I hadn't given much thought to why men wear pants until I started looking into why women wear skirts. The only known clothing to predate the skirt is the loincloth, which means that for a long time, skirts were unisex, as Valerie Steel, director of the museum at the Fashion Institute of Technology, told CBS News: "Men and women both wore skirts in ancient times. Once you had the initial idea of weaving a rectangle and tying it around yourself, you had basically the skirt. And if you look at say ancient, Egyptian paintings, you see men and women both wearing what's in effect a skirt."
So why and when did the skirt begat the pant (Fun fact: "pant" is derived from 4th-century Roman Saint Pantaleon)?