Shelter

Any built or found structure that puts a barrier between a human body and the weather — wind, rain, cold, and heat. At minimum it keeps you dry, blocks the wind, and lifts you off cold ground. Shelter is the body's first line of defense against the environment, older than agriculture, pottery, or the wheel.

A human body loses heat far faster than it can make it once wet and windblown, and can die of hypothermia in hours — long before thirst (days) or hunger (weeks) turns deadly. Managing that heat loss is what let humans spread out of the tropics into deserts, mountains, and ice. Every settled society begins with the problem of staying warm and dry.

The oldest shelters used whatever was at hand — branches, hides, bone, snow, or the mouth of a cave — arranged to break the wind and shed rain, over a bed of dry material to insulate the sleeper from the ground. The physics was understood intuitively long before it was named: trap a layer of still air, stay dry, and keep the wind out.

Survival outside the tropics, and with it the peopling of nearly every climate on Earth. Permanent shelter makes storage, settlement, and eventually the city possible; the hearth inside it is where cooking, warmth, and family life converge.