Scientists agree that the Earth formed about 4.6 billion years ago out of accreted matter from the solar nebula. All planets in the solar system formed the same way. Once the proto-Earth was formed, many changes occurred over vast timescales to produce the Earth of the 21st century.
The formation of Earth's atmosphere is attributed to early volcanic activity. The early Earth remained molten due to continued volcanic activity as well as frequent collisions with other celestial bodies. One of the biggest of these collisions is thought to have tilted the Earth on its axis while simultaneously creating the moon. Once the Earth cooled and formed a solid crust, liquid water was able to cover parts of this surface.
The earliest life appeared between 3.8 and 3.5 billion years ago, by way of a process that remains unknown. Life on Earth remained microscopic until approximately 580 million years ago, when the first multicellular life forms developed. Earth's life forms and geology are constantly changing, as new species evolve and others go extinct, and plate tectonics keep the structure of the Earth in a constant state of flux.