The first cells appeared on Earth about 3.5 billion years ago. Note that one of these is called a bacterium. Later bacteria evolved many new features. For example bacteria could swim. They used a long twisted whip-like tail called a flagellum fixed to a wonderful tiny rotating motor.
Bacteria have only one cell each.
Some bacteria benefit humans and other plants and animals. Bacteria differ from more advanced cells such of those found in animals and plants because they have no membrane around their nucleus nor any organelles . Simple cells like this are called prokaryotes . Bacteria make up the group which biologists call monera . Archaebacteria are probably living fossils, similar to the earliest bacteria.