# How Complex Cells Evolved > In 2026 researchers in Vienna filmed Asgard archaea, the closest known microbial relatives of complex cells, crawling on long arms that appear to rely on actin. This film explains the established story behind that news: the two basic kinds of cell, the bacterial traits of mitochondria and chloroplasts, Lynn Margulis's theory of endosymbiosis, the discovery of the Asgard archaea near Loki's Castle and the first one grown in a lab, the model in which an archaeon and a bacterium merged about two billion years ago, and what the crawling microbes can and cannot tell us. Interactive film: https://www.planetpast.com/stories/how-complex-cells-evolved/ Publisher: PlanetPast (https://www.planetpast.com/) · d => T('readPublished', l, { d: dateIn(d, l) }) 2026-11-02 · Biology ## Prologue ### The cell inside the cell *Biology* How complex cells began, and what a crawling microbe adds to the story. In a laboratory in Vienna, a microbe a thousand times smaller in volume than a human cell stretches out thin arms, grips the glass, and crawls. In 2026, a team led by the University of Vienna reported in Nature that these microbes, called Asgard archaea, crawl in a way seen before only in complex cells. Why does a crawling microbe excite biologists? Because these simple microbes are the closest known relatives of the cells that build every plant, animal and fungus, including you. To see why, we need the story of how complex cells began. - **1/1000**: the volume of a human cell, roughly - **2**: Asgard strains filmed crawling: one grown in Vienna, one in Japan ![An artist’s impression of a microbe reaching out thin arms. AI-generated still.](https://assets.planetpast.com/a/45/4504f5b776023f8c.jpg) ![A complex cell holds a thousand times the volume of a bacterium, or more, and keeps its parts in separate compartments. AI-generated still.](https://assets.planetpast.com/a/82/8270f9edd6a69de9.jpg) - Two kinds of cell - Bacteria living inside cells - The search for the host - Arms that crawl Prologue #### A microbe that crawls University of Vienna (30 September 2026): Philipp Radler and colleagues in Christa Schleper’s group, with JAMSTEC (Japan), IST Austria and HZI Braunschweig, report in Nature that two Asgard archaea, a Lokiarchaeon cultivated in Vienna and a Heimdallarchaeon cultivated in Japan, attach to surfaces and crawl using dynamic protrusions. Their cells are about 1,000 times smaller in volume than a human cell. The release calls Asgard archaea the closest known microbial relatives of eukaryotes. This new study is the hook; chapter IV returns to it. The rest of the film is long-established biology. The 3D scenes are simplified teaching models, not to scale. Sources: [University of Vienna (30 Sep 2026): Unexpected dynamics of ancient microbes discovered (Radler et al., Nature, doi 10.1038/s41586-026-11063-9)](https://www.univie.ac.at/en/news/press-room/press-releases/detail/unexpected-dynamics-of-ancient-microbes-discovered) ## Chapter I · Two kinds of cell ### Every living thing is made of cells, and cells come in two basic designs. Bacteria and archaea are simple cells. Many are about a micrometre thick, and their DNA lies loose inside, with no nucleus around it. Plants, animals, fungi and single-celled organisms such as amoebas are built from complex cells, called eukaryotes. They keep their DNA inside a nucleus, wrapped in its own membrane. Eukaryotic cells are usually much larger, and full of smaller compartments called organelles. Switch between the two designs. ![Rod-shaped bacteria, in the style of an electron micrograph. AI-generated still.](https://assets.planetpast.com/a/d4/d47b26b081d0d8fa.jpg) ![An amoeba: one cell, with a nucleus. AI-generated still.](https://assets.planetpast.com/a/30/3038b42d78f0fec7.jpg) - Prokaryotes (bacteria, archaea): no nucleus, no mitochondria - Eukaryotes: nucleus and organelles Chapter I · Two kinds of cell #### Prokaryotes and eukaryotes VAAM: prokaryotes (bacteria and archaea) have no nucleus, so their DNA is not enclosed in an envelope, and they lack mitochondria and chloroplasts; many are about one micrometre thick. Biology 1010 (13.2): life is divided into prokaryotes and eukaryotes, and eukaryotes include plants, animals, fungi and protists. Try it: show the simple cell, the complex cell, or both. The 3D scenes are simplified