Wool has been worn by humans for approximately ten thousand years. It was domesticated -- the practice of selectively breeding sheep for a dense, workable fleece -- sometime in the fourth millennium BCE in the ancient Near East. By the time of the Roman Empire, wool processing was one of the most important industries in the Mediterranean world. The Worsted cloth trade that defined English economic history in the 14th through 18th centuries was wool. The American Civil War was fought, in part, by soldiers wearing wool uniforms that the armies could not produce fast enough.
The properties of wool are the result of its fiber structure. Each wool fiber is a keratin protein covered with scales -- microscopic interlocking plates that give wool its characteristic surface. These scales cause fibers to felt when agitated with heat and water, which is why wool shrinks in a hot wash. But they also create tiny air pockets within the fiber bundle that insulate in both directions: trapping warm air next to the skin in cold weather and allowing warm, humid air to escape in warm weather.
Wool's most remarkable property is its behavior when wet. Unlike synthetic insulation and unlike cotton, wool maintains approximately 80% of its insulating value when saturated with water. This is because the keratin fiber absorbs moisture into the interior of the fiber rather than allowing it to sit on the surface -- the fiber gets heavier and darker but the air pockets within the bundle remain partially intact. A wet wool sweater keeps you alive in conditions where a wet cotton sweater is a hypothermia risk.
Merino wool -- from the Merino breed of sheep -- is the finest commercially available wool, with fiber diameters in the range of 15 to 24 microns. Human skin perceives fiber diameter as texture: fibers under about 19 microns feel soft; fibers over 22 microns feel scratchy. This is why Merino wool can be worn directly against skin without itching while coarser wool wools require a base layer. The Merino breed was originally developed in Spain and was a tightly controlled export commodity -- Spain prohibited the export of Merino sheep under penalty of death until the late 18th century.
The American wool tradition is primarily a Western story. The great sheep drives of the 19th century, the basque sheepherders in Idaho and Nevada, the Navajo weaving tradition -- these are the contexts in which American wool culture developed. The Hudson's Bay Company blanket, striped in colors originally indicating trade value in beaver pelts, is the most recognizable artifact of the North American wool trade.
The 20th century synthetic revolution produced fibers -- nylon, polyester, Gore-Tex laminates -- that replicated individual properties of wool at lower cost and higher consistency. Synthetic fleece insulates when wet; synthetic membranes shed water; Merino-blend fabrics provide softness at a lower price. But no synthetic material combines all of wool's properties in a single fiber: insulation, moisture management, odor resistance, fire resistance, and natural flexibility. Wool does not melt or burn like synthetic fabrics -- it chars and self-extinguishes. This is why the U.S. military still specifies wool for certain applications.
Care for wool is straightforward: cold water, gentle agitation, no heat. Machine wash on a wool or delicate cycle with cold water and a wool-specific detergent. Lay flat to dry. Heat is the enemy -- it causes the scales to interlock permanently and shrinks the garment. A wool item cared for correctly will last decades and emerge from each washing with its properties intact.