The Earth's atmosphere has not always been the same. Today, we are used to a living on a planet with a nitrogen- and oxygen-rich atmosphere that supports human life and most of the other forms of life on the planet.

However, the atmosphere was not always like this. In fact, the atmosphere we know is the third of series of atmospheres that have surrounded our planet.

The first atmosphere on Earth, the primary atmosphere, appeared soon after the planet was formed, about 4.5 billion years ago. This atmosphere would have consisted of gases captured from the Sun - hydrogen, helium, ammonia, methane and water vapor - and resembled the atmospheres of the gas giants. All of the terrestrial planets - Mercury, Venus, Earth and Mars - had similar primary atmospheres.

Earth's primary atmosphere did not last.  It was probably blown away by solar winds.

The secondary atmosphere of Earth was the result of intense volcanic activity as well as impacts from asteroids and comets. It probably consisted of gases such as nitrogen, methane, sulfur dioxide, carbon monoxide, carbon dioxide and water vapor. There would have been only a tiny amount of oxygen.

Earth's current, tertiary atmosphere, would never have come into existence if life had not developed on Earth.

Life appeared on Earth between around 3.5 and 4.1 billion years ago. The first living things on Earth were anaerobic. They did not require oxygen and, in fact, oxygen would have been deadly to some of them. They took in amino acids to obtain energy and gave off methane and carbon dioxide.

About 3 billion years ago, blue-green algae, or cyanobacteria, began obtaining energy by means of photosynthesis - a process in which sunlight is used to convert water and carbon dioxide into sugar, which is then used for energy.  Oxygen is a by-product of photosynthesis

The production of oxygen by blue-green algae completely changed the atmosphere's composition. While the Earth still has living creatures that thrive in the absence of oxygen, most of life as we know it today has evolved to live in an oxygen-rich world.

The vast increase in the amount of oxygen in the atmosphere led to the creation of an ozone layer in the atmosphere.

Oxygen molecules are made up of two oxygen atoms. Ozone consists of unstable molecules of three oxygen atoms.

Ultraviolet radiation broke down some oxygen molecules into individual atoms, and some of these atoms combined with other oxygen molecules to create ozone.  Ozone filters out ultraviolet radiation from the sun, so the ozone layer prevents too much ultraviolet radiation from reaching the Earth's surface, thus making the Earth a safe place for the forms of life that have since evolved here.

Our atmosphere is still changing.  The concentration of carbon dioxide has been increasing, particularly since the Industrial Revolution. Much of the increase in carbon dioxide in the atmosphere has been attributed to human activity - the destruction of forests and the burning of fossil fuels.

Carbon dioxide is associated with planetary warming.  A greenhouse gas, carbon dioxide absorbs heat and prevents heat loss.

An extreme example of the greenhouse effect occurs on Venus, where carbon dioxide makes up 95 per cent of the atmosphere and the average surface temperature is over 800 degrees Fahrenheit (400 degrees Celsius).