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Endosymbiotic Theory – Origin of the eukaryotic cell

What is Endosymbiotic Theory?

  • The Endosymbiotic Theory is a foundational concept in cellular biology, elucidating the evolutionary origin of eukaryotic organelles, specifically mitochondria and chloroplasts. This theory posits that these organelles originated from once free-living prokaryotic organisms that were engulfed by ancestral eukaryotic cells.
  • Historically, the evolutionary trajectory of organisms is believed to have commenced from a singular prokaryotic ancestor, akin to contemporary bacteria.
  • This primitive organism possessed a singular DNA strand encased within a plasma membrane. As evolutionary processes ensued, diversification among these bacteria led to the emergence of distinct metabolic pathways. For instance, certain bacteria evolved photosynthetic capabilities, enabling them to harness sunlight to synthesize sugars.
  • Concurrently, other organisms refined the oxidative phosphorylation process, facilitating the conversion of sugar into adenosine triphosphate (ATP), the cell’s primary energy currency.
  • These metabolic innovations conferred a reproductive advantage, allowing these organisms to proliferate more efficiently than their counterparts. However, some species, lacking these advanced metabolic pathways, faced potential extinction unless they evolved novel survival strategies. It is during this period that the phenomenon of endocytosis is believed to have emerged.
  • This process allowed certain cells to engulf others, leading to a symbiotic relationship where the engulfed cell, instead of being digested, was retained and utilized for its metabolic capabilities. Such an intracellular symbiotic relationship, where both entities derive mutual benefits, is termed endosymbiosis.
  • The Endosymbiotic Theory extends this understanding by suggesting that over evolutionary timescales, genetic material could be exchanged between the host cell and its endosymbiont. This genetic interplay is believed to be responsible for the distinct DNA types observed in eukaryotic organelles.
  • Notably, while most cellular functions are orchestrated by the nucleus, organelles like mitochondria and chloroplasts retain their own genetic machinery.
  • Furthermore, the presence of double membranes surrounding these organelles is postulated to be remnants of the engulfment process. Specifically, the theory suggests that the dual membranes represent the original membrane of the engulfed prokaryote and the engulfing eukaryote’s membrane.
  • Empirical evidence robustly supports the Endosymbiotic Theory. The distinct genetic signatures of mitochondria and chloroplasts, their double-membraned structure, and their resemblance to extant prokaryotes collectively validate this theory.
  • In essence, the Endosymbiotic Theory offers a compelling narrative on the evolutionary amalgamation of two distinct organisms, leading to the intricate cellular architecture observed in present-day eukaryotes.

Definition of Endosymbiotic Theory

The Endosymbiotic Theory posits that certain organelles in eukaryotic cells, specifically mitochondria and chloroplasts, originated from free-living prokaryotic cells that were engulfed by ancestral eukaryotic cells, leading to a mutually beneficial symbiotic relationship.

Origin of the eukaryotic cell

1. Gene Trees and the Complexity of Eukaryotic Origins

The evolutionary origins of eukaryotic cells have been a subject of intense scientific investigation and debate. One of the challenges in understanding eukaryotic evolution lies in the intricate nature of gene trees, which are not as straightforward as they might initially appear.

  1. Lateral Gene Transfer and Endosymbiosis: A comprehensive understanding of eukaryotic origins necessitates the consideration of lateral gene transfer (LGT) among prokaryotes, endosymbiosis, and the transfer of genes from organelles to the nucleus.
  2. Individual Gene Histories: It has become evident that each gene possesses its unique evolutionary history. To obtain a holistic view of eukaryotic evolution, it would be imperative to amalgamate individual gene trees into a unified diagram, factoring in the evolutionary relationships of the plastid, mitochondrion, and the host.
  3. Concatenated Phylogenies: The prevalent approach to understanding eukaryotic origins involves analyzing a concatenated set of genes, typically around 30, with the hope that the resulting tree would be representative of the entire genome. Most of these genes are involved in information processing, such as ribosomal proteins.
  4. Multiple Ribosomes in Eukaryotes: Due to endosymbiosis, eukaryotes can possess multiple sets of ribosomes with distinct evolutionary origins. For instance, the cytosol contains archaeal ribosomes, while mitochondria and plastids have bacterial ribosomes. However, most studies have predominantly focused on the archaeal component, potentially overlooking significant insights from the bacterial genes.
  5. Challenges with Core Gene Sets: An early study highlighted concerns with the core gene set approach, noting the limited sequence conservation across sites. There were also apprehensions about the potential incongruence in the evolutionary histories of individual genes within the set.
  6. Archaeal Component of Eukaryotes: Despite the realization that genes of archaeal origin in eukaryotes represent only a fraction of the genome, overshadowed by genes of bacterial origin, the focus has remained predominantly on the archaeal component. This focus might persist until methodologies evolve to effectively synthesize information from thousands of individual gene trees.
  7. Revised Phylogenetic Insights: Embley and colleagues, employing advanced phylogenetic methods, discerned that the archaeal component of eukaryotes branches within the archaea, specifically aligning with the TACK superphylum.

