Eukaryote
The word eukaryote comes from Greek roots meaning true kernel, referring to the nucleus inside a cell. This defining feature separates them from bacteria and archaea which lack such a structure. Inside these cells lies a membrane-bound nucleus that stores DNA in linear bundles called chromosomes. A microtubular spindle separates matching sets of chromosomes during nuclear division. Eukaryotic cells also contain various internal structures known as organelles. These include the endoplasmic reticulum and the Golgi apparatus. The cytoskeleton provides stiffening structure and points of attachment for motor proteins. Actin filaments and microtubules make up this dynamic network. Motor proteins like dynein and kinesin move materials within the cell. Flagella and cilia allow many eukaryotes to move or feed. These projections are composed mainly of tubulin and differ entirely from prokaryotic flagella.
Eukaryotes emerged within the archaeal phylum Promethearchaeota during the Paleoproterozoic era. Leading theory suggests they formed through symbiogenesis between an anaerobic Promethearchaeota archaeon and an aerobic proteobacterium. This merger created mitochondria which function as the powerhouse of the cell. Mitochondria have their own DNA with close structural similarities to bacterial DNA. They produce energy by oxidizing sugars or fats to create ATP molecules. A second episode of symbiogenesis involved a cyanobacterium creating chloroplasts in plants. Chloroplasts contain chlorophyll and produce organic compounds through photosynthesis. Some modern organisms still retain these organelles derived from ancient bacteria. Even species thought to lack mitochondria possess mitochondrion-derived organelles like hydrogenosomes. The last eukaryotic common ancestor likely had at least one centriole and flagellum. It also possessed facultatively aerobic mitochondria and sexual reproduction capabilities.
Aristotle and Theophrastus recognized two lineages of animals and plants in antiquity. Linnaeus gave these lineages the taxonomic rank of kingdom in the 18th century. He initially included fungi with plants though later scientists found them distinct enough for a separate kingdom. Georg A. Goldfuss coined the word Protozoa in 1818 to refer to organisms such as ciliates. Ernst Haeckel expanded this group into a kingdom called Protista in 1866. This system viewed protists as primitive forms united by their unicellular nature. DNA sequencing led Carl Woese, Otto Kandler, and Mark Wheelis to propose domains instead of kingdoms in 1990. They named the domain Eucarya uniting all eukaryote kingdoms. Lynn Margulis proposed replacing kingdoms and domains with inclusive names based on symbiosis in 1996. By the early 21st century phylogenomic studies placed most eukaryotes in clades dubbed Amorphea or Diphoda. A third major grouping called Excavata was abandoned as formal because it proved paraphyletic. Modern classification relies heavily on genomic similarities rather than visible shared characteristics.
The earliest multicellular eukaryote discovered so far is Qingshania magnifica from North China. It lived approximately 1.635 billion years ago during the Statherian period. Unicellular eukaryotes like Tappania plana appeared around 1.65 billion years ago. Some acritarchs are known from at least 1.65 billion years ago while Grypania may be an alga up to 2.1 billion years old. The problematic fossil Diskagma has been found in paleosols 2.2 billion years old. Structures called Francevillian biota date back 2.1 billion years but their status as fossils remains contested. The oldest valid biomarker records for eukaryotes appear only around 800 million years ago. Steranes previously indicated presence in Australian shales dated 2.7 billion years old but were later rebutted as contaminants. Molecular clock analysis suggests sterol biosynthesis emerged as early as 2.3 billion years ago. A massive increase in zinc composition of marine sediments occurred approximately a billion years after origin. This rise attributed to substantial populations of eukaryotes consuming and incorporating zinc relative to prokaryotes.
Eukaryotic cells typically have a volume around 10,000 times greater than those of bacteria or archaea. Organisms range from microscopic picozoans under 3 micrometres across to blue whales weighing up to 190 tonnes. Coast redwoods reach heights exceeding 100 meters while giant kelp extends over 60 meters long. Plants alone account for over 81% of Earth's total biomass totaling 468 gigatons. Prokaryotes collectively possess only 77 gigatons despite representing the majority of organism numbers. Multicellularity evolved independently at least 25 times within eukaryotes. Complex multicellular organisms exist within six lineages including animals brown algae red algae green algae land plants and symbiomycotan fungi. Many groups contain unicellular species alongside multicellular forms. The collective global biomass of eukaryotes far exceeds that of prokaryotes due to their larger size. Eukaryotes represent a small minority of organism count but dominate in terms of mass.
Eukaryotes reproduce both asexually through mitosis and sexually through meiosis with gamete fusion. Sexual reproduction involves alternating between haploid phase with one chromosome copy and diploid phase with two copies. Haploid gametes like eggs and spermatozoa fuse to form a zygote which grows into a body. Meiosis reduces chromosome number creating genetic variability essential for evolution. Core genes functioning in meiosis appear in Trichomonas vaginalis and Giardia intestinalis suggesting ancient origins. Species once thought asexual such as Leishmania parasites actually possess sexual cycles. Amoebae previously regarded as asexual may be anciently sexual though present-day groups could have arisen recently. Facultative sex was likely present in the common ancestor of all eukaryotes according to phylogenetic analysis by Dacks and Roger. Eukaryotes exhibit lower metabolic rates and longer generation times than prokaryotes because they are larger. This results in a smaller surface area to volume ratio affecting their biological processes.
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Common questions
What is the origin of the word eukaryote?
The word eukaryote comes from Greek roots meaning true kernel, referring to the nucleus inside a cell. This defining feature separates them from bacteria and archaea which lack such a structure.
When did eukaryotes emerge within the archaeal phylum Promethearchaeota?
Eukaryotes emerged during the Paleoproterozoic era through symbiogenesis between an anaerobic Promethearchaeota archaeon and an aerobic proteobacterium. This merger created mitochondria which function as the powerhouse of the cell.
Who proposed domains instead of kingdoms for eukaryotes in 1990?
Carl Woese, Otto Kandler, and Mark Wheelis proposed domains instead of kingdoms in 1990. They named the domain Eucarya uniting all eukaryote kingdoms.
How old are the earliest multicellular eukaryote fossils discovered so far?
The earliest multicellular eukaryote discovered so far is Qingshania magnifica from North China. It lived approximately 1.635 billion years ago during the Statherian period.
Why do eukaryotic cells have lower metabolic rates than prokaryotes?
Eukaryotes exhibit lower metabolic rates and longer generation times because they are larger. This results in a smaller surface area to volume ratio affecting their biological processes.