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Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's surface is land consisting of continents and islands. The remaining 71% is covered with water, mostly by oceans but also lakes, rivers and other fresh water, which together constitute the hydrosphere. Much of Earth's polar regions are covered in ice. Earth's outer layer is divided into several rigid tectonic plates that migrate across the surface over many millions of years. Earth's interior remains active with a solid iron inner core, a liquid outer core that generates Earth's magnetic field, and a convecting mantle that drives plate tectonics.
According to radiometric dating estimation and other evidence, Earth formed over 4.5 billion years ago. Within the first billion years of Earth's history, life appeared in the oceans and began to affect Earth's atmosphere and surface, leading to the proliferation of anaerobic and, later, aerobic organisms. Some geological evidence indicates that life may have arisen as early as 4.1 billion years ago. Since then, the combination of Earth's distance from the Sun, physical properties and geological history have allowed life to evolve and thrive. In the history of life on Earth, biodiversity has gone through long periods of expansion, occasionally punctuated by mass extinctions.
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Knoxville is a city in, and the county seat of, Knox County in the U.S. state of Tennessee. As of July 1, 2019, Knoxville's population was 187,603, making it the largest city in the East Tennessee Grand Division, and the state’s overall third largest city after Nashville and Memphis. Knoxville is the principal city of the Knoxville Metropolitan Statistical Area, which had an estimated population of 869,046 in 2019.First settled in 1786, Knoxville was the first capital of Tennessee. The city struggled with geographic isolation throughout the early 19th century. The arrival of the railroad in 1855 led to an economic boom. During the Civil War, the city was bitterly divided over the secession issue, and was occupied alternately by both Confederate and Union armies. Following the war, Knoxville grew rapidly as a major wholesaling and manufacturing center. The city's economy stagnated after the 1920s as the manufacturing sector collapsed, the downtown area declined and city leaders became entrenched in highly partisan political fights.