Eukaryote

The eukaryotes () are cellular organisms of the domain Eukaryota or Eukarya, which have a distinct cell nucleus (formed by a nuclear membrane), membrane-bound organelles and organization supported by a cytoskeleton. Extant eukaryotes are traditionally grouped into four kingdoms: animals, plants, fungi and protists. "Protists" are a basal paraphyletic group that includes all algae, amoebae, flagellates and ciliates; however, this is now considered a wastebasket taxon and has been replaced with newer classifications that better represent the phylogeny of these organisms. As part of the prevailing three-domain system, eukaryotes constitute a major group of organisms alongside the two domains of prokaryotes: Bacteria and Archaea. Cladistically, eukaryotes emerged within the archaeal phylum Promethearchaeota, colloquially also known as "Asgard archaea". Ignoring mitochondrial DNA (which comes from a bacterial lineage rather than an archaeal one), this would imply only two domains of life, Bacteria and Archaea, with eukaryotes incorporated as a clade under the Archaea. Eukaryotes first emerged during the Paleoproterozoic, and the leading evolutionary theory is that they were the result of a metabolically mutualistic symbiogenesis between an anaerobic Promethearchaeota archaeon and an endosymbiont aerobic alphaproteobacterium (i.e. proto-mitochondrion) in response to the oxidative stress following the Great Oxygenation Event. A second symbiogenetic event between an eukaryotic host (a protist) and a beta-cyanobacteria endosymbiont during the Mesoproterozoic led to the latter becoming chloroplasts, and resulted in the evolution of the photosynthetic clade Archaeplastida, which includes all green algae, red algae, glaucophytes and land plants. Eukaryotes can be unicellular or multicellular, and their cell sizes are significantly (hundreds of times) larger than prokaryotes. They have biomembranes enclosing their nucleus and organelles such as the endoplasmic reticulum, Golgi apparatus, ribosome, vacuole and lysosome, as well as endosymbiont-derived organelles (which are bound by two or more layers of membrane) such as mitochondria and various plastids. Eukaryotes...