Along with all the other planets of the Solar System, the Earth was formed about 4.6 billion years ago. In its early years, it had no water or life. It was just a ball of barren rock. Meteorites (large boulders), rained down from space, punching large craters out of the ground. Life probably arose in the Earth's oceans about 3800 million years ago.

The Solar System began life as billions of small rocks whirling round a dense ball of hot gas: what would become the Sun. Over millions of years, the rock fragments gradually "snowballed” into boulders and then into large, rocky globes. Scientists call them planetesimals. One of them became the Earth.

Young Earth

Chunks of rock, called meteorites, kept on crashing into the Earth (see illustration 1, top left). This made its surface so hot it became a sea of molten rock (2). Eventually the bombardment eased and the surface cooled to a rocky shell.

To begin with, the Earth was a barren planet, rather like the Moon is today. Volcanoes started to blast out gases that were trapped beneath its outer shell (3). The gases, carbon dioxide and nitrogen, collected to form the atmosphere. (It did not then contain oxygen, like it does today). 

Scientists think that water was brought to Earth carried on meteorites. Water vapour formed clouds that surrounded the planet. As the Earth cooled, rain started to fall. Over millions of years, the basins in the land gradually filled to become the​​​​​​​ oceans (4).

All forms of life need water to survive. So the first living things appeared in the ocean. No one knows exactly how or where. Many scientists think that life may have arisen on the ocean bed, in those places where jets of hot water, filled with chemicals, gush through cracks in the rocks, as they do today. The jets spurt through chimneys called "black smokers". The chemicals provide food for microscopic living things.