Eukaryote

The eukaryotes () are cellularorganisms of the domainEukaryota or Eukarya, which have a distinct cell nucleus (formed by a nuclear membrane), membrane-bound organelles and often complex cellular architecture supported by cytoskeleton. Extant eukaryotes are traditionally grouped into four kingdoms, namely animals, plants, fungi and protists, the lattermost being a basalparaphyletic group that includes all algae, amoebae, flagellates and ciliates but now considered a wastebasket taxon and replaced with newer classifications. As part of the prevailing three-domain system, eukaryotes constitute a major group of life forms alongside the two domains of prokaryotes: the Bacteria and the Archaea.

Cladistically, eukaryotes emerged within the archaeal phylumPromethearchaeota, 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 mutualisticsymbiogenesis between an anaerobicPromethearchaeota archaeon and an endosymbiontaerobicalphaproteobacterium (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 can reproduce both asexually through mitosis, and sexually through meiosis and gamete fusion (fertilization). Due to their larger size and more complex cellular architecture, eukaryotes are capable of morphogenesis and phagocytosis, which enables microscopicpredation of other unicellular organisms. Multicellularity in some eukaryotes led to cell differentiation and functional specialization, which led to the formation of various organs and tissues and the development of organ systems. Although eukaryotes represent only a small minority of the total number of organisms on Earth, due to their generally much larger sizes, their collective global biomass is much greater than that of prokaryotes.