Heather Scoville Updated January 09, The endosymbiotic theory is the accepted mechanism for how eukaryotic cells evolved from prokaryotic cells. It involves a cooperative relationship between two cells which allow both to survive—and eventually led to the development of all life on Earth. Endosymbiotic Theory History First proposed by Boston University biologist Lynn Margulis in the late s, the Endosymbiont Theory proposed that the main organelles of the eukaryotic cell were actually primitive prokaryotic cells that had been engulfed by a different, bigger prokaryotic cell. Margulis and other scientists continued work on the subject, however, and now her theory is the accepted norm within biological circles. While this sounded like a far-fetched idea at first, the data to back it up is undeniable.
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Lynn Margulis and the Question of How Cells Evolved excerpts from the book "Doing Biology" by Joel Hagen et al Modern biology inherited two great theories from the nineteenth century: evolutionary theory and cell theory. Surprisingly, these theories, so central to our understanding of the living world, have had a rather uneasy relationship. Until quite recently, most cell biologists ignored evolution, and most evolutionary biologists ignored cells. The exception to this historical generalization was the chromosomes, which both evolutionary biologists and cell biologists studied.
But what about the cytoplasm, the contents of the cell outside the nucleus? Could knowing about other cellular structures organelles add anything to evolutionary theory? Could evolutionary theory suggest interesting questions about the structure or function of organelles? For most biologists, the answer to these questions was no. The cytoplasm added little to understanding evolutionary theory, and vice versa. Occasionally, some biologists tried to bridge the theoretical gap, but they usually met with derision.
For example, during the. According to Wallin, the former bacteria and their host cells evolved together to establish an inseparable symbiotic partnership.
He even claimed to have removed mitochondria from cells and grown them in isolation. According to his critics, evolution by symbiosis was as improbable as that other great pseudoscientific idea of the s: continental drift. With the benefit of hindsight it is easy to smile at the comparison between continental drift and endosymbiosis, two great scientific heresies that later revolutionized the way we look at the natural world.
The criticisms were, however, justified. No one, then or now, has verified his claim that mitochondria can be grown outside of cells. PROBLEM Assuming that mitochondria really did evolve from free-living bacteria, why might it be difficult or impossible to experimentally grow them outside of the host cell? Both the structure and the function of mitochondria were mysteries in The internal anatomy of bacteria was also almost totally unknown.
The evidence Wallin needed to support his theory required the electron microscope and other sophisticated laboratory techniques developed only after World War II. As in the case of continental drift, the theory of symbiosis in cellular evolution that was finally accepted during the s was very different from the one suggested by Wallin in the s. A prolific writer and dynamic speaker, Margulis captivates audiences and often irritates more traditional biologists with her unorthodox ideas.
Although many biologists continue to disagree with some of her ideas, everyone takes endosymbiosis seriously. Margulis entered biology during a particularly exciting period. A few years later, when she was a graduate student, two of her professors discovered DNA in chloroplasts. Other scientists reported finding DNA in mitochondria. Because these early reports were hotly disputed, searching for DNA outside the nucleus was not the sort of research project that most graduate students would have chosen.
Despite warnings, Margulis plunged into the controversial problem for her Ph. Using radioactively labeled nucleotides, she convincingly demonstrated the presence of DNA in the chloroplasts of Euglena gracilis, one of the curious unicellular organisms that shares both plant and animal characteristics. Margulis wrote her first article on the endosymbiotic theory in , two years after she completed her Ph.
At the time, she was a single mother without a permanent teaching position. She was also writing her first book on endosymbiosis, which sparked a lively controversy when it was published in Although it initially brought Margulis notoriety, the controversy over cellular evolution was rather short lived.
By the time she published a second book on endosymbiosis in , most biologists accepted important parts of her theory. As a result, Margulis became a scientific celebrity whose success was publicized in both popular and professional magazines.
