How were prehistoric shelters built?

Around the world, the first shelters that people used depended on two things: their livelihood and their environment. Typically pre-historic people were hunter-gatherers first. This required lots of movement to chase game. As a result, they needed to either utilize natural shelter such as caves or bring shelter with them as in a tent. Even today nomadic and hunter-gathering peoples will carry tent like structures. Examples would be Mongolian yurts and American Indian Teepees. As these cultures moved toward cultivation of crops, they required more permanent structures. Again they tried leverage whatever natural opportunities for shelter were available with the minimum of additional work and material to enclose. They would therefore find a natural cave and enclose the only open wall or find a natural depression in the ground and add a roof.