Earth's water came from different places. Condensing water vapour, and comets and asteroids hitting Earth, made the oceans. Within a billion years (that is at about 3.6 billion years ago) the first life evolved, in the Archaean era. Some bacteria developed photosynthesis, which lets plants make food from the Sun's light and water. This released a lot of oxygen, which was first taken up by iron in solution. Eventually, free oxygen got into the atmosphere or air, making Earth's surface suitable for aerobic life (see Great Oxygenation Event). This oxygen also formed the ozone layer which protects Earth's surface from bad ultraviolet radiation from the Sun. Complex life on the surface of the land did not exist before the ozone layer.