ABSTRACT
Roots originated during the early phase of the diversification of plants on land in the Devonian Period, some 363 million to 409 million years ago. This was a time of enormous changes in plant life that were to have far-reaching consequences for the evolution of land animals and for the chemical economy of life on earth (Epstein, 1977; DiMichele et al., 1992; Kenrick and Crane, 1997b; Algeo and Scheckler, 1998; Bateman et al., 1998; Berner, 1998). From small rootless organisms a few centimeters tall, plants evolved into large shrubs and trees with a range of specialized rooting structures. Roots combined with a fully integrated vascular system were essential to the evolution of large plants, enabling them to meet the requirements of anchorage, water, and nutrient acquisition. Large roots with secondary wood were widespread by the Late Devonian (378Ma) (Retallack, 1986; Algeo and Scheckler, 1998), possibly earlier (Elick et al., 1998), and this development coincided with the appearance of the earliest forest ecosystems. The impact of roots on the evolution of soils was enormous. Physical effects, such as the fracturing of rock, the binding of loose particles, and the introduction of large quantities of organic material, combined with the chemical consequences of actively pumping solutes through the system, led to the development of soils with modern profiles. Plants ploughed, tilled, and fertilized the land, and in so doing had a lasting influence on Earth geochemistry. The origin of roots set in motion the silent and unobtrusive cycling of minerals from soil to biosphere on a truly prodigious scale (Epstein, 1977), and root-mediated weathering of silicates is thought to have had worldwide consequences for the carbon cycle (Berner, 1998).
