Eukaryotic cells likely began as an archaeal merger
Eukaryotes keep their DNA in a membrane-wrapped nucleus and fill their cells with organelles held in shape by a cytoskeleton. The leading theory says they arose in the Paleoproterozoic when an Asgard archaeon teamed up with an oxygen-using bacterium that became the mitochondrion.
The name joins Greek eu, true, and karyon, kernel, for the nucleus. That partnership is thought to have helped the host cope with oxidative stress after the Great Oxygenation Event. Family-tree studies place eukaryotes inside the archaeal phylum Promethearchaeota, so if mitochondrial DNA is set aside, life has only two domains, Bacteria and Archaea. A second merger in the Mesoproterozoic, when a protist host engulfed a cyanobacterium, produced chloroplasts and the lineage that includes green and red algae and land plants.
Eukaryotic cells hold roughly 10,000 times the volume of a typical bacterium. Their DNA sits on linear chromosomes wound around histone proteins, and a spindle of microtubules pulls copies apart during mitosis; they can also reproduce sexually through meiosis and fertilisation. Proteins made on the rough endoplasmic reticulum are shipped in vesicles to the Golgi apparatus, while lysosomes digest unwanted molecules. Mitochondria, which make ATP on folded inner membranes, still carry circular DNA like that of their bacterial ancestors.
The group spans picozoans under 3 micrometres wide to 190-tonne blue whales and redwoods 120 metres tall. Multicellularity has evolved independently at least 25 times, but complex multicellular life appears in only six lineages: animals, certain fungi, brown, red and green algae, and land plants. The old term protist, covering algae, amoebae, flagellates and ciliates, is now considered a wastebasket and is being replaced by groupings based on genomes.
Eukaryotes are a small minority of all organisms, yet their global biomass of about 468 gigatons dwarfs the 77 gigatons of prokaryotes, and plants alone account for over 81 percent of Earth's biomass.
Source: Eukaryote
