Earth 101 | National Geographic

• [Narrator] Earth, the only planet known to maintain life. A product of scientific phenomena and sheer chance. This blue speck in space holds the past, present, and future of our very existence. (instrumental music) Approximately 4.5 billion years ago, the Earth formed from particles left over from the creation of our sun. Gravity drew these particles together to form pebbles, which then formed boulders, and eventually, the Earth.

At its heart is a solid inner core covered by a liquid outer core. Above this sits the mantle, made of flowing silicate rocks, and a rocky crust. This rocky mass is the third planet from the sun, orbiting the star from an average distance of about 93 million miles. It's close enough to the sun to be warm, unlike the cold gas giants. But not so close that its surface is exposed to extreme heat and solar radiation as is the case with Mercury.

Earth's unique position in the solar system allows it to house phenomena yet to be found anywhere else in the universe, particularly liquid surface water and life. According to one theory, much of Earth's water is as old as its rocks, both of which having formed during the Earth's earliest days. Because of Earth's unique distance from the sun, the planet is able to contain water in all of its forms, liquid, ice, and gas, rather than have them permanently frozen or evaporated into space.

But Earth is the only known place in the universe with liquid water on the surface, thereby having unique cascading effects on the planet. It hydrates the land, helping create nutrient-rich soil. It collects and pools to form oceans and freshwater systems. And it cycles upward to add moisture to Earth's protective atmosphere. And where there is liquid water, there is life.

About 3.8 billion years ago, in Earth's oceans, primitive life existed in the form of microbial organisms. They and the ensuing billions of years gave rise to a range of more advanced life forms that survived in Earth's seas, lands, and skies. As the only world known to harbor life, Earth's biodiversity is expansive in nature. An estimated 1.5 million species of plants, animals, bacteria, fungi, and others have been cataloged, with potentially millions, if not billions more yet to be discovered.

Home to life and fueled by water, Earth houses a unique global ecosystem as curious and as grand as the astronomical events that made them possible.