Page within: Evolution

The Archaea comprise a group of single-celled microorganisms that, like bacteria, are prokaryotes and having no cell nucleus or any other organelles within their cells. Consequently, they were once considered to be an unusual group of bacteria and named archaebacteria. However, it in now known that Archaea have an independent evolutionary history and have numerous differences in their biochemistry to other forms of life. The differences are so great that they are now classified as a distinctly separate domain in the three-domain system. Carl Woese introduced the three main branches of evolutionary descent as the Archaea, Eukaryota and Bacteria. Classifying Archaea remains difficult, since the vast majority of these organisms have never been studied in the laboratory and have only been detected by analysis of their nucleic acids in environmental samples.

Archaea are an ancient form of life, possibly the most ancient. Putative fossils of archaean cells in stromatolites have been dated to almost 3.5 billion years ago, and the remains of lipids that may be either archaean or eukaryotic have been detected in shales dating from 2.7 billion years ago. Since most prokaryotes do not have distinct morphologies, the shapes of fossils cannot be used to identify them as Archaea. Instead, chemical fossils, in the form of the unique lipids found in archaea are used, and such lipids have now been detected in rocks dating back to the Precambrian. The oldest known traces of these isoprene lipids have been found in Greenland, which include sediments formed 3.8 billion years old and are the oldest on Earth; some scientists, however, dispute this claim.

The theory of endosymbiosis proposes that Eukaryotic life evolved from the Archaea. That is, the theory explains that organelles such as mitochondria and chloroplasts in eukaryotic cells evolved from certain types of bacteria that prokaryotic cells engulfed through endophagocytosis. These cells and the bacteria trapped inside subsequently evolved a symbiotic relationship. In this endosymbiotic relationship, the bacteria lived within the other prokaryotic cells.