teaching models, not to scale. Sources: [VAAM, Association for General and Applied Microbiology: What are microbes?](https://vaam.de/en/microbiology-portal/short-question/what-are-microbes/); [Biology 1010 (OpenStax-based, Utah Education Network) 13.2: Eukaryotic Origins](https://uen.pressbooks.pub/biology1010revision/chapter/eukaryotic-origins/) ### Two kinds of organelle matter most to this story. Mitochondria use oxygen to release energy from food. Plant and algal cells also have chloroplasts, which capture sunlight in photosynthesis. Compare an animal cell with a plant cell. Simple cells appear about three and a half billion years ago. For well over a billion years after that, every cell on Earth was simple. Complex cells appear about two billion years ago. Where did their nucleus and organelles come from? - **1–10 µm**: long: about the size of a bacterium ![Leaf cells crowded with chloroplasts. AI-generated still.](https://assets.planetpast.com/a/79/7961aa3b32550b87.jpg) - ~3.5 billion years ago: simple cells - ~2 billion years ago: complex cells - today: both designs still thrive ![An ancient shallow sea with microbial mats; no animals or plants yet. AI-generated still.](https://assets.planetpast.com/a/ff/ffbaad38fcc49a37.jpg) Chapter I · Two kinds of cell #### Organelles and deep time Biology 1010 (13.2): prokaryotes appear about 3.5 billion years ago and eukaryotes about 2.1 billion years ago; mitochondria are 1 to 10 micrometres long. University of Vienna (2026): current models place the first eukaryotes around 2 billion years ago. Dates this old come with wide error margins; “about” matters. The 3D scenes are simplified teaching models, not to scale. Sources: [Biology 1010 (OpenStax-based, Utah Education Network) 13.2: Eukaryotic Origins](https://uen.pressbooks.pub/biology1010revision/chapter/eukaryotic-origins/); [University of Vienna (30 Sep 2026): Unexpected dynamics of ancient microbes discovered (Radler et al., Nature, doi 10.1038/s41586-026-11063-9)](https://www.univie.ac.at/en/news/press-room/press-releases/detail/unexpected-dynamics-of-ancient-microbes-discovered) ## Chapter II · Cells within cells ### Look closely at a mitochondrion and it looks surprisingly like a bacterium. It has two membranes. Inside, it carries its own small loop of DNA, circular like a bacterium’s, separate from the DNA in the nucleus. It makes some of its own proteins on its own ribosomes, the cell’s protein-making machines, which resemble bacterial ones, and it starts them with the same chemical signal that bacteria use. New mitochondria come only from old ones, which divide in two much as a bacterium does. Even an antibiotic agrees. Streptomycin, which stops bacteria making proteins, also stops mitochondria, but not the rest of the cell. ![Folded inner membrane and a ring of DNA. AI-generated still.](https://assets.planetpast.com/a/5a/5ada3bc2e68c7bd9.jpg) - **16,569**: base pairs in one small ring - Two membranes - Circular DNA, like bacteria - Bacteria-sized ribosomes (70S) - Same protein start signal as bacteria (fMet) - Divide like bacteria (binary fission) - Never made from scratch by the cell - Streptomycin: stops bacteria and mitochondria - Diphtheria toxin: stops the cell’s own ribosomes, not theirs Chapter II · Cells within cells #### Why mitochondria look like bacteria Kimball 18.11: mitochondria have their own single circular DNA molecule without histones (16,569 base pairs in humans), their own bacteria-like protein-making machinery, start proteins with fMet as bacteria do, arise only from pre-existing mitochondria, and are blocked by streptomycin, which spares the eukaryotic cytoplasm. BioNinja: double membrane, 70S ribosomes and division resembling binary fission. Try it: watch it divide. The 3D scenes are simplified teaching models, not to scale. Sources: [LibreTexts, Biology (Kimball) 18.11: Endosymbiosis](https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Biology_%28Kimball%29/18%3A_Evolution/18.11%3A_Endosymbiosis); [BioNinja: Endosymbiosis (evidence)](https://ib.bioninja.com.au/a-endosymbiosis/) ### In 1967 the biologist Lynn Margulis, then publishing as Lynn Sagan, set out a bold explanation. Mitochondria, she argued, began as free-living bacteria that came to live inside another cell. Over many generations the guest became part of its host. The idea is called