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 percent of all species of life (over five billion) 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 percent have not yet been described. Over 7.3 billion humans depend upon its biosphere and minerals. Earth’s human population is divided among about two hundred sovereign states which interact through diplomacy, conflict, travel, trade and communication media.