In conclusion, the study of gene trees in the context of eukaryotic evolution underscores the intricate and multifaceted nature of cellular evolution. While significant strides have been made in understanding the origins of eukaryotes, the field remains ripe for further exploration, especially as methodologies advance and allow for a more nuanced synthesis of vast genomic data.

2. The Origin of the Nucleus in Eukaryotic Cells

Models describing the origin of the nucleus in eukaryotes.
Models describing the origin of the nucleus in eukaryotes. Image Source:

The nucleus stands as a hallmark feature of eukaryotic cells, and its evolutionary origin has been a subject of extensive scientific discourse.

  1. Historical Theories:
    • Endosymbiotic Theory: Mereschkowsky postulated that the nucleus evolved from a prokaryote (specifically a mycoplasma) that was engulfed by an amoeboid cell, leading to the formation of the nucleus.
    • Plasma Membrane Invagination: Cavalier-Smith proposed that the nuclear and endoplasmic reticulum (ER) membranes originated from invaginations of a prokaryotic cell’s plasma membrane. This theory suggests that ribosomes attached to the plasma membrane became internalized, forming the rough ER and subsequently the nuclear envelope.
    • Endospore Formation: Gould & Dring theorized that the nucleus originated during endospore formation in Gram-positive bacteria, where a cell engulfs a portion of its cytoplasm, leading to the formation of a double-membraned nucleus.
  2. Endokaryotic Models:
    • These models generally propose that a eubacterial host engulfed an archaebacterial endosymbiont, which then transformed into the nucleus.
    • Fuerst & Webb observed a structure in the bacterium Gemmata obscuriglobus that seemed to resemble a nucleus. However, subsequent studies clarified that this structure was an invagination of the plasma membrane.
    • Lake & Rivera proposed an endosymbiotic event where a bacterium engulfed an archaeon, leading to the origin of eukaryotes.
    • Moreira & López-García introduced the principle of anaerobic syntrophy, suggesting a fusion between δ-proteobacteria and a methanogenic archaebacterium, resulting in the nucleus’s evolution.
  3. Viral Origin and Other Theories:
    • Bell proposed a viral origin for the nucleus involving poxviruses.
    • Horiike suggested that the nucleus emerged from an archaeal endosymbiont engulfed by a γ-proteobacterium.
    • Some theories even posited that eukaryotes (and their nucleus) originated before prokaryotes.
  4. Recent Developments:
    • Forterre introduced a new variant of the endokaryotic hypothesis, suggesting a fusion involving planctomycetes, a thaumarchaeon, and viruses. However, there’s no molecular evidence linking planctomycetes with eukaryotes.
    • The recognition that eukaryotes’ common ancestor had a mitochondrion challenges many of the existing theories, especially those that do not account for the origin of mitochondria.
  5. Challenges with Existing Models:
    • Many models that propose the nucleus’s origin from an endosymbiont struggle to explain the nucleus’s unique features, such as its distinct membrane topology, permeability, and division mechanism.
    • The thermoreduction hypothesis, which suggests that prokaryotes evolved from eukaryotes, faces challenges, especially regarding the origin of mitochondria.

In summary, the origin of the nucleus in eukaryotic cells remains an intricate puzzle. While various theories have been proposed over the years, each comes with its set of challenges and unanswered questions. The nucleus’s evolution is a testament to the complexity of cellular evolution and the myriad factors that have shaped the diverse life forms we observe today.

3. The Origin of Mitochondria and Chloroplasts: A Synthesis of Endosymbiotic Theories

Models describing the origin of mitochondria and/or chloroplasts in eukaryotes.
Models describing the origin of mitochondria and/or chloroplasts in eukaryotes. Image Source:

The cellular architecture of eukaryotes is distinguished by the presence of mitochondria and chloroplasts, organelles that play pivotal roles in energy production and photosynthesis, respectively. The origin of these organelles has been a subject of extensive scientific inquiry, with the endosymbiotic theory emerging as the most widely accepted explanation.

