Leather was likely the first material employed as a helmet shell. It essentially served as a second skin, reducing damage from edged weapons. Leather—even heavily padded leather—leaves much to be desired when it comes to impact attenuation. However, it is readily available, as well as comparatively affordable and easy to fabricate into a finished product (easier, that is, than smelting ore and casting or forging metals).
Metal replaced leather as the premier helmet shell material sometime during the fourth millennia BC and this remained true into the early 1700s. Leather helmets enjoyed a revival in military applications, however, from the mid 1700s until the early 20th century, when artillery shrapnel made metal a suitable choice for helmet shells again.
Leather helmets, however, did survive for quite a long time in sporting applications. The first (American) football helmets were constructed in the 1890s. Within a few years leather football helmets became de rigueur and continued to be until the early 1950s. Early motorsport helmets were also constructed of leather—usually with cotton or other organic materials sandwiched between leather layers to provide a degree of impact protection. In the world of cycling, padded leather “hairnet” style bike helmets were the norm well into the 1980s.