Why do we wear pants? Peter Turchin looks at their cultural evolution:
The basic garment worn by the Greeks was the chiton (basically, same as the Roman tunic). And wearing ‘sacks’ around their legs was something that only barbarians did. The Romans of the Classical Age felt the same way. Citizens were required to wear togas for any official functions, and at other times (e.g., for war) they wore tunics.
So if you go back to Italy of the Classical Age, nobody (apart from barbarians) is wearing trousers. Fast forward a thousand years to medieval Italy and all men are wearing a kind of trousers (hose).
Why did the Italians switch from tunics to pants? The answer is the horse. Not only are the horses responsible for why we live in complex, large-scale societies (or, at least, how such large-scale societies first evolved), they are also the reason why males have to swelter in pants in summer, instead of wearing the cool kilt.
We wear pants because of the rise of cavalry:
While classical Greece and Rome produced excellent heavy infantry (hoplites), their cavalry was really pathetic. Yes, some of them (usually, the wealthy) rode horses. Among the Romans the upper class was even called ‘knights’ — equites, from equus, the Latin word for horse, but these ‘knights’ served mostly as officers and perhaps messengers. They never played a decisive role in battle.
On the other hand, the greatest enemy of the Romans, Hannibal, knew how to use cavalry. As long as the Numidian horse riders fought on the side of Carthaginians, they trounced Romans, again and again. The Romans only won one major battle in that war, the Battle of Zama, which ended the war. Interestingly enough, the Numidians switched sides just prior to the battle…
The Romans eventually realized that they had to acquire reasonably efficient cavalry. At first, cavalry was an auxiliary force, manned by non-Roman citizens. During the Empire (from the first century AD on), the Romans began to employ cavalry more effectively. But riding a horse while wearing a tunic is not very comfortable. So Roman cavalrymen started wearing pants, or braccae as they called them (borrowing a Celtic term; this word eventually became ‘breeches’).
After the collapse of the Roman Empire, Europe fell under the rule of warriors who fought from the horseback — the knights (this transition actually occurred during the Carolingian times, roughly eighth century AD). So wearing pants became associated with high-status men, and gradually spread to other males. By the way, I am talking here about the Mediterranean cultures. In northern Europe, of course, pants were worn by both Celtic and Germanic people at least from the Iron Age on.
The same pattern holds elsewhere:
In Japan, for example, the traditional dress is kimono, but the warrior class (samurai) wore baggy pants (sometimes characterized as a divided skirt), hakama.
Before the introduction of horses by Europeans (actually, re-introduction — horses were native to North America, but were hunted to extinction when humans first arrived there), civilized Amerindians wore kilts. But when the Plains Indians started riding horses they also adopted pants. Another correlation is that typically only men wear pants (or men are first to switch to wearing pants).
One striking exception to this rule is the Amazons, who are, of course, famous for their horse-riding and archery skills.