endosymbiosis, which means living together, one inside the other. More than a dozen journals turned her paper down before the Journal of Theoretical Biology accepted it. Today the idea that mitochondria and chloroplasts began as bacteria is widely accepted. > On the Origin of Mitosing Cells (Lynn Sagan (Margulis), Journal of Theoretical Biology) ![A large cell wrapping around a smaller bacterium. AI-generated still.](https://assets.planetpast.com/a/a0/a085a2f50ea8d13c.jpg) - endo: inside - sym: together - biosis: living - **12+**: journals rejected it, partly because it crossed many fields ![Some single-celled hunters today keep live algae inside them and live partly on what the algae make. AI-generated still.](https://assets.planetpast.com/a/22/22602e05917aa3b6.jpg) Chapter II · Cells within cells #### Margulis and endosymbiosis Understanding Science (UC Berkeley): in 1967 Lynn Margulis, then Lynn Sagan, published “On the Origin of Mitosing Cells”; more than a dozen journals rejected it, not because the science was poor but because it did not fit one field, before the Journal of Theoretical Biology accepted it; a book followed in 1970. Biology 1010 (13.2): the endosymbiotic hypothesis is now widely accepted. Try it: replay the merger. The 3D scenes are simplified teaching models, not to scale. Sources: [Understanding Science, UC Berkeley: Cells within cells, the community reacts](https://undsci.berkeley.edu/cells-within-cells-an-extraordinary-claim-with-extraordinary-evidence/the-community-reacts/); [Biology 1010 (OpenStax-based, Utah Education Network) 13.2: Eukaryotic Origins](https://uen.pressbooks.pub/biology1010revision/chapter/eukaryotic-origins/) ### The same story explains chloroplasts. They descend from cyanobacteria, the blue-green bacteria that photosynthesise. Like mitochondria, chloroplasts carry their own circular DNA and make proteins on bacteria-like ribosomes. The chloroplast came second. Every known complex cell has mitochondria, or stripped-down organelles descended from them, but only some have chloroplasts. So a cell that already had mitochondria took in a cyanobacterium. That is why a leaf cell carries two kinds of former bacteria, and yours carry one. ![Chains of small blue-green cells. AI-generated still.](https://assets.planetpast.com/a/bb/bb40e1a73bea5a65.jpg) - **121,024**: base pairs, about seven times a human mitochondrion’s - first: a bacterium becomes the mitochondrion - later: a cyanobacterium becomes the chloroplast ![Each green oval descends from a once free-living cyanobacterium. AI-generated still.](https://assets.planetpast.com/a/79/7961aa3b32550b87.jpg) Chapter II · Cells within cells #### Chloroplasts Kimball 18.11: chloroplasts descend from an endosymbiotic cyanobacterium that lived inside a eukaryotic cell that already had mitochondria; they have their own circular DNA (121,024 base pairs in the liverwort Marchantia) and bacteria-like protein synthesis. Biology 1010 (13.2): mitochondria likely came before plastids, because all eukaryotes have mitochondria or mitochondria-like organelles but only some have plastids. Try it: replay the merger. The 3D scenes are simplified teaching models, not to scale. Sources: [LibreTexts, Biology (Kimball) 18.11: Endosymbiosis](https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Biology_%28Kimball%29/18%3A_Evolution/18.11%3A_Endosymbiosis); [Biology 1010 (OpenStax-based, Utah Education Network) 13.2: Eukaryotic Origins](https://uen.pressbooks.pub/biology1010revision/chapter/eukaryotic-origins/) ## Chapter III · Who was the host? ### That leaves a question. If the mitochondrion was the guest, who was the host? Many genes that copy and read the DNA in our nucleus are closely related to those of archaea, the other kind of simple cell. So the host was probably an archaeon. In 2015 a team led by Thijs Ettema at Uppsala University in Sweden found DNA from a new group of archaea in sea-floor mud near a hydrothermal vent field called Loki’s Castle, in the Arctic seas between Greenland and Norway. They called it Lokiarchaeota. The wider group, named after the home of the Norse gods, is the Asgard archaea. Their genes put them closer to complex cells than any other known microbe, and many researchers think eukaryotes branched off from within them. - Guest: a bacterium (the mitochondrion) - Host: ? ![Bacteria, archaea