Arid and semiarid soils occupy about one-third of the Earth's ice-free land surface. They are sources and sinks of atmospheric CO₂, sources and sinks (mainly sources) of global dust, and substrate that support high biodiversity of plants and animals. Arid and semiarid soils uniquely accumulate secondary minerals, such as calcite and gypsum, as the result of low rainfall and limited leaching. Because of sparse vegetative cover and high susceptibility to wind and water erosion, many arid and semiarid soils have low resistance and low resilience to disturbance. Hence land degradation (i.e., desertification) is common on most continents with arid and semiarid soils. These dryland soils have been a factor of primary importance in human history. The oldest hominid fossils are found in east Africa in sediments with paleosols containing pedogenic carbonate indicting an arid or semiarid climate. The transition from hunting-and-gathering to agriculture took place in arid and semiarid Mesopotamia about 10,000 years ago. The production of surplus food on the floodplain soils of the Euphrates and Nile gave rise to early civilizations in Sumeria, Babylonia, and Egypt. Today several large urban centers are located on arid and semiarid soils around the world that have adequate groundwater or river water supplies. If irrigated, arid and semiarid soils are an important source of local and global food production. Still, most arid and semiarid land is sparsely populated, open, and often wilderness land.