Earth, also known as the World,[26] Terra,[28] or Gaia,[30] 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 celestial body known to accommodate life. The Earth's biodiversity has evolved over hundreds of million years, expanding continually except when punctuated by mass extinctions.[31] It is home to over eight million species.[32][33] There are over 7.2 billion humans[34] who depend upon its biosphere and minerals. The Earth's human population is divided among about two hundred independent states that interact through diplomacy, conflict, travel, trade, and media.