  1. Historical Foundations:
    • Endosymbiotic Theory: Mereschkowsky proposed the symbiotic relationship between ‘chromatophores’ (plastids) and a heterotrophic amoeboid cell. He envisioned chromatophores as symbionts that established a mutualistic relationship with the host cell. He further postulated multiple independent origins for the plant kingdom based on the diverse pigmentation of algal plastids. Although this multi-origin view was later refuted, the endosymbiotic origin of plastids from cyanobacteria gained consensus.
  2. Mitochondrial Origins:
    • Early Theories: Wallin suggested that mitochondria descended from endosymbiotic bacteria. However, like Portier, he believed that mitochondria could be cultured outside their host cells, a view that was later debunked.
    • Symbiotic Evolution: Lynn Sagan (later Margulis) rejuvenated the endosymbiotic theory, proposing that both chloroplasts and mitochondria evolved from distinct endosymbiotic events. She hypothesized that an aerobic bacterium was first engulfed, leading to mitochondria, followed by the engulfment of a spirochaete, resulting in the eukaryotic flagellum.
    • Alternative Models: Raff & Mahler proposed a non-symbiotic origin for mitochondria, suggesting that membrane-bound vesicles evolved from invaginations of the inner cell membrane. Other models, like those of Bogorad and Cavalier-Smith, proposed non-symbiotic origins or different sequences of endosymbiotic events.
  3. Chloroplast Origins:
    • Mereschkowsky’s theory emphasized the symbiotic relationship between cyanobacteria and a host cell, leading to the evolution of chloroplasts. This view was later supported by molecular evolutionary studies.
  4. Recent Developments:
    • Hydrogen Hypothesis: This theory posits a symbiotic relationship mediated by hydrogen transfer between an anaerobic archaebacterium (host) and a eubacterium (endosymbiont), leading to the evolution of mitochondria.
    • Inside-Out Theory: David & Buzz Baum proposed that eukaryotic evolution was driven by a mutualistic association between an archaeal host and an epibiotic α-proteobacterium. The host cell’s protrusions and bleb enlargements increased the contact area, leading to the evolution of the eukaryotic cell structure.
  5. Challenges and Considerations:
    • Predation and phagocytosis have been frequently cited as mechanisms facilitating endosymbiotic events. However, these processes are more about digestion than symbiosis. The true essence of endosymbiosis lies in the chemistry and mutualistic interactions between the host and the symbiont.

In conclusion, the origin of mitochondria and chloroplasts in eukaryotic cells is intricately tied to the endosymbiotic theory, which posits that these organelles evolved from once free-living prokaryotic organisms. Over the years, various models have been proposed to elucidate the specifics of these symbiotic events, reflecting the complexity and significance of this evolutionary milestone.

4. Anaerobes and the Mitochondrial Origin in a Prokaryotic Host

Mitochondrial origin in a prokaryotic host.
Mitochondrial origin in a prokaryotic host. Image Source:
  • The intricate evolutionary journey of eukaryotic cells has been a subject of scientific intrigue for decades. Central to this discourse is the endosymbiotic theory, which postulates the origin of eukaryotic organelles, notably mitochondria, from a symbiotic relationship between distinct prokaryotic organisms. This article delves into the nuances of this theory, emphasizing the role of anaerobes and the mitochondrial origin within a prokaryotic host.
  • Historically, the endosymbiotic theory has been rooted in comparative physiology, particularly in the realms of core carbon and energy metabolism. Various formulations of this theory have been proposed over the years, each with its strengths and limitations. However, a standout proposition that accounts for anaerobic mitochondria is the hydrogen hypothesis. This hypothesis, grounded in comparative physiology, suggests that the host for the origin of mitochondria was an archaeon, not a eukaryote. This perspective has gained traction over the years, with the consensus now leaning towards the ubiquity of mitochondria across eukaryotic lineages.
  • A pivotal aspect of the hydrogen hypothesis is the nature of host-symbiont interactions at the onset of mitochondrial symbiosis. It posits an anaerobic syntrophic relationship, wherein the host is a H2-dependent archaeon and the symbiont, a facultative anaerobe. This symbiotic relationship is metabolically intertwined, with the host relying on the symbiont for hydrogen production, a crucial component for its survival.
  • The hydrogen hypothesis also underscores the significance of gene transfer from the symbiont to the host. This transfer not only facilitates the host’s metabolic needs but also carries with it group II introns, which are believed to have played a pivotal role in the origin of the nucleus. These introns, upon entering the eukaryotic lineage, spread across the host’s chromosomes, leading to a situation where splicing became a slow process compared to the rapid pace of translation. This disparity necessitated the physical separation of splicing from translation, giving rise to the nuclear membrane and, by extension, the eukaryotic cell’s distinct nucleus.
  • Furthermore, the transition from an autotrophic to a heterotrophic lifestyle by the host, facilitated by the symbiont’s gene transfer, is of paramount importance. This shift allowed the host to harness energy more efficiently, paving the way for the myriad of eukaryotic novelties. However, these innovations came at an energetic cost, which was offset by the mitochondria.
  • In conclusion, the evolution of eukaryotic cells, with their distinct nucleus and organelles, is a complex interplay of symbiotic relationships, gene transfers, and metabolic shifts. The hydrogen hypothesis offers a compelling perspective on this evolutionary journey, placing anaerobes and the mitochondrial origin at the heart of eukaryogenesis. The intricate balance of energy demands and innovations underscores the indispensable role of mitochondria in shaping the eukaryotic cell’s identity.

