When did Men Start Wearing Pants?
Why did humans start wearing pants? To answer this question it’s important to understand two things - first, what were the earliest forms of clothing and how did they evolve into pants and secondly, why did a need for pants develop? It is also helpful to define what is meant by pants - specifically a bifurcated garment for the bottom half of the body which covers from waist to the lower leg. This definition helps to differentiate from the earlier leggings which were often pieces of cloth or skins which were wrapped around the legs and then tied on with straps. Leggings were comprised of two separate garments. Ötzil the Ice Man, perhaps the most famous archaeological find of prehistoric human remains from the northern regions, was found wearing leggings.
From archaeological evidence, it is known that the earliest types of clothing were wrap skirts or aprons for both genders. The oldest known woven example is a fragment made from woven reeds found in Armenia and dating from approximately 2900 BCE. While this is just a fragment the construction hints to what the complete style would have looked like with a waistband woven in the opposite direction from the skirt. This is likely stylistically based off of earlier versions made from hides which do not survive to the modern day. Even earlier examples were of so-called string skirts which were comprised of a waistband with strings or pieces of grass hanging down - these skirts often tied like an apron and depictions can be found in art dating back nearly 20,000 years. In the present day this style is still seen in southeast Asian and other countries, for example, the sarong, a traditionally unisex garment. In colder climates, these could be paired with the previously mentioned leggings and a T-shaped tunic. These are all very simple garment that requires limited construction and materials. 
The development of pants came alongside the domestication of horses and served as an indicator of class and profession. People who rode horses needed to have a way of protecting their legs and remaining clothed as a simple wrap garment would not remain on the body. Some early variants involved using the same single pieces of cloth and tying it through the legs to create trousers. Horses were initially domesticated in Central Asia sometime between 3500 and 3000 BCE. Horses were a signifier of prestige, and in many cultures horses and the equipment used in riding them or in using them to drive chariots were included in the tombs of the elite. In these earliest horse riding cultures then trousers, as a form of clothing connected to horses, also served as a sign of prestige. 
The earliest existing pair of pants was found in China and dates to around 1000 BCE - within 1000 years of the time when it is believed horses were initially domesticated. These pants are constructed of wool with legs made of multiple panels. Experts believe due to the construction style of these garments that they were designed specifically for horseback riding rather than for warmth or protection from the underbrush. These garments show a more complex pieced construction than those tunics and skirts, with far more small parts stitched together instead of being a simple wrap or comprised of two simple shapes sewn to one another. These were garments which were specifically designed for a task rather than to protect the wearer from the elements. In this way, pants helped to define a class and social divisions, as wearing them visually identified the wearer with the tasks which they completed.
While the development of horseback riding led to the immediate beginnings of pants they were not instantly accepted as a garment across the entire world at that time. Instead, they spread gradually and often faced challenges among cultures that considered themselves more “civilized” than those who were wearing pants.
Development in the Western World
Scythian warriors, both male, and female, from Central Eurasia, are depicted wearing tight fitting trousers in Greek art dating from the 6th centuries BCE. Similar styles, consisting of a tunic and trousers have been found surviving in tombs. The Greeks wore a wrapped garment, the chiton, and viewed the wearing of trousers as something done by foreigners and females - some historians believe that the Amazons of Greek myth were at least partially based off of the female warriors of the Scythians. In these myths, the trousers come to stand as just one of the ways these warriors buck tradition.
In the Roman world, the toga was the typical wrap garment for men on formal occasions. Casual wear consisted of a tunic. Earlier members of the military didn’t wear trousers, seeing them as effeminate like their Greek predecessors - however, the combination of being defeated by the trouser wearing Teutons, continued northern exploration, and increased usage of cavalry. The spread of the Romans also helped in spreading trousers throughout much of the area where they conquered. Many of these areas were at that point still wearing the separate leggings with a tunic and mantle - normally made of heavy wool.
Men Wearing Pants in the Modern Era
After pants were accepted by the Romans they became a more standard mode of dress across the Western world. As centuries went on it became those who did not wear pants who stood out, such as Scottish soldiers who wore kilts into battle up into the 20th century. Even as in previous civilizations pants had served as a designator of completing a specific task for the upper class of later Western civilizations they serve to show modernity and how the wearer fits the mold of masculinity. By being prepared to carry out physical activities and not being constrained by tight clothing or billowing robes, new fashions showed a change in the cultural mindset as to what was appropriate for these men to accomplish. To show how this change occurred it helps to look at the specific cases of Imperial Russia and Regency England.
The final victory for trousers over breeches in England and those nations which looked to England for fashion was the rise of the popularity of the menswear suit as known today, heavily influenced by Beau Brummell in the early 1800s. Brummell popularized a style characterized by simplicity and good tailoring, in contrast to the most flamboyant styles of previous decades. Previous styles had featured tight fitting knee-breeches and stockings along with heeled shoes, styles which showcased the wearers high rank in that they were not practical for doing a vigorous physical activity.  On the other hand, the style brought to the forefront by Brummell with both its looser long trousers and flat boots and shoes allowed the wearer to participate in more activities matching a social change in the concept of masculinity. He also preferred dark colors, normally black with a white shirt, over the bright colors worn previously. This style was adopted by upper-class men and became the norm for western society through to modern day.
The development of pants allowed for a greater range of freedom and movement. While this initially was just for the warrior and lower classes, specifically the males in many societies, over history the wearing of pants has come to symbolize not only a necessity of movement (as seen when worn by warriors or working peasants) but the choice to be active and to enjoy physical freedoms. Wearing pants showed cultural and societal changes not only in the ideas of what is masculine and what is feminine but also in what is expected of all members of society.
Related DailyHistory.org Articles
- Douglas A. Russell, Costume history and style (Englewood Cliffs, NJ: Prentice-Hall, 1983), 2
- "What We Theorize - When and Where Domestication Occurred," What We Theorize - When and Where Domestication Occurred | International Museum of the Horse, http://www.imh.org/exhibits/online/what-we-theorize-when-and-where-domestication-occurred.
- Ulrike Beck et al., "The invention of trousers and its likely affiliation with horseback riding and mobility: A case study of late 2nd millennium BC finds from Turfan in eastern Central Asia," Quaternary International 348 (2014): , doi:10.1016/j.quaint.2014.04.056.
- Susan Brown, Fashion: the definitive history of costume and style (New York, NY: DK Publishing, 2012), 15.
- Mayor, Adrienne. The Amazons: lives and legends of warrior women across the ancient world. Princeton: Princeton University Press, 2014, 96.
- Douglas A. Russell, Costume history and style (Englewood Cliffs, NJ: Prentice-Hall, 1983), 77
- Mila Contini, Fashion, from ancient Egypt to the present day (New York: Odyssey Press, 1965), 183.
- "PETER'S DECREES ON WESTERN DRESS AND SHAVING, 1701 AND 1705," http://wayback.archive-it.org/6473/20160819151435/https://tspace.library.utoronto.ca/citd/RussianHeritage/6.PG/6.L/7.X.30.html.
- C. Willett Cunnington and Phillis Cunnington, The history of underclothes (New York: Dover Publications, Inc., 1992), 95.
- Brian Dillon, "Inventory/A Poet of Cloth," Cabinet, Spring 2006.