Earth, also called the world and, less frequently, Gaia and Terra in some works of science fiction, is the third planet from the Sun, the densest planet in the Solar System, the largest of the Solar System's four terrestrial planets, and the only astronomical object known to accommodate life.
The earliest life on Earth arose at least 3,5 billion years ago. Earth's biodiversity has expanded continually except when interrupted by mass extinctions. Although scholars estimate that over 99% of all species of life that ever lived on Earth are extinct, there are still an estimated 10-14 million extant species, of which about 1,2 million have been documented and over 86% have not yet been described.
Over 7,3 billion humans live on Earth and depend on its biosphere and minerals for their survival. Earth's human population is divided among about two hundred sovereign states which interact through diplomacy, conflict, travel, trade and communication media.
According to evidence from radiometric dating and other sources, Earth was formed about 4, 54 billion years ago. Within its first billion years, life appeared in its oceans and began to affect its atmophere and surface, promoting the proliferation of aerobic as well as anaerobic organisms and causin the formation of the atmophere's ozone layer.
Earth was a personified goddess in Germanic paganism: the Angles were listed by Tacitus as among the devotees of Nerthus, and later Norse mythology included Jöro, a giantess often given as the mother of Thor.
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