4000 – 3900 million years ago

Year seven
The first living organisms develop from chemicals

The first living organisms develop from chemicals

The oldest rocks are around 4000 million years old, but there is no evidence of life in them.

The oldest living organisms lived in the oceans. They were bacteria belonging to a type called 'chemoautotrophs'. They were adapted to living in conditions without oxygen. If there was oxygen in the water, it would have killed them.

For life to evolve there must be certain chemicals: carbon, hydrogen, oxygen, nitrogen, phosphorus and calcium. There must also be water, and water seems to have formed on the planet very early in Earth's history.

It is likely that before living things evolved, there must have been a long period of chemical evolution in the water. This seems to have happened between Earth's second and seventh birthdays (between 4400 and 3900 million years ago).

We do not know when life began because there are no known rocks of the right kind and the right age, but it is likely that life had evolved from the earlier chemicals when the Earth was about six or seven (about 4000 or 3900 million years ago).

What is life?

It is usually supposed that all organisms eat (take in energy) and excrete (produce waste products), respire (or breathe in some way), grow, reproduce (give rise to young), respond to external stimuli (e.g. react to the heat or cold) and move (although this may be to a limited extent in the case of, say, a tree). The bacterium is the most primitive kind of living organism, and although the virus may be considered more primitive in that it seems to span the prebiological/biological boundary, it does not show any life signs independent of a host cell.

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