Short answers, pulled from the story.
The oldest preserved shirt in the world is a linen shirt dated to around 3000 BC, discovered by Flinders Petrie in a First Dynasty Egyptian tomb at Tarkan. It features finely pleated shoulders and sleeves and a decorative fringe at the neck opening and side seam.
Hannah Montague, a housewife in upstate New York, invented the detachable collar in 1827. She cut the collars off her husband's shirts and devised a way to reattach them after washing, avoiding the need to launder the entire shirt each time.
European and American women began wearing shirts in 1860. The Garibaldi shirt, a red garment associated with freedom fighters under Giuseppe Garibaldi, was popularized by Empress Eugenie of France and drove the trend.
Colored shirts were considered casual wear fit only for lower-class workers before the twentieth century. For a gentleman, wearing a sky-blue shirt was unthinkable in 1860, but had become standard by 1920 and commonplace by 1980.
Fascist and political movements across Europe and the Americas adopted distinctive shirt colors as symbols of allegiance. Italian fascists wore black shirts, German Nazi SA members wore brownshirts, and various movements in Ireland, Portugal, Spain, France, Mexico, and the United States each claimed their own colors including blue, green, gold, silver, and grey.
Descamisados is a Spanish word meaning "the shirtless," used to describe the masses of the poor. The term was used in nineteenth-century Spain and in Argentina during the era of Juan Peron, making the absence of a shirt a political and economic marker.