Bacteria are one of the earliest living organisms that evolved on the Earth. This is before plants and animals. From a study of fossils scientists estimate the earliest bacteria to have evolved around 3.5 billion years ago.
The cell structure of bacteria is simple and very different from that of the cells of plants and animals.
Bacterial cells do not have any organelles that are bound by membranes except ribosome which contain granules of RNA and where protein synthesis takes place.
A bacterial cell is filled with cytoplasm in which molecules of DNA float freely as they do not have a nucleus. In addition to this the only membrane bound organelle is the ribosome.