Bacteria are some of the oldest life forms on Earth, with fossil evidence placing their emergence at around 3.5 billion years ago. They are classified into two kingdoms, Archaea and Eubacteria, based on rRNA sequencing. While Archaea and Eubacteria both lack nuclei, they differ in their cell wall structure - Archaea have no peptidoglycan but special lipids, while Eubacteria contain peptidoglycan. Eubacteria are further classified by their shape as cocci, bacilli, or spirilla and differentiated as Gram positive or Gram negative based on their staining properties related to cell wall thickness.

1. Bacteria