Eukaryote
Eukaryotes are organisms with a membrane-bound nucleus and organelles.
Gáspár Jékely · CC BY-SA 4.0
Eukaryotes are cellular organisms of the domain Eukaryota or Eukarya, characterized by a distinct cell nucleus enclosed by a nuclear membrane, membrane-bound organelles, and a cytoskeleton. They are one of the three major domains of life alongside Bacteria and Archaea, and include animals, plants, fungi, and protists. Eukaryotes are thought to have first emerged during the Paleoproterozoic, though the exact timing is debated, likely through a symbiogenetic event between an Asgard archaeon (related to Lokiarchaeota) and an aerobic alphaproteobacterium, leading to the evolution of mitochondria. A later symbiogenesis with a cyanobacterium gave rise to chloroplasts and the photosynthetic clade Archaeplastida. Eukaryotes range from microscopic unicellular organisms to massive multicellular forms like blue whales and coast redwoods, and their collective global biomass far exceeds that of prokaryotes.
- emergence
- Paleoproterozoic (debated)
Quick Facts
- Taxon
- Eukaryota
Facts from the source article.
Lore & Background
Eukaryotes are defined by their membrane-bound nucleus, which houses linear chromosomes packaged into chromatin by histone proteins. Their cells contain a variety of internal membranes, including the endoplasmic reticulum, Golgi apparatus, lysosomes, and vesicles, forming the endomembrane system. Mitochondria, derived from an endosymbiotic alphaproteobacterium, provide energy via aerobic respiration and contain their own circular DNA. Some eukaryotes have secondarily lost mitochondria but retain mitochondrion-derived organelles like hydrogenosomes or mitosomes. Plastids, such as chloroplasts, originated from cyanobacterial endosymbionts and are found in plants and various algae; some eukaryotes acquire plastids through secondary endosymbiosis or kleptoplasty. The cytoskeleton, composed of microfilaments and microtubules, enables cell movement, shape changes, and transport, while flagella and cilia provide motility.
Reader's Guide
Eukaryotes represent a fundamental branch of life, distinguished from prokaryotes by their complex cellular architecture and nucleus. Their emergence during the Paleoproterozoic, driven by symbiogenesis between an archaeon and a bacterium, marked a major evolutionary transition that enabled larger cell sizes and greater genetic complexity. This complexity allowed for the evolution of multicellularity, cell differentiation, and the formation of organs and organ systems in animals, plants, and fungi. Eukaryotes are responsible for the majority of Earth's biomass, with plants alone accounting for over 81% of the total. Despite representing a small minority of organisms by number, their ecological and evolutionary impact is immense, from microscopic predators to giant kelp forests and blue whales. The traditional four-kingdom classification (animals, plants, fungi, protists) has been refined by genomic studies, which reveal that protists are a paraphyletic wastebasket taxon, and that eukaryotes cladistically emerged within the archaeal phylum Promethearchaeota. This challenges the three-domain system, suggesting that if mitochondrial DNA is ignored, life may consist of only two domains: Bacteria and Archaea, with eukaryotes as a clade under Archaea.
Did You Know?
- Eukaryotes are thought to have first emerged during the Paleoproterozoic, though the exact timing is debated, likely through a symbiogenetic event between an Asgard archaeon (related to Lokiarchaeota) and an aerobic alph
- The word 'eukaryote' comes from Greek 'eu' (true) and 'karyon' (nut or kernel), referring to the cell nucleus.
- Multicellularity in some form has evolved independently at least 25 times within eukaryotes.
- Eukaryotes represent only a small minority of organisms by number, but their collective global biomass (468 gigatons) is far larger than that of prokaryotes (77 gigatons).
Gallery
Frequently Asked Questions
What is Eukaryote?
Eukaryote is the domain of life whose members build their cells around a membrane-wrapped nucleus plus a suite of internal membrane-bound compartments. It stands alongside Bacteria and Archaea as one of the three great branches of all living things.
How did Eukaryote's origin story go down?
The prevailing narrative places Eukaryote's emergence in the Paleoproterozoic, though the exact timing is still hotly debated. Most researchers picture a symbiogenetic merger in which an Asgard archaeon (in the Lokiarchaeota lineage) engulfed an oxygen-breathing alphaproteobacterium, and that captured bacterium eventually became the mitochondrion.
What are Eukaryote's signature traits or 'powers'?
Every eukaryotic cell is organized around a true nucleus sealed by a double membrane, a network of membrane-bound organelles, and an internal cytoskeletal framework that provides shape and motility. These features collectively set the domain apart from the structurally simpler prokaryotic lineages.
What major 'factions' or kingdoms fall under Eukaryote?
The domain encompasses the familiar kingdoms of animals, plants, and fungi, as well as the vast and morphologically diverse group of protists. Together these lineages represent the full breadth of eukaryotic diversity.
Why is Eukaryote important in the bigger evolutionary picture?
Eukaryote is the branch that gave rise to every multicellular animal, plant, and fungus on Earth, making it the source of essentially all complex life. Its origin through symbiogenesis is also one of the most dramatic turning points in the history of life, showing how two unrelated lineages fused to spawn an entirely new domain.
More in Higher-Level Taxonomy & Evolution 1-24
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