A relatively sparse fossil record is available to help discern what the first members of each of these lineages looked like, so it is possible that all the events that led to the last common ancestor of extant eukaryotes will remain unknown. Figure 1. They are about 3.5 billion years old and are recognizable because of their relatively complex structure and, for prokaryotes, relatively large cells. Koonin, E. V., J. Dacks, W. Doolittle, et al. The origin and cellular complexity of eukaryotes represent a major enigma in biology. The iconic rooted 3-domains tree of life shows eukaryotes and archaebacteria as separate groups that share a common ancestor to the exclusion of eubacteria. Endosymbiotic events probably contributed to the origin of the last common ancestor (LCA) of today’s eukaryotes. 2017. (a) Red algae and (b) green algae (visualized by light microscopy) share similar DNA sequences with photosynthetic cyanobacteria. One of these functions is to generate clusters of iron and sulfur that are important cofactors of many enzymes. As cell biology developed in the twentieth century, it became clear that mitochondria were the organelles responsible for producing ATP using aerobic respiration. The green algal endosymbiont also exhibits a stunted vestigial nucleus. Image credit: Christa Schleper, University of Vienna. Over the years, the origin of eukaryotes has been rewritten several times as scientists encounter new species or develop new tools for genome analysis. The link was not copied. The origin of eukaryotes is a huge enigma and a major challenge for evolutionary biology [1–3]. Although Margulis’ work initially was met with resistance, this once-revolutionary hypothesis is now widely (but not completely) accepted, with work progressing on uncovering the steps involved in this evolutionary process and the key players involved. In fact, it appears that chlorarachniophytes are the products of an evolutionarily recent secondary endosymbiotic event. Numerous microscopic and genetic studies have supported this conclusion. 1. The nuclear genome of eukaryotes is related most closely to the Archaea, so it may have been an early archaean that engulfed a bacterial cell that evolved into a mitochondrion. When such cells are carrying out photosynthesis, their plastids are rich in the pigment chlorophyll a and a range of other pigments, called accessory pigments, which are involved in harvesting energy from light. 2013. Eventually, the amount of photosynthetic oxygen built up in some environments to levels that posed a risk to living organisms, since it can damage many organic compounds. Biologists are almost certain that … Archaea and the origin of eukaryotes. Studies of the breadth of eukaryotic diversity help paint a convincing picture of a last eukaryotic common ancestor possessed of mitochondria, a complete cytoskeleton and trafficking machinery. Therefore, although these organelles are highly integrated into the eukaryotic cell, they still reproduce as if they are independent organisms within the cell. However, mitochondria cannot survive outside the cell. However, many of the genes for respiratory proteins are located in the nucleus. During Margulis' research on the origin of eukaryotic cells, she studied data on prokaryotes, eukaryotes, and organelles, finally proposing that similarities between prokaryotes and organelles, combined with their appearance in the fossil record, was best explained by something called "endosymbiosis" (meaning "to cooperate inside.") Gould, S. B., R. F. Waller, and G. I. McFadden. The few eukaryotes with chromosomes lacking histones clearly evolved from ancestors that had them. The chloroplasts of red and green algae, for instance, are derived from the engulfment of a photosynthetic cyanobacterium by an early prokaryote. (b) In this micrograph of Elodea sp., the chloroplasts can be seen as small green spheres. Each mitochondrion measures 1 to 10 or greater micrometers in length and exists in the cell as an organelle that can be ovoid to worm-shaped to intricately branched (Figure 1). As the atmosphere was oxygenated by photosynthesis, and as successful aerobic prokaryotes evolved, evidence suggests that an ancestral cell with some membrane compartmentalization engulfed a free-living aerobic prokaryote, specifically an alpha-proteobacterium, thereby giving the host cell the ability to use oxygen to release energy stored in nutrients. Not all plastids in eukaryotes are derived directly from primary endosymbiosis. This hypothesis was also championed by Lynn Margulis. (credit: Louise Howard). Gould, et al. In a primary endosymbiotic event, a heterotrophic eukaryote consumed a cyanobacterium. This is the single characteristic that is both necessary and sufficient to define an organism as a eukaryote. These organelles were first observed by light microscopists in the late 1800s, where they appeared to be somewhat worm-shaped structures that seemed to be moving around in the cell. Analyzes the origins of key eukaryotic protein regulatory