and eukaryotes. AI-generated still.](https://assets.planetpast.com/a/05/0581f2c97669fb2c.jpg) ![Mineral chimneys on a dark sea floor, like those near Loki’s Castle. AI-generated still.](https://assets.planetpast.com/a/c3/c3f4dd88e3c6f2b2.jpg) - **2015**: genes first, no living cells yet - Three domains: eukaryotes a separate branch - Two domains: eukaryotes grew from within the archaea Chapter III · Who was the host? #### Finding the host Biology 1010 (13.2): many nuclear genes of eukaryotes and their replication and expression machinery appear closely related to archaea, while energy-harvesting genes came from bacteria. Kimball 18.11 describes the host as an archaeal cell. Wikipedia (Promethearchaeum): Lokiarchaeota were reported in 2015 by Thijs Ettema’s team at Uppsala University from near the Loki’s Castle vent. Earth.com (2025): Asgard archaea, named from Norse mythology, were identified in 2015 from gene fragments; some scientists argue eukaryotes evolved from within them, which would leave two primary domains. The tree on screen is simplified; branch lengths mean nothing. The 3D scenes are simplified teaching models, not to scale. Sources: [Biology 1010 (OpenStax-based, Utah Education Network) 13.2: Eukaryotic Origins](https://uen.pressbooks.pub/biology1010revision/chapter/eukaryotic-origins/); [LibreTexts, Biology (Kimball) 18.11: Endosymbiosis](https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Biology_%28Kimball%29/18%3A_Evolution/18.11%3A_Endosymbiosis); [Wikipedia: Promethearchaeum (Lokiarchaeota 2015; Imachi et al., Nature, January 2020)](https://en.wikipedia.org/wiki/Promethearchaeum); [Earth.com (24 Mar 2025): Tiny microbes may have given rise to complex life](https://earth.com/news/tiny-microbes-may-have-given-rise-to-complex-life) ### But genes in the mud are not a living cell. Nobody had seen what an Asgard archaeon looked like. A Japanese team led by Hiroyuki Imachi took about twelve years to grow one, from sea-floor mud collected about two and a half kilometres deep off Shikoku. Reported in 2020, it would only grow alongside partner microbes. Its cells, about half a micrometre wide, sprouted long, branching arms. Its DNA held about eighty genes once thought to belong only to complex cells. A real Asgard archaeon was finally under the microscope. ![Sediment cores like those that held the microbe. AI-generated still.](https://assets.planetpast.com/a/5d/5d4548a2fff42f26.jpg) - **~12 years**: to grow it, publish in Nature, January 2020 - **~0.55 µm**: cell width; arms about 5 µm long - **80**: “eukaryotic signature” genes Chapter III · Who was the host? #### The first one grown Wikipedia (Promethearchaeum): Hiroyuki Imachi and colleagues reported in Nature in January 2020 the culture of Promethearchaeum syntrophicum from deep-sea sediment of the Nankai Trench off Shikoku, at about 2,500 m, after about 12 years; it grows only with partners (a Methanogenium archaeon and a Halodesulfovibrio bacterium), its cells are about 550 nm across, its tentacle-like protrusions, some branched, average about 5 µm long, and its genome holds 80 eukaryotic signature genes. University of Vienna (2022): the first images of Asgard cells came from this Japanese group in 2020. The 3D scenes are simplified teaching models, not to scale. Sources: [Wikipedia: Promethearchaeum (Lokiarchaeota 2015; Imachi et al., Nature, January 2020)](https://en.wikipedia.org/wiki/Promethearchaeum); [University of Vienna via idw (21 Dec 2022): On the trail of the origin of complex life (Rodrigues-Oliveira et al., Nature, doi 10.1038/s41586-022-05550-y)](https://idw-online.de/en/news807035) ### Put the pieces together and you get the leading model. About two billion years ago, a cell related to today’s Asgard archaea joined with a bacterium. The bacterium lived on inside it and became the mitochondrion. Over a very long time the partners came to depend on each other. Many proteins a mitochondrion needs are now made from genes in the nucleus. How the bacterium got inside, and what each side gained, is still debated. This is the model the evidence favours, not a film of what happened. - Host: an Asgard-like archaeon - Guest: a bacterium - Later, in some lines: a cyanobacterium ![Two billion years is most of the history of life; the first animals turn up only in the last fifth of it. AI-generated