3200 – 3100 million years ago

Year fifteen
A fossil of the bacterium Eobacterium isolatum, from the Fig Tree rocks of South Africa

Eobacterium — early fossil bacteria

When Earth was 15 (about 3100 million years ago), the Fig Tree Formation of South Africa was deposited in a shallow sea. The Fig Tree rocks are the oldest rocks that are definitely known to contain fossils. These are fossil bacteria and they are very, very, very, very, very small! Have a look at a millimetre on a ruler. Imagine cutting that millimetre into 1000 little pieces! Now take just one of those little pieces and cut it in half again! For hundreds of millions of years, all living organisms were microscopic.

Each of the tiniest pieces that make up our skin, bones, blood and everything else in our bodies is called a cell. Mammals, like elephants, mice, you and me, are made of millions and millions of cells. But for the first 37 or 38 years (3700 to 3800 million years) of Earth's life, all living things were made of just one cell.


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