How did life start on this planet?
Planet Earth is 4·5 billion years old. Scientists believe that for the first half a billion years asteroids and meteors smashed holes in the surface so frequently that life stood little chance of survival. Scientists estimate the Earth became more habitable 4 billion years ago, and life emerged 200 million years later.
Exactly how life started is uncertain. Maybe dormant deep-frozen micro-organisms hitched a ride on a comet or asteroid. Maybe basic life first started deep beneath the surface, where conditions were more stable, and moved up through cracks and fissures as conditions above ground improved. Maybe the chemical soup in the oceans favoured simple self-replicating carbon-based molecules. These molecules might have increased in complexity and number over millions of years, until we have the diversity of life we see around us today. It is almost certain that all life developed from the same single source, as all life discovered relies on has the same complex molecule - DNA.
For most of Earth's history, there were only single-celled organisms. Scientists have found fossilised single-celled micro-organisms in rocks 3·5 billion years old. The earliest date for anything more complex is from rocks less than 1 billion years old.