5. Rounding Out The Picture: The Plastid in Eukaryotic Evolution

Evolution of anaerobes and the plastid.
Evolution of anaerobes and the plastid.
  • The evolutionary narrative of eukaryotic cells is incomplete without acknowledging the significant role of plastids, organelles that originated from an endosymbiotic relationship with cyanobacteria. This article seeks to elucidate the intricacies surrounding the origin and significance of plastids in the eukaryotic lineage.
  • Figure 4 delineates the evolutionary trajectory of the plastid. The ancestral eukaryote, from an energy metabolism perspective, was a facultative anaerobe. This organism underwent diversification, adapting to both aerobic and anaerobic environments, leading to the emergence of specialized eukaryotes. The presence of enzymes for anaerobic energy metabolism in a wide array of eukaryotes, including algae like Chlamydomonas, suggests that the host for the origin of plastids was indeed a facultative anaerobe.
  • The inception of plastids is rooted in endosymbiotic theory. A eukaryote, already equipped with a mitochondrion, acquired a cyanobacterium as an endosymbiont. The metabolic interplay between the host and the cyanobacterium could have revolved around carbohydrate production, oxygen generation, or nitrogen supply by the plastid. Phylogenetic analyses indicate that the cyanobacterial ancestor of the plastid was likely a large-genomed, nitrogen-fixing form. As with mitochondria, gene transfer from the endosymbiont to the host’s chromosomes was pivotal, facilitating the integration of the two entities while preserving the biochemical identity of the endosymbiont. This led to the divergence of the three primary plastid lineages: chlorophytes, rhodophytes, and glaucocystophytes. Subsequent evolutionary events saw secondary endosymbioses involving both green and red algal endosymbionts.
  • The traditional view of endosymbiotic theory, as proposed by Mereschkowsky, posits the cyanobacterial endosymbiont as the foundational event leading to primary plastids. However, a recent variant of this theory suggests that the plastid symbiosis began with a chlamydial infection of a eukaryotic cell, which was subsequently “cured” by the cyanobacterium. This chlamydial hypothesis, though gaining traction in some scientific circles, has been met with skepticism. Critical analyses by researchers such as Deschamps and Domman et al. have highlighted potential phylogenetic artefacts that challenge the validity of the chlamydial theory. Given the complexities of phylogenetic interpretations and the prevalence of lateral gene transfer among prokaryotes, it is imperative to approach such hypotheses with caution and rely on a holistic array of evidence.
  • In conclusion, the plastid, with its cyanobacterial origins, stands as a testament to the intricate and multifaceted evolutionary journey of eukaryotic cells. While the exact nuances of its origin remain a subject of debate, its significance in shaping the eukaryotic landscape is undeniable.

