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)? In a historical nutshell, men wear pants because they started riding horses into battle. Evolutionary biologist Peter Turchin noticed a peculiar 1,000-year transition from a global preference robes, tunics and skirts to the lasting reign of the pant and discovered pugilistic rationale behind the commonplace garment. He pointed out, for instance, that fierce Japanese samurai wore baggy trousers that permitted mounted warfare, and 1st-century Roman soldiers traded in their robes for trousers to trounce Hannibal. Due to the high status of pants-wearing soldiers, the clothing gradually became a signal of desirable masculinity and modernity, as evidenced by Peter the Great's 1701 decree that Russian men of all classes retire their robes and skirts in an effort to get with the Western times.
It was also a shift in transportation that initially planted the sartorial seed for women wearing pants centuries later. In the mid-1800s, the dress reform movement had begun calling for women to be relieved of their heavy, floor-length skirts that not only restricted movement, but also doubled as walking dustbins and fire hazards. And when the bicycle took off at the turn of the century, some radical women traded in their skirts for billowy bloomers. But it would take, yet again, warfare -- World War II, specifically -- to decisively bring pants into the average female wardrobe. While the male soldiers were away, women donned their domestic trouser uniform, and the rest is fashion history.