modules using comparative genomics. Summarizes the diversity of models for the origin of the nuclear compartment, arguing against nuclear endosymbiotic models. Structures this size, which might be fossils, appear in the geological record about 2.1 billion years ago. There is also evidence of secondary endosymbiotic events. On the origin of eukaryotes: When cells got complex New light on a crucial moment in the evolution of life: When cells acquired mitochondria Date: For instance, it is not known whether the endosymbiotic event that led to mitochondria occurred before or after the host cell had a nucleus. Eukaryotic cells may contain anywhere from one to several thousand mitochondria, depending on the cell’s level of energy consumption. this page. Molecular and morphological evidence suggest that the chlorarachniophyte protists are derived from a secondary endosymbiotic event. Archaebacteria (Archaea) and the origin of the eukaryotic nucleus. Introduction The appearance of the eukaryotes (all organisms that have cells in which the nucleus is enclosed within a membrane) represents one of the most fundamental milestones in the history of life (Butterfield, 2015). In addition to the aerobic generation of ATP, mitochondria have several other metabolic functions. On the age of eukaryotes: Evaluating evidence from fossils and molecular clocks. Over many generations, a symbiotic relationship can result in two organisms that depend on each other so completely that neither could survive on its own. (a) This chloroplast cross-section illustrates its elaborate inner membrane organization. Plastid evolution. Without oxygen, aerobic respiration would not be expected, and living things would have relied on fermentation instead. Sex, a process of genetic recombination unique to eukaryotes in which diploid nuclei at one stage of the life cycle undergo meiosis to yield haploid nuclei and subsequent karyogamy, a stage where two haploid nuclei fuse together to create a diploid zygote nucleus. 2015 and Baum 2015. In both cases, the presence of enzymes is necessary, and their origin is not fully understood. At some point before, about 3.5 billion years ago, some prokaryotes began using energy from sunlight to power anabolic processes that reduce carbon dioxide to form organic compounds. Cyanobacteria also have the peptidoglycan wall and lipopolysaccharide layer associated with Gram-negative bacteria. The resulting organelle became a plastid in modern chlorarachniophytes. Eukaryotes (/ juːˈkærioʊts, - əts /) are organisms whose cells have a nucleus enclosed within a nuclear envelope. The origins of eukaryotes have always been, and may always be elusive. Mitochondria have their own (usually) circular DNA chromosome that is stabilized by attachments to the inner membrane and carries genes similar to genes expressed by alpha-proteobacteria. Organisms that did not acquire aerobic respiration had to remain in oxygen-free environments. Cold Spring Harbor Perspectives in Biology 6:a016139. 2008 focuses on the acquisition of plastids and subsequent additional endosymbiotic events. Archaea and the origin of eukaryotes Woese and Fox's 1977 paper on the discovery of the Archaea triggered a revolution in the field of evolutionary biology by showing that life was divided into not only prokaryotes and eukaryotes. Koonin, et al. 2013 summarizes phylogenetic arguments for the phylogenetic model that dominates current thinking, namely that archaea/eukarya and bacteria represent the two primary domains of life. Such organisms would be among the extinct precursors of the last common ancestor of eukaryotes. Several lines of evidence support that chlorarachniophytes evolved from secondary endosymbiosis. Current data support scenarios in which an archaeal host cell and an alphaproteobacterial (mitochondrial) endosymbiont merged together, resulting in the first eukaryotic cell. It also differentiates the 1. Origin of Eukaryotes Time Period: Proterozoic. In a secondary endosymbiotic event, the cell resulting from primary endosymbiosis was consumed by a second cell. These two competing views are relevant to the origin of the eukaryotes since they each suggest different characteristics of the eukaryotic progenitor. Mitochondria appear to have originated from an alpha-proteobacterium, whereas chloroplasts originated as a cyanobacterium. Eme, L., S. C. Sharpe, M. W. Brown, et al. Williams, et al. Expand or collapse the "in this article" section, Historical Perspectives on the Chimeric Ancestry of Eukaryotes, Cooperation and Conflict: Microbes to Humans, Cooperative Breeding in Insects and Vertebrates. Discusses models for eukaryogenesis in the light of newly discovered and characterized archaeal lineages. Describe the hypothesized steps in the origin of eukaryotic cells. The process of secondary endosymbiosis is not unique to chlorarachniophytes. Prokaryotes contribute as decomposers and recyclers to such … However, later findings suggest that reduced organelles are found in most, if not all, anaerobic eukaryotes, and