An important early stage in the development of a habitable terrestrial environment was the formation of a viable ground cover of soil. Soil is composed of two kinds of materials. The first kind comes from rocks. When rocks are broken up by geological processes such as erosion and weathering, they can form soil particles. Mineral elements and compounds that include silicon, aluminum, iron, oxygen, calcium, magnesium, sodium, and potassium can all be found in varying amounts in soil composed of rock fragments. The other materials in soil are organic. The organic components of soil include the remains of animals and plants that have been broken down by microorganisms that live in the soil.
The first soils formed during the Ordovician Period. These soils only sparsely covered the mostly barren rock surfaces or sandy deposits of dry land. Early soils were largely mineral-based because they were almost entirely composed of weathered rocks. By the middle of the Ordovician, however, algal films had begun to reach onto the land from the oceans. Soils were probably thin but held in place by matlike, complex communities of fungi, algae, and bacteria known as microbiotic crusts. Examples of microbiotic-crust soils can be found today in arid regions of the world. These soils are composed of a filamentous structure that traps small particles of sand and silt to form a soil surface. Microbiotic-crust communities may have been the first inhabitants of land.
Trace evidence of microbiotic crusts goes back to Precambrian times. The development of a true soil, or humus, to underlie this crust required many more millions of years. Nutrient-rich humus consists of a mixture of minerals and organic compounds left by dead and decaying organisms. Burrowing creatures such as earthworms, ants, centipedes, and millipedes are instrumental in mixing these elements to create a rich substrate capable of encouraging the spread of plant life. The first evidence of burrowing creatures in the terrestrial fossil record occurs in the Late Ordovician Epoch. The fossil evidence suggests that ancestral centipedes and millipedes may have made these early burrows. Fossil spores found in terrestrial rocks dating from the end of the Ordovician Period indicate that by that time, the first plants had successfully begun to take root on land.
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