Organisms
About 4 billion years ago, the first life to appear on Earth was very simple. The first living things on our planet were tiny, single-celled organisms. Scientists have a special word for this kind of life: prokaryotes (pro-KAR-ee-yo-tees). Many of these are bacteria.
We can't see them, but bacteria are so common that they make up one of the three 'domains', or empires, of life. Your body is home to about 1 trillion bacteria. A trillion is 1,000,000,000,000 - about 150 times the number of people on the planet.
Not just nasty germs
Bacteria get a lot of bad press. Most people think of them as nasty. Some of them are. But with 10 times as many of them on and in our bodies as cells, they can't be all bad. Of course, it is true that bad bacteria in food can poison us. Everyone experiences eating something 'bad' at some point. That's bacteria our body does not want.
But we also deliberately introduce bacteria and other organisms called yeasts to certain foods to transform them in a process called fermentation. We add types of bacteria to milk, for example, to make yoghurt or cheese. Kimchi or soy sauce would be impossible to make without bacteria. They even play a role in making chocolate: cocoa fruit must be fermented before their beans can be processed.
