For many years biologists supposed that one group of microorganisms, which they called archaebacteria, were an ancient and primitive type of bacteria. Following biochemical analysis of their RNA and other cell components, it soon became clear that their distinct features merited classification in a separate domain, the archea. From an evolutionary viewpoint, they are of considerable interest, not being in any way dependent on photosynthesis but deriving their energy from chemical reactions or heat sources. On the primitive Earth, archean cells, acting as hosts, ingested and retained active photosynthetic and aerobic bacteria. This resulted in the formation of more complex (eukaryotic) cells, with chloroplasts, derived from photosynthetic cyanobacteria and mitochondria originating from aerobic ones. Had these two one-off processes not occurred, life on Earth might not have evolved beyond the unicellular stage. Without it, or something like it, multicellular life forms might not have evolved elsewhere in the universe. (Contains 4 figures and 2 online resources)