Those who knew about it usually dismissed it. In order to succeed, Margulis had to carefully distinguish her ideas from the discredited theory proposed by Ivan Wallin half a century earlier. She also had to overcome a basic assumption about evolution held by nearly all biologists at the time. According to the traditional view, evolution usually occurs gradually; endosymbiosis, however, is based on the idea of rather sudden evolutionary changes.
Finally, Margulis had to convince biologists to take DNA in the cytoplasm seriously. Although evidence for DNA in chloroplasts and mitochondria was growing stronger, the idea that some genes reside outside the nucleus remained unorthodox.
Indeed, the book convinced many biologists that cellular evolution was an exciting, if controversial, field. How had cell biology changed during the 50 years after Wallin proposed his unsuccessful theory? Much more was known about the internal structure of cells in than in Unlike Wallin, who knew little about the internal structure or function of mitochondria, Margulis had access to a great deal of information about the intemal structure of cells when she wrote her book.
Powerful electron microscopes, perfected after World War II, allowed scientists to study the previously hidden parts of organelles. Using new biochemical techniques, scientists were able to discover many details of cellular activities. Mitochondria, long an enigma, were now known to be important sites of adenosine triphosphate ATP production, and for the first time scientists were beginning to under stand how this critical process occurred on mitochondrial membranes.
By biologists also became aware of major differences between prokaryotic bacteria, which lack nuclei and most other organelles, and eukaryotic cells, which have both.
What other similarities and differences might be found between the two types of cells? How had eukaryotic cells evolved? What was the evolutionary significance of the DNA found in some organelles? These were the questions that Margulis set out to answer in The smaller partners invaded larger host cells and eventually evolved into three different kinds of organelles: mitochondria, chloroplasts, and flagella. Like other evolutionary biologists, Margulis believes that life first appeared on the earth about four billion years ago.
The first organisms were extremely simple--microscopic droplets of water containing a few genes and enzymes surrounded by a membrane. Like some modern bacteria, early prokaryotic cells extracted energy from these molecules by fermentation, using various forms of metabolism that do not require oxygen. Luckily for the fermenters, there was almost no oxygen in the atmosphere. If there had been, the primitive cells would have been poisoned by this highly reactive gas.
Later, as the supply of energy-rich molecules in the watery environment began to be depleted, other types of bacteria evolved which used solar energy to synthesize their own supplies of large, organic molecules. These early photosynthetic bacteria were also anaerobic. In other words, they did not use oxygen and their primitive photosynthetic reactions did not produce oxygen as a by-product.
For over a billion years, primitive ecosystems included only two types of prokaryotic organisms: simple photosynthetic bacteria and fermenting bacteria. Perhaps 2. A byproduct of this water-splitting reaction was oxygen gas. This was a catastrophic event in the history of life.
Oxygen is such a reactive element that it easily destroys delicate biological structures. Some survivors retreated to areas of brackish water or other oxygen-depleted habitats, where their anaerobic descendants still flourish today. A few prokaryotes became aerobic by evolving various mechanisms to detoxify oxygen. The most successful of these processes was respiration, which not only converted toxic oxygen back into harmless water molecules, but also generated large quantities of ATP.
According to the SET, the photosynthetic production of oxygen gas and the subsequent evolution of respiration set the stage for the evolution of all eukaryotic cells. This evolutionary process occurred in several separate symbiotic events. The first eukaryotic organelles to evolve were mitochondria--structures found in almost all eukaryotic cells. Like some bacteria today Bdellovibrio , these early parasites burrowed through the cell walls of their prey and invaded their cytoplasm.
Either the host or the parasite was often killed in the process, but in a few cases the two cells established an uneasy coexistence. The mutual benefits to the partners are obvious. The respiring parasite, which actually required oxygen, would allow its host to survive in previously uninhabitable, oxygen-rich environments.
Perhaps the parasite also shared with its host some of the ATP that it produced using oxygen. In exchange, the host provided sugar or other organic molecules to serve as fuel for aerobic respiration.
