Origin and diversity of eukaryotes

Origin

Life on Earth is classed into three phylogenetic domains: the Archaea, the Bacteria and the Eucarya. Eukaryotic cells are less diverse than prokaryotes metabolically, but have a complex cellular architecture comprising a nucleus, a cytoskeleton (a proteinic network structuring the cytoplasm to facilitate intracellular traffic, endo- and exo-cytosis and amoeboid locomotion; Cavalier-Smith, 2002), an endomembrane system (a system of internal membranes subdivided into several organelles, and used for synthesis, processing, packaging and transport of macromolecules such as lipids and proteins) and organelles such as mitochondria (or derived organelles) and chloroplasts in photosynthetic eukaryotes. Archaea and Bacteria are called prokaryotes because their cells do not contain a nucleus or organelles, and because transcription and translation are coupled, i.e. they do not occur in different cellular compartments. The prokaryotes have diverse and complex metabolisms, but simpler cellular architecture. They possess proteins playing the role of cytoskeleton, but not the motor proteins involved in intracellular transport as in eukaryotes (Moller-Jensen and Lowe, 2005; Cabeen and Jacobs-Wagner, 2005) and some (the planctomycetales) may possess endomembrane systems (e.g. Fuerst, 2005).

The eukaryotic genome is seen as a mosaic of bacterial genes (involved in energy and carbon metabolism) and archaeal genes (related to DNA replication, transcription and translation), but it also contains a core set of genes and proteins unique to eukaryotes (e.g. Kurland et al., 2006), while the membrane lipids are closer to those of the bacteria (Rivera et al., 1998).