still.](https://assets.planetpast.com/a/cd/cd25233f1ea7f069.jpg) - **2 genomes**: a mitochondrion relies on its own DNA and the nucleus - Swallowed, or slowly wrapped by arms? - Which came first: nucleus or mitochondrion? - What each partner traded Chapter III · Who was the host? #### Two cells become one University of Vienna (2026): current models suggest the first eukaryotes arose around 2 billion years ago from a fusion of a bacterium with an Asgard-like ancestor. Kimball 18.11: nuclear genes encode many of the proteins in mitochondria and chloroplasts, so the organelles can arise only from pre-existing ones. Earth.com (2025): researchers propose that an Asgard archaeon may have used its tentacle-like arms to capture the bacterium that became the mitochondrion, a hypothesis. Try it: replay the merger. The 3D scenes are simplified teaching models, not to scale. Sources: [University of Vienna (30 Sep 2026): Unexpected dynamics of ancient microbes discovered (Radler et al., Nature, doi 10.1038/s41586-026-11063-9)](https://www.univie.ac.at/en/news/press-room/press-releases/detail/unexpected-dynamics-of-ancient-microbes-discovered); [LibreTexts, Biology (Kimball) 18.11: Endosymbiosis](https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Biology_%28Kimball%29/18%3A_Evolution/18.11%3A_Endosymbiosis); [Earth.com (24 Mar 2025): Tiny microbes may have given rise to complex life](https://earth.com/news/tiny-microbes-may-have-given-rise-to-complex-life) ## Chapter IV · Arms that crawl ### In 2022, Christa Schleper’s group in Vienna found that one Asgard archaeon is threaded with a network of actin filaments, a scaffold like those seen before only in complex cells. In complex cells, actin can push out the edge of a cell so that it crawls. Some human immune cells move this way. Now Philipp Radler and colleagues have filmed living cells of two Asgard strains, kept free of oxygen: one from the Loki branch, grown in Vienna, and one from the Heimdall branch, grown in Japan. The cells changed shape about every minute, stretched arms up to twenty times their body length, stuck to the glass and crawled. When the team added drugs that block actin, the cells lost control of their arms and stopped moving. Try it. - **actin**: filament network inside Lokiarchaeum ossiferum ![Complex cells such as amoebas push out their edges to move. AI-generated still.](https://assets.planetpast.com/a/30/3038b42d78f0fec7.jpg) - Lokiarchaeon: grown in Vienna - Heimdallarchaeon: grown in Japan (JAMSTEC) - **20×**: arm length versus body - **~1 min**: between big shape changes - Normal: arms reach, attach, pull - Actin blocked: arms out of control, no crawling Chapter IV · Arms that crawl #### Arms that crawl University of Vienna via idw (2022): Schleper’s group, with ETH Zürich, grew Lokiarchaeum ossiferum from sediment off Piran, Slovenia, and found an extensive actin network previously seen only in eukaryotes. University of Vienna (2026): Radler et al. used live-cell microscopy in oxygen-free conditions on a Lokiarchaeon and a Heimdallarchaeon; cells changed shape drastically about every minute, protrusions reached up to 20 times the cell body, cells attached and crawled, a motility previously seen only in complex cells such as human immune cells; actin inhibitors stopped the movement. Try it: add the actin inhibitor. The 3D scenes are simplified teaching models, not to scale. Sources: [University of Vienna via idw (21 Dec 2022): On the trail of the origin of complex life (Rodrigues-Oliveira et al., Nature, doi 10.1038/s41586-022-05550-y)](https://idw-online.de/en/news807035); [University of Vienna (30 Sep 2026): Unexpected dynamics of ancient microbes discovered (Radler et al., Nature, doi 10.1038/s41586-026-11063-9)](https://www.univie.ac.at/en/news/press-room/press-releases/detail/unexpected-dynamics-of-ancient-microbes-discovered) ### What does this tell us? The authors suggest that crawling, a hallmark of complex cells, may be older than complex cells themselves. If so, the archaeal host may already have had a working actin skeleton before it took in the bacterium that became the mitochondrion. But today’s Asgard archaea are cousins, not ancestors. They have kept evolving for two billion years too, so they are a living model, not a time machine. And only a handful have ever been grown and watched alive, so the conclusions rest on