6. Organelles and the Retention of Genomes: A Redox Regulation Perspective

  • The presence of DNA within organelles has been a cornerstone observation that bolstered the endosymbiotic theory, a theory suggesting that certain organelles originated from symbiotic bacteria that were engulfed by ancestral eukaryotic cells. This discovery of DNA within organelles, such as mitochondria and plastids, was pivotal in challenging alternative theories that sought to explain the existence of DNA in these cellular compartments without invoking endosymbiosis.
  • A pressing question that arises from this observation is: Why have organelles retained their own DNA? The most compelling answer to this query is encapsulated in John F. Allen’s CoRR hypothesis, which stands for “co-location for redox regulation.” This hypothesis proposes that the retention of genomes within organelles is crucial for the regulation of components involved in the respiratory and photosynthetic electron transport chains. This regulation ensures the maintenance of redox balance within the bioenergetic membrane of the organelle.
  • Redox balance, in essence, refers to the uninterrupted flow of electrons through the electron transport chain. Both mitochondria and chloroplasts possess electron transport chains that create proton gradients, which in turn drive ATP synthesis. A critical component of these chains is the quinols and quinones, which are membrane-soluble electron carriers. In situations where the electron flow is disrupted, these carriers can non-enzymatically transfer electrons to oxygen, leading to the formation of the superoxide radical (O2−), a precursor for reactive oxygen species (ROS). An imbalance in this flow, either due to insufficient downstream components or overactive upstream components, can elevate the concentration of quinols, subsequently increasing ROS production.
  • The CoRR hypothesis underscores the convergence observed in the gene content of plastid and mitochondrial genomes. Both organelles have evolved to predominantly encode genes associated with their respective electron transport chains and the necessary ribosomal components for their expression. An intriguing aspect of this convergence is the similar set of ribosomal proteins encoded by both organelles, which is postulated to be essential for ribosome biogenesis within the organelle.
  • An interesting corollary of the CoRR hypothesis pertains to hydrogenosomes, organelles that resemble mitochondria but lack the respiratory chain in their inner membrane. Given that the CoRR hypothesis attributes the retention of organelle genomes to the need for redox balance, the absence of an electron transport chain in hydrogenosomes implies that there’s no selective pressure to maintain their DNA. This prediction aligns with observations that hydrogenosomes in various lineages, including trichomonads, ciliates, fungi, and amoeboflagellates, have lost their genomes.
  • In conclusion, the retention of genomes within organelles is not a mere evolutionary relic but serves a critical function in maintaining the redox equilibrium within these cellular compartments. The CoRR hypothesis offers a robust framework to understand this phenomenon, emphasizing the intricate interplay between organelle genetics and cellular bioenergetics.

7. Eukaryotic Origins and the Archaeal Phylogenetic Landscape

  • The evolutionary relationship between eukaryotes and archaea has been a subject of intense scientific scrutiny. Recent discussions have centered around the TACK superphylum, encompassing Thaumarchaeota, Aigarchaeota, Crenarchaeota, and Korarchaeota, as potentially housing the closest ancestors of the host that eventually acquired the mitochondrion. Several phylogenetic trees have been proposed to elucidate this relationship, revealing intriguing patterns.
  • One notable observation from these trees is the placement of eukaryotic informational genes within the crenarchaeotes. Such placements often root the archaea within euryarchaeotes or methanogens. This rooting is significant, given that methanogens possess characteristics that render them strong contenders for the most ancient among the archaeal lineages. Methanogenesis, the production of methane, is the oldest biological process with evidence in the geological isotope record, dating back approximately 3.5 billion years. This ancient metabolic pathway was considered primordial even before the discovery of archaea.
  • Supporting the antiquity of methanogenesis, abiotic methane production has been observed at serpentinizing hydrothermal vents. Among known geochemical reactions, only serpentinization mirrors the core bioenergetic reactions of some extant microbial cells. If we accept that the ancestral state of archaeal carbon and energy metabolism is methanogenesis, it implies that all archaea are ancestrally methanogenic and dependent on hydrogen. This hydrogen dependence is pivotal for models of eukaryotic origins that suggest an anaerobic syntrophy, where a hydrogen-dependent archaeal host gave rise to mitochondria.
  • Recent studies suggest that many archaeal lineages, including haloarchaea, evolved from methanogenic ancestors through gene transfers. This transition from an anaerobic hydrogen-dependent state to a facultatively anaerobic heterotrophic state mirrors the transformation posited for eukaryotic origins. The primary distinction lies in the location of the respiratory chain: in haloarchaea, it’s in the archaeal cytoplasmic membrane, while in eukaryotes, it’s in the mitochondria’s internalized bioenergetic membranes. This distinction is pivotal, as it demarcates the metabolic energy boundaries that allowed eukaryotes to evolve novel protein families and cellular traits.
  • In conclusion, as we refine our understanding of eukaryotes’ position within the archaeal tree, the root’s position among archaea becomes clearer. The recurring theme seems to be multiple evolutionary transitions from an ancestrally hydrogen-dependent state, with bacterial gene transfers facilitating access to alternative electron and energy sources. The early chapters of evolution are intricate, and the emergence of eukaryotes stands out as a pivotal event, marking the advent of complex life.

Lynn Margulis And Endosdymbiotic Genesis Theory

The evolutionary origins of eukaryotic cells have been a subject of scientific intrigue and debate. Until the 1970s, the prevailing hypothesis posited that eukaryotic cells evolved from prokaryotic predecessors through a gradual process, wherein the organelles of the eukaryotic cell became increasingly sophisticated over time. However, this perspective underwent a paradigmatic shift due to the groundbreaking work of Lynn Margulis, a scientist based at Boston University.

