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Studies of ancient rock fossils reveal that life probably began about 4 billion years ago, when the Earth was very young. Nobody knows how life began on our planet. Most scientists think it originated in liquid water. But was it in pools on the surface, underground, or at the bottom of the oceans that volcanic activity creates hot springs?

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Experiments show that the fundamental chemical components of life can form in space or on Earth. Many of them must have reached our planet with the fall of meteorites and comets. Somehow these simple 'organic' chemicals came together and formed more advanced molecules. Then they began to copy themselves and grow.

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What we do know is that, once it is formed, life is incredibly tough. Live bacteria have been found in frozen Antarctica, in boiling water and within rocks located up to five kilometers underground. We also know that these organisms can survive for years in the harsh environment of space.

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Once the vegetation spread across the planet, the air began to receive large amounts of oxygen. Earth became the only planet in our Solar System with an atmosphere rich in this element. By searching for oxygen on other worlds, perhaps one day we will find evidence of extraterrestrial life.