Renaissance Clothing and the Evolution of Underwear
While times and clothes have changed so often and so radically, most of the concepts that birthed various articles of clothing have more or less remained the same. The concept behind underwear was to uphold a degree of modesty. Like now, back then there was no universal underwear rule and people wore what was comfortable, available-or nothing at all.
Ancient Bare Necessities
Not much documentation or clothing survives (woolen and linen garments rot after a few hundred years and no one ever thought writing about underwear was important). Getting a definite idea of what people wore beneath their clothes before medieval times can only come from what surviving mosaics or works of art there are, which aren't yet very realistic.
But in ancient times, Imperial Rome set the trend in everything, including what one wore beneath one's outer wear. Men and women alike were known to wear loin-cloths, probably made of linen. Women might have worn a lengthy band wrapped around their chests called a strophium or mamillare.