Margulis championed the endosymbiotic theory, which postulated a radically different mechanism for the evolution of eukaryotic cells. According to this theory, certain organelles within eukaryotic cells, notably mitochondria and chloroplasts, originated from independent prokaryotic cells. These smaller prokaryotic cells were believed to have been engulfed by a larger host cell, subsequently establishing a symbiotic relationship within the host’s cytoplasm.

The term “endosymbiont” aptly describes this relationship, as it encapsulates the idea of one organism living inside another in a mutually beneficial arrangement. This theory suggests that the genesis of complex eukaryotic cells was not merely a result of incremental evolutionary changes but was facilitated by symbiotic interactions between distinct cellular entities.

The ancestral prokaryotic organisms were thought to be anaerobic heterotrophs. Being anaerobic, they derived energy from organic matter without utilizing molecular oxygen. As heterotrophs, they were incapable of synthesizing organic compounds from inorganic precursors and relied on obtaining preformed organic compounds from their surroundings.

A proposed model for the endosymbiotic evolution of eukaryotic cells is as follows:

  1. A large anaerobic, heterotrophic prokaryote engulfs a smaller aerobic prokaryote. The engulfed prokaryote is believed to have been related to modern-day rickettsia, a group of bacteria known for causing diseases like typhus.
  2. Over time, the engulfed aerobic prokaryote evolves into what we recognize today as a mitochondrion.
  3. Concurrently, the host cell undergoes structural changes, with invaginations of the plasma membrane evolving into the nuclear envelope and the endoplasmic reticulum.
  4. This primitive eukaryote then diverges into two evolutionary paths. In one trajectory, it evolves into nonphotosynthetic protists, fungi, and animals.
  5. In the alternative trajectory, the primitive eukaryote engulfs a photosynthetic prokaryote. This endosymbiont eventually evolves into a chloroplast, giving rise to photosynthetic eukaryotic cells.

In conclusion, Lynn Margulis’s contributions to the endosymbiotic theory have profoundly reshaped our understanding of eukaryotic evolution. By positing a symbiotic origin for key organelles, she provided a compelling framework that explains the complexity and diversity of eukaryotic cells.

Lynn Margulis And Endosdymbiotic Genesis Theory
Lynn Margulis And Endosdymbiotic Genesis Theory

Key Points of Endosymbiotic Theory

The Endosymbiotic Theory provides a comprehensive framework elucidating the evolutionary origins of eukaryotic cells. Drawing from the provided content, the following are the pivotal tenets of this theory:

  1. Initial Symbiotic Event: A significant event postulated by the theory is the ingestion of a small, aerobic prokaryote by a larger, anaerobic, heterotrophic prokaryote. Instead of being digested, the smaller prokaryote established itself as a permanent endosymbiont within the host cell’s cytoplasm. As the host cell proliferated, the endosymbiont did likewise, leading to a colony of composite cells.
  2. Evolution of Mitochondria: Over successive generations, these endosymbionts underwent evolutionary changes. Many of their original traits, which became redundant within the host environment, were lost. Consequently, these oxygen-respiring microbes gradually evolved into entities resembling modern-day mitochondria.
  3. Emergence of Eukaryotic Features: The composite cells, having acquired their endosymbionts, underwent further evolutionary transformations. They developed hallmark eukaryotic features, including intricate membrane systems (like the nuclear membrane, endoplasmic reticulum, and Golgi complex), a sophisticated cytoskeleton, and a mitotic mechanism for cell division. These features are believed to have evolved progressively and not through a singular endosymbiotic event.
  4. Origin of Internal Membranes: The theory posits that certain internal membranes, such as the endoplasmic reticulum and nuclear membranes, evolved from portions of the cell’s external plasma membrane. This external membrane underwent internalization and subsequent modifications, giving rise to distinct membrane structures.
  5. Ancestry of Heterotrophic Eukaryotes: Cells possessing these evolved internal compartments represent the ancestral lineage of heterotrophic eukaryotic cells, encompassing entities like fungi and certain protists. Fossil evidence indicates that the earliest eukaryotic remnants date back approximately 1.8 billion years.
  6. Acquisition of Chloroplasts: A subsequent endosymbiotic event is believed to have transformed an early heterotrophic eukaryote into a photosynthetic ancestor. This transformation was facilitated by the engulfment of a cyanobacterium, which eventually evolved into chloroplasts. This event, estimated to have occurred around one billion years ago, marked the genesis of photosynthetic eukaryotes, including green algae and plants. Notably, while chloroplasts are exclusive to plants and algae, mitochondria or evidence of their ancestral presence is ubiquitous across all known eukaryotic groups.

