Earth is the third planet from the Sun in the Solar System. It is currently the only planet in the universe known to contain life. The only natural satellite of Earth is the Moon. The Earth has a mean radius of 6,371.0 km and a surface area of 510,000,000 square kilometers. Of this, 148,900,000 sq. km. is land, while the rest is ocean. There are 3 major landmasses on Earth: Afro-Eurasia (including the continents of Africa, Asia, and Europe), the Americas (including North America and South America), and Australia. Although all of Earth's oceans connect to form one world ocean, they are conventionally divided into the Atlantic, Pacific, Indian, Arctic, and Southern oceans.
Earth is estimated to be 4.5 billion years old. The geologic history of Earth is divided into 4 eons: Hadean, Archean, Proterozoic, and Phanerozoic. Although life is believed to have begun in the Archean eon, the Cambrian explosion occurred in the first period of the Proterozoic. Since then, there have been several major extinction events that have wiped out a majority of life on Earth. The most recent of these events, the Cretaceous-Paleogene extinction event, eradicated most of the dinosaurs and set the stage for the dominance of mammals.