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.