In summary, the Endosymbiotic Theory offers a structured narrative detailing the evolutionary journey of eukaryotic cells, emphasizing the pivotal role of symbiotic relationships in shaping cellular complexity.

Importance of Endosymbiotic Theory

The Endosymbiotic Theory holds paramount importance in the realm of biology for several reasons:

  1. Evolutionary Significance: The theory provides a comprehensive explanation for the evolutionary transition from simple prokaryotic cells to complex eukaryotic cells. It offers insights into how eukaryotic cells, with their intricate organelle structures, evolved from ancestral prokaryotic forms.
  2. Understanding Cellular Complexity: It elucidates the origins of vital eukaryotic organelles, namely mitochondria and chloroplasts. By understanding their prokaryotic ancestry, we gain insights into their unique functions, genetic material, and double-membraned structures.
  3. Genetic Implications: The theory highlights the phenomenon of horizontal gene transfer between the engulfed prokaryotes and their eukaryotic hosts. This has led to the unique genetic configuration observed in eukaryotic organelles, where they retain some of their own DNA while other genes have been integrated into the host cell’s nucleus.
  4. Biological Diversity: The Endosymbiotic Theory underscores the significance of symbiotic relationships in driving evolutionary innovation. Such relationships have played a pivotal role in shaping the vast diversity of life forms on Earth.
  5. Foundation for Further Research: The theory has paved the way for extensive research in cellular biology, genetics, and evolutionary biology. It provides a framework for studying the co-evolution of symbiotic partners and the intricacies of cellular processes.
  6. Implications for Extraterrestrial Life: Understanding the evolutionary processes that led to complex life on Earth, such as endosymbiosis, can inform hypotheses and explorations related to the potential existence and nature of life on other planets.

In essence, the Endosymbiotic Theory has reshaped our understanding of cellular evolution, emphasizing the interconnectedness and interdependence of life forms and highlighting the adaptive strategies that have driven the diversification of life on Earth.

Endosymbiotic Theory Evidence

The Endosymbiotic Theory, which postulates the origin of eukaryotic organelles from ancestral prokaryotic cells, has garnered substantial empirical support over the years. This support primarily stems from advancements in molecular biology, particularly DNA sequencing, and the structural analysis of cells. Here, we elucidate the key pieces of evidence that bolster this theory.

  1. DNA Sequencing and Molecular Homology: The advent of DNA sequencing has enabled precise comparisons of DNA molecules across diverse organisms. When sequences of DNA are identical or highly similar between two organisms, it suggests a shared evolutionary lineage rather than independent evolution. Analyses of mitochondrial DNA (mtDNA) and chloroplast DNA have revealed striking similarities with certain bacterial DNA sequences. For instance, mtDNA shares sequence homology with the DNA of Rickettsiaceae bacteria, which are obligate intracellular parasites. Such molecular congruence underscores the theory’s proposition of a shared ancestral lineage.
  2. Circular DNA in Organelles: Unlike the linear DNA configuration typical of eukaryotic nuclei, both mtDNA and chloroplast DNA are circular, mirroring the DNA structure observed in bacteria. Moreover, the genes associated with nucleotide, lipid, and amino acid biosynthesis are conspicuously absent in these organelles, aligning with the theory’s assertion that endosymbiotic entities would gradually lose functions redundant to those provided by the host cell.
  3. Protein and RNA Analysis: Proteins, RNA, and DNA within organelles like mitochondria and chloroplasts exhibit characteristics distinct from the eukaryotic cell’s nucleus. Some proteins within these organelles are too hydrophobic to traverse the organelle’s external membrane, suggesting that their genes could not have been transferred to the nucleus. Furthermore, mitochondria and chloroplasts possess their own genetic code and ribosomes, which are essential for protein synthesis. Notably, the ribosomes in these organelles are more akin to the smaller bacterial ribosomes than the larger eukaryotic counterparts, reinforcing the theory’s premise of a bacterial origin.
  4. Organelle Structure and Positioning: The structural configuration of mitochondria, chloroplasts, and nuclei, all encapsulated by double membranes, is evocative of bacterial cells. The central positioning of their DNA within the cytoplasm further mirrors bacterial cellular architecture. While the nucleus’s origin remains a subject of ongoing research, both mitochondria and chloroplasts exhibit remarkable structural resemblance to specific intracellular bacterial species.
  5. Existence of Contemporary Endosymbionts: Present-day ecosystems abound with endosymbiotic bacteria residing within various organisms. These bacteria have carved out specialized niches within host cells, exemplifying the symbiotic relationships postulated by the Endosymbiotic Theory.