that all eukaryotes appear to carry some genes in their nuclei that are of mitochondrial origin. Modern accounts of eukaryogenesis entail an endosymbiotic encounter between an archaeal host and a proteobacterial endosymbiont, with subsequent evolu Press. However, not enough is known about eukaryotes’ cell walls and their development to know how much homology exists among them. In 1967, Margulis introduced new work on the theory and substantiated her findings through microbiological evidence. Genome Biology 11:209. An archaeal origin of eukaryotes supports only two primary domains of life. Much still remains to be discovered about the origins of the cells that now make up the cells in all living eukaryotes. Users without a subscription are not able to see the full content on Mapping the characteristics found in all major groups of eukaryotes reveals that the following characteristics must have been present in the last common ancestor, because these characteristics are present in at least some of the members of each major lineage. Figure 3. In order to understand eukaryotic organisms fully, it is necessary to understand that all extant eukaryotes are descendants of a chimeric organism that was a composite of a host cell and the cell(s) of an alpha-proteobacterium that “took up residence” inside it. Therefore, most biologists accept that the last common ancestor of eukaryotes had mitochondria. Most mitochondria are shaped like alpha-proteobacteria and are surrounded by two membranes, which would result when one membrane-bound organism was engulfed into a vacuole by another membrane-bound organism. Your current browser may not support copying via this button. Each chloroplast is surrounded by two membranes. 2014 links molecular data, which drive much of the field, to fossil evidence. What evidence is there that mitochondria were incorporated into the ancestral eukaryotic cell before chloroplasts? The origin of the eukaryotes must have appeared before because the fossil is of a relative complex single-celled organism. Rather, they revealed that prokaryotes comprise two distinct types of organisms, the Bacteria and the Arch … Fossil records indicate that eukaryotes evolved from prokaryotes somewhere between 1.5 to 2 billion years ago. Plastids are derived from cyanobacteria that lived inside the cells of an ancestral, aerobic, heterotrophic eukaryote. (credit b: modification of work by Brandon Zierer; scale-bar data from Matt Russell). If the last common ancestor could make cell walls, it is clear that this ability must have been lost in many groups. Mitosis is universally present in eukaryotes. Given the great structural chasm between these cell types, the question of eukaryotic origins is one of the most enduring mysteries in modern biology. Fossils older than this all appear to be prokaryotes. This is currently rectified as a sequential loss of a membrane during the course of evolution. It became widely present among prokaryotes, including in a group we now call alpha-proteobacteria. There are cases where tertiary or higher-order endosymbiotic events are the best explanations for plastids in some eukaryotes. Alpha-proteobacteria are a large group of bacteria that includes species symbiotic with plants, disease organisms that can infect humans via ticks, and many free-living species that use light for energy. Some appear to lack organelles that could be recognized as mitochondria. When these genes are compared to those of other organisms, they appear to be of alpha-proteobacterial origin. 2008. Eukaryotic cells arose through endosymbiotic events that gave rise to the energy-producing organelles within the eukaryotic cells such as mitochondria and chloroplasts. Cyanobacteria are a group of Gram-negative bacteria with all the conventional structures of the group. Mitosis, a process of nuclear division wherein replicated chromosomes are divided and separated using elements of the cytoskeleton. The Lokiarchaea are surprisingly similar to modern eukaryotes, suggesting they share a relatively recent common ancestor. In search of cell history: The evolution of life’s building blocks. Photosynthetic plastids are called chloroplasts (Figure 2). There is also, as with the case of mitochondria, strong evidence that many of the genes of the endosymbiont were transferred to the nucleus. This major theme in the origin of eukaryotes is known as endosymbiosis, one cell engulfing another such that the engulfed cell survives and both cells benefit. Eukaryotic cells arose through endosymbiotic events that gave rise to the energy-producing organelles within the eukaryotic cells such as mitochondria and chloroplasts. What evidence is there that mitochondria were incorporated into the ancestral eukaryotic cell before chloroplasts? By contrast, the eocyte hypothesis has eukaryotes originating within the archaebacteria and sharing a common ancestor with a particular group called the Crenarchaeota or eocytes. American Journal of Botany 102:1954–1965. The first eukaryote may have originated