and the spread of temperature controlled environments, our descendents' dress will be probably be reduced to body paint, tattoos and some version of the Brazilian tapa-sexo, that strip of cloth that covers the genitalia on an otherwise nude dancer. That is something none of us will see for ourselves, but as a society we have already dispensed with coats and jackets, and the dress shirt is on its way to oblivion. The tee shirt leaves us with no further layers that can be removed.