In summation, the confluence of molecular, structural, and ecological evidence robustly supports the Endosymbiotic Theory. This theory not only offers a coherent narrative for the evolution of eukaryotic organelles but also underscores the intricate interplay and co-evolution of diverse life forms.


What is the primary premise of the endosymbiotic theory?
a) Eukaryotic cells evolved from prokaryotic cells.
b) Eukaryotic cells evolved from other eukaryotic cells.
c) Mitochondria and plastids originated as free-living bacteria that were engulfed by a host cell.
d) All cells have a common ancestor that was neither prokaryotic nor eukaryotic.

Which organelle is primarily believed to have originated from a cyanobacterium according to the endosymbiotic theory?
a) Nucleus
b) Mitochondria
c) Plastid
d) Ribosome

Which evidence supports the endosymbiotic theory?
a) Mitochondria and plastids have their own DNA.
b) Eukaryotic cells lack ribosomes.
c) Prokaryotic cells have multiple organelles.
d) Eukaryotic cells reproduce only by binary fission.

Which organelle is believed to have been the first to be engulfed according to the endosymbiotic theory?
a) Plastid
b) Nucleus
c) Mitochondria
d) Lysosome

The DNA of mitochondria and plastids is similar to the DNA of which type of cells?
a) Eukaryotic cells
b) Archaea
c) Viruses
d) Bacteria

Which of the following is NOT a characteristic shared by mitochondria and bacteria?
a) Circular DNA
b) Binary fission
c) 80S ribosomes
d) Double membrane

Which scientist is credited with proposing the endosymbiotic theory in the early 20th century?
a) Charles Darwin
b) Gregor Mendel
c) Konstantin Mereschkowski
d) Rosalind Franklin

Which event is believed to have occurred after the engulfment of a prokaryotic cell by a host cell, according to the endosymbiotic theory?
a) The engulfed cell became a parasite.
b) The engulfed cell was digested.
c) The engulfed cell formed a mutualistic relationship with the host.
d) The host cell died.

Which organelle is NOT believed to have originated from endosymbiosis?
a) Mitochondria
b) Plastids
c) Golgi apparatus
d) Chloroplasts

The endosymbiotic theory suggests that eukaryotic cells evolved from a symbiotic relationship between different species of which type of cells?
a) Eukaryotic cells only
b) Prokaryotic cells only
c) Both eukaryotic and prokaryotic cells
d) Neither eukaryotic nor prokaryotic cells


What is the endosymbiotic theory?

The endosymbiotic theory proposes that certain organelles in eukaryotic cells, specifically mitochondria and plastids, originated from free-living bacteria that were engulfed by ancestral eukaryotic cells and formed a mutualistic relationship.

Who proposed the endosymbiotic theory?

The theory was first proposed by the American biologist Lynn Margulis in the 1960s, although earlier versions were suggested by Konstantin Mereschkowski in the early 20th century.

What evidence supports the endosymbiotic theory?

Several pieces of evidence support this theory, including the presence of circular DNA in mitochondria and plastids, similarities in ribosomes between these organelles and bacteria, and the double-membrane structure of these organelles.

Why is the endosymbiotic theory important?

The theory provides a comprehensive explanation for the origin of eukaryotic cells and the evolution of complexity in life. It highlights the significance of mutualistic relationships in evolution.

Do all eukaryotic cells support the endosymbiotic theory?

While most eukaryotic cells have mitochondria, not all have plastids. However, the presence of mitochondria in diverse eukaryotic lineages supports the theory’s premise.

How does the endosymbiotic theory explain the origin of chloroplasts?

The theory suggests that chloroplasts originated from free-living cyanobacteria that were engulfed by ancestral eukaryotic cells, leading to a mutualistic relationship.

Why do mitochondria and plastids have their own DNA?

This is a key piece of evidence for the endosymbiotic theory. It suggests that these organelles were once independent organisms with their own genetic material before becoming part of eukaryotic cells.

How does the endosymbiotic theory relate to the evolution of multicellular organisms?

The development of eukaryotic cells with complex organelles, as explained by the endosymbiotic theory, paved the way for the evolution of multicellular organisms by providing increased metabolic versatility and efficiency.

Are there any criticisms or challenges to the endosymbiotic theory?

While the endosymbiotic theory is widely accepted, some details and mechanisms are still debated. For instance, the exact sequence of endosymbiotic events and the number of times such events occurred are topics of ongoing research.

What are the implications of the endosymbiotic theory for modern biology?

The theory underscores the importance of symbiotic relationships in evolution and provides a framework for understanding the origin and evolution of eukaryotic cell complexity. It also has implications for studies in cell biology, genetics, and evolutionary biology.


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