from an ancestral prokaryote that had undergone membrane proliferation, compartmentalization of cellular function (into a nucleus, lysosomes, and an endoplasmic reticulum), and the establishment of endosymbiotic relationships with an aerobic prokaryote, and, in some cases, a photosynthetic prokaryote, to form mitochondria and chloroplasts, respectively. Over the course of the 20th century, advances in cytology, the characterization of DNA as the universal genetic material, and pioneering work on phylogenies of ribosomal RNA all combined to establish a common origin for all life and identified a deep split in the prokaryotic world between the domains of archaea (once called Archaebacteria) and bacteria (once called Eubacteria). Apart from the convincing demonstration that plastids, of which chloroplasts are the most familiar, and mitochondria are derived from endosymbiotic bacteria, the field of eukaryotic origins remains full of uncertainties and controversy. Cell biological arguments have been used to support a bewildering variety of models for the origins of the nucleus and other aspects of eukaryotic cellular organization. Eme, L., A. Spang, J. Lombard, C. W. Stairs, and T. J. G. Ettema. Annual Review of Plant Biology 59:491–517. Apart from the convincing demonstration that plastids, of which chloroplasts are the most familiar, and mitochondria are derived from endosymbiotic bacteria, the field of eukaryotic origins remains full of uncertainties and controversy. Oxford Bibliographies Online is available by subscription and perpetual access to institutions. For reviews of eukaryogenesis, refer to Martin, et al. All other living things that lack a nucleus, mitochondria, and the eukaryote LEGO-like skeleton are known as prokaryotes. Oxygen levels similar to today’s levels only arose within the last 700 million years. The best evidence is that this has happened twice in the history of eukaryotes. Also, recall that all extant eukaryotes descended from an ancestor with mitochondria. Figure \(\PageIndex{1}\): Endosymbiosis: Modern eukaryotic cells evolved from more primitive cells that engulfed bacteria with useful properties, such as energy production. Phylogenetic Comparative Methods and Tests of Macroevoluti... Quantitative Genetic Variation and Heritability. The last common ancestor may have had a cell wall for at least part of its life cycle, but more data are needed to confirm this hypothesis. The last common ancestor of today’s Eukarya had several characteristics, including cells with nuclei that divided mitotically and contained linear chromosomes where the DNA was associated with histones, a cytoskeleton and endomembrane system, and the ability to make cilia/flagella during at least part of its life cycle. It was aerobic because it had mitochondria that were the result of an aerobic alpha-proteobacterium that lived inside a host cell. Eukaryotes belong to the domain Eukaryota or Eukarya; their name comes from the Greek εὖ (eu, "well" or "good") and κάρυον (karyon, "nut" or "kernel"). The process of aerobic respiration is found in all major lineages of eukaryotes, and it is localized in the mitochondria. One of the major features distinguishing prokaryotes from eukaryotes is the presence of mitochondria. These features all support that mitochondria were once free-living prokaryotes. The oldest eukaryotic fossil is approximately 1.5 billion years old. Endosymbiotic events likely contributed to the origin of the last common ancestor of today’s eukaryotes and to later diversification in certain lineages of eukaryotes (Figure 4). Hydrogen, derived from various sources, was captured using light-powered reactions to reduce fixed carbon dioxide in the Calvin cycle. However, comparative biology of extant organisms and the limited fossil record provide some insight into the history of Eukarya. Although the origin of the eukaryotic cell has long been recognized as the single most profound change in cellular organization during the evolution of life on earth, this transition remains poorly understood. Flagella and cilia, organelles associated with cell motility. The hypothesized process of endosymbiotic events leading to the evolution of chlorarachniophytes is shown. Their cells contain, in addition to the standard eukaryotic organelles, another kind of organelle called a plastid. 2014. Endosymbiotic theories for eukaryote origin. The most widely used identification of a eukaryotic cell is the presence of … Various metabolic processes evolved that protected organisms from oxygen, one of which, aerobic respiration, also generated high levels of ATP. Prior to the origin of eukaryotes, all life on Earth was prokaryotic (lacking nucleus or other membrane-bound organelles). While today’s atmosphere is about one-fifth molecular oxygen (O2), geological evidence shows that it originally lacked O2. Most other prokaryotes have small cells, 1 or 2 µm in size, and would be difficult to pick out as fossils. (credit a: modification of work by Ed Bierman; credit b: modification of work by G. Fahnenstiel, NOAA; scale-bar data from Matt Russell). The origin of eukaryotes is a major evolutionary transition for which we lack much information about intermediate stages. Data from these fossils have led comparative biologists to the conclusion that living eukaryotes are all descendants of a single common ancestor. At the close of the 20th century, phylogenetic data had been used to support either the three-domain view of life (monophyletic Bacteria, Archaea, and Eukarya) or a competing two-domain model, which features a paraphyletic archaeal grade from which the eukaryotes emerged. 