The bacteria are assigned to the kingdom Monera, also called Prokaryotae. On average, the prokaryotic cells of bacteria are much smaller, and much simpler in structure, than eukaryotic cells. Most prokaryotes are one-celled, although many kinds occur as filaments (strings) or clumps of independent cells.
Some bacterial fossils date from 3.5 billion years ago, long before the evolution of cells recognizable as eukaryotes of the modern type. Today, bacteria are still by far the most numerous organisms, flourishing in almost every conceivable habitat, including our own bodies.