Eukaryote

Lore & Background

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. 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.

Reader's Guide

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 microscopic predation 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. Eukaryotes represent only a small minority of the total number of organisms on Earth, but due to their generally much larger sizes, their collective global biomass is much greater than that of prokaryotes.

Did You Know?

• The word eukaryote is derived from the Greek words 'eu' (εὖ) meaning 'true' or 'good' and 'karyon' (κάρυον) meaning 'nut' or 'kernel', referring to the nucleus of a cell.
• Eukaryotes are organisms that range from microscopic single cells, such as picozoans under 3 micrometers across, to animals like the blue whale, weighing up to 190 tonnes and measuring up to 33.6 meter long, or plants li
• Eukaryotic cells are typically much larger than those of prokaryotes—the bacteria and the archaea—having a volume of around 10,000 times greater.
• Eukaryotes represent a small minority of the number of organisms, but, as many of them are much larger, their collective global biomass (468 gigatons) is far larger than that of prokaryotes (77 gigatons), with plants alo