Wikijunior:Biology/Kingdoms/Archaea
Archaea
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Archaea (Archaeon if there's only one) are a kingdom of creatures made up of single cells. They have been on Earth for a long, long time - longer than plants, animals, or protists. Their name means "old" in Ancient Greek. They used to be grouped in with bacteria and called "archaeabacteria", but they were later put into their own kingdom because they're so different.
Like bacteria, archaea are tiny, microscopic creatures, far too small to be seen with just your eyes. They also have no nucleus, like bacteria as well. Unlike bacteria, however, archaea can live in super hot and super cold climates, and also can't give you any nasty diseases.
Archaea can live inside of animals - some even inside of humans. Don't worry though, they can't hurt you. Many archaea live inside your intestines, feeding on matter your body can't process. Sometimes, this can even help with digestion! Some other animals use archaea inside them in other ways, too; for example, archaea living inside of termites help them process the wood they chew through.