Chapter The EarthEarth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Terra. Home to millions of species, including humans, Earth is the only place in the universe where life is known to exist. Scientific evidence indicates that the planet formed 4.54 billion years ago, and life appeared on its surface within a billion years. Since then, Earths biosphere has significantly altered the atmosphere and other abiotic conditions on the planet, enabling the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earths magnetic field, blocks harmful radiation, permitting life on land. Earths outer surface is divided into several rigid segments, or tectonic plates, that gradually migrate across the surface over periods of many millions of years. About 71% of the surface is covered with salt-water oceans, the remainder consisting of continents and islands; liquid water, necessary for all known life, is not known to exist on any other planets surface. Earths interior remains active, with a thick layer of relatively solid mantle, a liquid outer core that generates a magnetic field, and a solid iron inner core.Earth interacts with other objects in outer space, including the Sun and the Moon. At present, Earth orbits the Sun once for every roughly 366.26 times it rotates about its axis. This length of time is a sidereal year, which is equal to 365.26 solar days. The Earths axis of rotation is tilted 23.4 away from the perpendicular to its orbital plane, producing seasonal variations on the planets surface with a period of one tropical year (365.24 solar days). Earths only known natural satellite, the Moon, which began orbiting it about 4.53 billion years ago, provides ocean tides, stabilizes the axial tilt and gradually slows the planets rotation. A cometary bombardment during the early history of the planet played a role in the formation of the oceans. Later, asteroid impacts caused significant changes to the surface environment.Part HistoryScientists have been able to reconstruct detailed information about the planets past. Earth and the other planets in the Solar System formed 4.54billionyears ago out of the solar nebula, a disk-shaped mass of dust and gas left over from the formation of the Sun. Initially molten, the outer layer of the planet Earth cooled to form a solid crust when water began accumulating in the atmosphere. The Moon formed soon afterward, possibly as the result of a Mars-sized object with about 10% of the Earths mass impacting the Earth in a glancing blow. Some of this objects mass would have merged with the Earth and a portion would have been ejected into space, but enough material would have been sent into orbit to form the Moon.Outgassing and volcanic activity produced the primordial atmosphere. Condensing water vapor, augmented by ice and liquid water delivered by asteroids and the larger proto-planets, comets, and trans-Neptunian objects produced the oceans. The highly energetic chemistry is believed to have produced a self-replicating molecule around 4billionyears ago, and half a billion years later, the last common ancestor of all life existed. The development of photosynthesis allowed the Suns energy to be harvested directly by life forms; the resultant oxygen accumulated in the atmosphere and resulted in a layer of ozone (a form of molecular oxygen O3) in the upper atmosphere. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes. True multicellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized the surface of Earth. Beginning with almost no dry land, the total amount of surface lying above the oceans has steadily increased. During the past two billion years, for example, the total size of the continents has doubled. As the surface continually reshaped itself, over hundreds of millions of years, continents formed and broke up. The continents migrated across the surface, occasionally combining to form a supercontinent. Roughly 750millionyears ago (mya), the earliest known supercontinent, Rodinia, began to break apart. The continents later recombined to form Pannotia, 600540mya, then finally Pangaea, which broke apart 180mya. Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580mya, during the Neoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has been termed Snowball Earth, and is of particular interest because it preceded the Cambrian explosion, when multicellular life forms began to proliferate. Following the Cambrian explosion, about 535 mya, there have been five mass extinctions. The last extinction event occurred 65 mya, when a meteorite collision probably triggered the