2. EUKARYOTIC Meaning: "characterized by well-defined cells (with nuclei and cell walls)," 1957, from French eucaryote (1925),… See definitions of eukaryotic. Drawing on diverse aspects of cell biology and phylogenetic … Chlorophyll is a component of these membranes, as are many of the proteins of the light reactions of photosynthesis. Specifically, mitochondria are not formed from scratch (de novo) by the eukaryotic cell; they reproduce within it and are distributed with the cytoplasm when a cell divides or two cells fuse. eukaryotes evolved through a process whereby different types of free-living prokaryotes became incorporated inside larger prokaryotic cells and eventually developed into mitochondria, chloroplasts, and possibly other organelles origin of the eukaryotes. A Microbial Death Star. The last few years in particular have generated a great deal of excitement, as newly discovered archaeal genomes, which includes a complete genome from the recently cultured Asgard archaeon Prometheoarchaeum syntrophicum, have shifted the consensus steadily toward models of eukaryotes emerging from the symbiosis of an Asgard-like archaeal host and a protomitochondrial bacterial cell. Figure 2. Whether this host had a nucleus at the time of the initial symbiosis remains unknown. The word is Greek for “true kernel,” referring to the nucleus. Reviews the molecular composition of archaeal and bacterial phospholipid membranes and consequences for models of eukaryogenesis. Nature Reviews Microbiology 10:507–515. Although the larger eukaryotes—namely plants, animals, and fungi—dominate our visual landscapes, microbial lineages compose the greater part of both genetic diversity and biomass, and contain many evolutionary innovations. A crucial date in the history of eukaryotes lies around 2.4 billion years ago, when molecular oxygen started rising in the Earth's atmosphere 4. Identifying the Genomic Basis Underlying Phenotypic Variat... Natural Selection in the Genome, Detecting, New Zealand, Evolutionary Biogeography of. “It’s the deepest divide in the living world,” says William Martin of the University of Düsseldorf in Germany. In parallel, newly sequenced bacterial and archaeal genomes have revealed prokaryotic homologues for many genes originally deemed eukaryotic “inventions,” reducing the perceived gap between prokaryotic and eukaryotic complexity. Refer to Figure 4. Aerobic respiration is also found in many lineages of prokaryotes, but it is not present in all of them, and many forms of evidence suggest that such anaerobic prokaryotes never carried out aerobic respiration nor did their ancestors. Nature 504:231–236. This is called primary endosymbiosis, and plastids of primary origin are surrounded by two membranes. Like mitochondria, plastids appear to have an endosymbiotic origin. Mitochondria divide independently by a process that resembles binary fission in prokaryotes. The oldest fossil evidence of eukaryotes is about 2 billion years old. Current Opinion in Microbiology 8:630–637. Over many generations, a symbiotic relationship can result in two organisms that depend on each other so completely that neither could survive on its own. Others have not “kept” any remnants. Figure 4. endosymbiosis:  engulfment of one cell within another such that the engulfed cell survives, and both cells benefit; the process responsible for the evolution of mitochondria and chloroplasts in eukaryotes, endosymbiotic theory:  theory that states that eukaryotes may have been a product of one cell engulfing another, one living within another, and evolving over time until the separate cells were no longer recognizable as such, plastid:  one of a group of related organelles in plant cells that are involved in the storage of starches, fats, proteins, and pigments, http://cnx.org/contents/[email protected]:1/Biology, List the unifying characteristics of eukaryotes, Describe what scientists know about the origins of eukaryotes based on the last common ancestor. In the 1960s, American biologist Lynn Margulis developed endosymbiotic theory, which states that eukaryotes may have been a product of one cell engulfing another, one living within another, and evolving over time until the separate cells were no longer recognizable as such. Abstract. Some extant eukaryotes have very reduced remnants of mitochondria in their cells, whereas other members of their lineages have “typical” mitochondria. All extant eukaryotes