Eventually, as often occurs with parasites, the protomitochondria lost many metabolic functions provided by the host cell. Similarly, as oxygen in the atmosphere continued to increase, the host became more and more dependent upon its pro-tomitochondria to detoxify the gas. What began as a case of opportunistic parasitism evolved into an obligatory partnership.
The small respiratory bacteria eventually evolved into the mitochondria of eukaryotic cells. Although virtually all eukaryotic cells contain mitochondria, only those of plants and certain protists contain chloroplasts.
Therefore, it seems likely that chloroplasts evolved in only a few lines of eukaryotic cells, and this event occurred after mitochondria were already well established. How did this new evolutionary partnership evolve? With higher metabolic rates, cells containing mitochondria were more efficient than anaerobic cells. Some of these newer, unicellular organisms grew larger and evolved into predators capable of eating smaller cells.
Their prey undoubtedly included cyanobacteria. In rare cases, these small photosynthetic cells may have resisted digestion after being engulfed. Inside the predator, they set up a semi-independent existence and eventually evolved into chloroplasts. Although such a scenario may seem far-fetched, we know that similar partnerships exist today. For example, the unusual ciliate Paramecium bursaria is host to many unicellular green algae in the genus Chlorella. These "pseudochloroplasts" produce sugar molecules that are shared with the host.
If the Chlorella are experimentally removed, both partners continue to exist independently. Without its photosynthetic partners, however, the Paramecium becomes totally dependent , upon external sources of food. Provided the opportunity, the Paramecium will eat Chlorella but will not digest them, thus reestablishing the symbiotic partnership. Many other organisms, including several multicellular animals, also play host to photosynthetic algae or cyanobacteria.
Endosymbiotic Theory: How Eukaryotic Cells Evolve
Talk it over, and list it here: II. The Endosymbiotic Theory Although now accepted as a well-supported theory, both she and the theory were ridiculed by mainstream biologists for a number of years. Thanks to her persistance, and the large volumes of data that support this hypothesis gathered by her and many other scientists over the last 30 years, biology can now offer a plausible explanation for the evolution of eukaryotes. Margulis was doing reserarch on the origin of eukaryotic cells.
Meet extraordinary women who dared to bring gender equality and other issues to the forefront. From overcoming oppression, to breaking rules, to reimagining the world or waging a rebellion, these women of history have a story to tell. Margulis was raised in Chicago. Soon after, she married American astronomer Carl Sagan , with whom she had two children; one, Dorion, would become her frequent collaborator.
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Lynn Margulis and the Question of How Cells Evolved excerpts from the book "Doing Biology" by Joel Hagen et al Modern biology inherited two great theories from the nineteenth century: evolutionary theory and cell theory. Surprisingly, these theories, so central to our understanding of the living world, have had a rather uneasy relationship. Until quite recently, most cell biologists ignored evolution, and most evolutionary biologists ignored cells. The exception to this historical generalization was the chromosomes, which both evolutionary biologists and cell biologists studied. But what about the cytoplasm, the contents of the cell outside the nucleus? Could knowing about other cellular structures organelles add anything to evolutionary theory? Could evolutionary theory suggest interesting questions about the structure or function of organelles?
The chloroplasts of glaucophytes like this Glaucocystis have a peptidoglycan layer, evidence of their endosymbiotic origin from cyanobacteria. Weathering constant criticism of her ideas for decades, Margulis was famous for her tenacity in pushing her theory forward, despite the opposition she faced at the time. This is one of the great achievements of twentieth-century evolutionary biology, and I greatly admire her for it. Neo-Darwinism, which insists on [the slow accrual of mutations by gene-level natural selection], is in a complete funk. I noticed that all kinds of bacteria produced gases. Oxygen, hydrogen sulfide, carbon dioxide, nitrogen, ammonia—more than thirty different gases are given off by the bacteria whose evolutionary history I was keen to reconstruct.