very few kinds of cell. - Crawling may predate complex cells - Actin may have mattered before the merger ![An idea to test, not a finding. AI-generated still.](https://assets.planetpast.com/a/45/4504f5b776023f8c.jpg) - Today’s archaea are not the ancestor - Two strains, not the whole group - How the merger actually happened - **2020, 2023**: first two Asgard specimens cultivated, per the University of Vienna Chapter IV · Arms that crawl #### What it means, and what it doesn’t University of Vienna (2026): the authors suggest that complex cell motility may be much older than assumed, and that these traits may have mattered even before the first eukaryotes and the symbiosis that gave rise to mitochondria; the release frames this as a possibility (“may”, “suggests”). The first two Asgard specimens were cultivated in 2020 and 2023, at JAMSTEC and in Schleper’s lab. Martin Pilhofer (ETH Zürich), quoted by Earth.com (2025): “We still have a lot of unanswered questions about Asgard archaea, especially regarding their relation to eukaryotes.” “Cousins, not ancestors” is the standard caution when living species are used to picture the past. The 3D scenes are simplified teaching models, not to scale. Sources: [University of Vienna (30 Sep 2026): Unexpected dynamics of ancient microbes discovered (Radler et al., Nature, doi 10.1038/s41586-026-11063-9)](https://www.univie.ac.at/en/news/press-room/press-releases/detail/unexpected-dynamics-of-ancient-microbes-discovered); [Earth.com (24 Mar 2025): Tiny microbes may have given rise to complex life](https://earth.com/news/tiny-microbes-may-have-given-rise-to-complex-life) ## Epilogue ### Every breath you take carries oxygen to the mitochondria in your cells, descendants of a bacterium that moved in about two billion years ago. Complex life seems to have begun not with one cell defeating another, but with two very different cells living together. How a small archaeon first took in its partner is still one of biology’s great open questions. - ~2 billion years ago: archaeon and bacterium merge - 1967: Margulis’s endosymbiosis paper - 2015: Lokiarchaeota found from DNA - 2020: first Asgard archaeon grown - 2026: Asgard archaea filmed crawling ![Two very different cells, one future. AI-generated still.](https://assets.planetpast.com/a/a0/a085a2f50ea8d13c.jpg) - How did the bacterium get inside? - Where did the nucleus come from? Epilogue · Sources #### Sources Every fact in this film comes from the sources below. The chapter stills and pictures are AI-generated and show no real cells or people; they are artist’s impressions. The cells, mitochondrion, merger, tree and crawling archaeon in the 3D scenes are simplified teaching models, not to scale. The 2026 crawling study is new research, presented as its authors frame it: as a suggestion about the past, not proof. Sources: [University of Vienna (30 Sep 2026): Unexpected dynamics of ancient microbes discovered (Radler et al., Nature, doi 10.1038/s41586-026-11063-9)](https://www.univie.ac.at/en/news/press-room/press-releases/detail/unexpected-dynamics-of-ancient-microbes-discovered); [University of Vienna via idw (21 Dec 2022): On the trail of the origin of complex life (Rodrigues-Oliveira et al., Nature, doi 10.1038/s41586-022-05550-y)](https://idw-online.de/en/news807035); [Earth.com (24 Mar 2025): Tiny microbes may have given rise to complex life](https://earth.com/news/tiny-microbes-may-have-given-rise-to-complex-life); [Wikipedia: Promethearchaeum (Lokiarchaeota 2015; Imachi et al., Nature, January 2020)](https://en.wikipedia.org/wiki/Promethearchaeum); [Understanding Science, UC Berkeley: Cells within cells, the community reacts](https://undsci.berkeley.edu/cells-within-cells-an-extraordinary-claim-with-extraordinary-evidence/the-community-reacts/); [LibreTexts, Biology (Kimball) 18.11: Endosymbiosis](https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Biology_%28Kimball%29/18%3A_Evolution/18.11%3A_Endosymbiosis); [BioNinja: Endosymbiosis (evidence)](https://ib.bioninja.com.au/a-endosymbiosis/); [Biology 1010 (OpenStax-based, Utah Education Network) 13.2: Eukaryotic Origins](https://uen.pressbooks.pub/biology1010revision/chapter/eukaryotic-origins/); [VAAM, Association for General and Applied Microbiology: What are microbes?](https://vaam.de/en/microbiology-portal/short-question/what-are-microbes/)