Earth is the only place in the universe where life is known to exist. The planet formed 4.54 billion years ago and life appeared on its surface within a billion years. Since then, Earth's biosphere has significantly altered the atmosphere and other abiotic conditions on the planet, enabling the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth's magnetic field, blocks harmful radiation, permitting life on land.
The physical properties of the Earth, as well as its geological history and orbit, allowed life to persist during this period. The world is expected to continue supporting life for another 1.5 billion years, after which the rising luminosity of the Sun will eliminate the biosphere.
Scientists are still trying to unravel one of the greatest mysteries of earth: When did "life" first appear and how did it happen? It is estimated that the first life forms on earth were primitive, one-celled creatures that appeared about 3 billion years ago. Then suddenly those single celled organisms began to evolve into multicellular organisms. Then an unprecedented profusion of life in incredibly complex forms began to fill the oceans...
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