have these cytoskeletal elements. The nuclear genome of eukaryotes is related most closely to the Archaea, so it may have been an early archaean that engulfed a bacterial cell that evolved into a mitochondrion. Originally, oxygen-rich environments were likely localized around places where cyanobacteria were active, but by about 2 billion years ago, geological evidence shows that oxygen was building up to higher concentrations in the atmosphere. Recall that the first fossils that we believe to be eukaryotes date to about 2 billion years old, so they appeared as oxygen levels were increasing. Mitochondria that carry out aerobic respiration have their own genomes, with genes similar to those in alpha-proteobacteria. The first two have prokaryotic cells, and the third contains all eukaryotes. However, their reproduction is synchronized with the activity and division of the cell. Secondary plastids are surrounded by three or more membranes, and some secondary plastids even have clear remnants of the nucleus of endosymbiotic alga. Scientists speculate that, in a process called endosymbiosis, an ancestral prokaryote engulfed a photosynthetic cyanobacterium that evolved into modern-day chloroplasts. There is a sharp divide in the organizational complexity of the cell between eukaryotes, which have complex intracellular compartmentalization, and even the most sophisticated prokaryotes (archaea and bacteria), which do not [ 4 – 6 ]. The fossil record doesn't tell us much about their origin. Mitochondria arise from the division of existing mitochondria; they may fuse together; and they may be moved around inside the cell by interactions with the cytoskeleton. Reviews the phylogenetic and fossil evidence on the age of eukaryotes. Summarizes support for having only two primary domains of life, with eukaryotes being embedded within a paraphyletic Archaea. The outer membrane surrounding the plastid is thought to be derived from the vacuole in the host, and the inner membrane is thought to be derived from the plasma membrane of the symbiont. Much remains to be clarified about how this relationship occurred; this continues to be an exciting field of discovery in biology. Eme, et al. Chlorarachniophytes are rare algae indigenous to tropical seas and sand that can be classified into the rhizarian supergroup. 2010 and Lombard, et al. Surveys models of eukaryogenesis with a historical slant, focusing on origins of the nuclear and mitochondrial compartment as well as metabolic considerations. The bulk of the diversity of eukaryotic life is microbial. Eme, et al. 2. Figure 5. Phylogenomic analyses that included members of the Asgard superphylum strongly suggested that eukaryotes originated from within the Asgard archaea or … A comparison of autogenous theories for the origin of eukaryotic cells. By the end of this section, you will be able to: Living things fall into three large groups: Archaea, Bacteria, and Eukarya. Further reading. Chromosomes, each consisting of a linear DNA molecule coiled around basic (alkaline) proteins called histones. Stacks of thylakoid membranes compartmentalize photosynthetic enzymes and provide scaffolding for chloroplast DNA. The divergence of Lokiarchaeota and Eukaryota may have coincided with a merger between this common ancestor and a bacteria. Provides a comprehensive overview of plastid evolution, encompassing primary and secondary endosymbioses, protein targeting to plastids and plastid metabolism. Some living eukaryotes are anaerobic and cannot survive in the presence of too much oxygen. A cytoskeleton containing the structural and motility components called actin microfilaments and microtubules. Lombard, J., P. López-García, and D. Moreira. Today prokaryotes are found everywhere life exists on Earth and greatly outnumber all eukaryotes combined. This theory is a very well-known theory which describes about the way organisms arose and evolved. In addition, like mitochondria, plastids are derived from the division of other plastids and never built from scratch. These two competing views are relevant to the origin of the eukaryotes since they each suggest different characteristics of the eukaryotic progenitor. This major theme in the origin of eukaryotes is known as endosymbiosis, one cell engulfing another such that the engulfed cell survives and both cells benefit. Today’s eukaryotes are very diverse in their shapes, organization, life cycles, and number of cells per individual. 2015. The origin and well-resolved primary bifurcation of eukaryotes probably occurred in the Cryogenian Period, about 850 million years ago, much more recently than suggested by unwarranted backward extrapolations of molecular 'clocks' or dubious interpretations as 'eukaryotic' of earlier large microbial fossils or still more ancient steranes. Such functions are often associated with the reduced mitochondrion-derived organelles of anaerobic eukaryotes. Chloroplasts of primary origin have thylakoids, a circular DNA chromosome, and ribosomes similar to those of cyanobacteria.