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.
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Endosymbiosis: Lynn Margulis Margulis and others hypothesized that chloroplasts bottom evolved from cyanobacteria top. The Modern Synthesis established that over time, natural selection acting on mutations could generate new adaptations and new species. But did that mean that new lineages and adaptations only form by branching off of old ones and inheriting the genes of the old lineage?
Some researchers answered no. Evolutionist Lynn Margulis showed that a major organizational event in the history of life probably involved the merging of two or more lineages through symbiosis. In the late s Margulis left studied the structure of cells. Mitochondria, for example, are wriggly bodies that generate the energy required for metabolism.
To Margulis, they looked remarkably like bacteria. She knew that scientists had been struck by the similarity ever since the discovery of mitochondria at the end of the s. Some even suggested that mitochondria began from bacteria that lived in a permanent symbiosis within the cells of animals and plants.
There were parallel examples in all plant cells. Algae and plant cells have a second set of bodies that they use to carry out photosynthesis. Known as chloroplasts , they capture incoming sunlight energy. The energy drives biochemical reactions including the combination of water and carbon dioxide to make organic matter.
Chloroplasts, like mitochondria, bear a striking resemblance to bacteria. When one of her professors saw DNA inside chloroplasts, Margulis was not surprised. Margulis spent much of the rest of the s honing her argument that symbiosis see figure, below was an unrecognized but major force in the evolution of cells.
In she published her argument in The Origin of Eukaryotic Cells. Mitochondria are thought to have descended from close relatives of typhus-causing bacteria. The genetic evidence In the s scientists developed new tools and methods for comparing genes from different species. The DNA in mitochondria, meanwhile, resembles that within a group of bacteria that includes the type of bacteria that causes typhus see photos, right.
Margulis has maintained that earlier symbioses helped to build nucleated cells. For example, spiral-shaped bacteria called spirochetes were incorporated into all organisms that divide by mitosis. Tails on cells such as sperm eventually resulted. Most researchers remain skeptical about this claim. It has become clear that symbiotic events have had a profound impact on the organization and complexity of many forms of life.
Algae have swallowed up bacterial partners, and have themselves been included within other single cells. Nucleated cells are more like tightly knit communities than single individuals. Evolution is more flexible than was once believed.
Phylogenetic analyses based on genetic sequences support the endosymbiosis hypothesis.
Like most subjects in science, an accepted theory is developed from the combined efforts of multiple researchers. The Endosymbiotic Theory is no different. Many in the scientific community claim that Dr. Lynn Margulis had been the first to propose the Endosymbiotic Theory. Many before her breakthrough during the late 20th century also had contributions to the entire formation of the theory.
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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?
Endosymbiotic Theory: How Eukaryotic Cells Evolve
How important is endosymbiosis? Although Jeon watched his amoebae become infected with the x-bacteria and then evolve to depend upon them, no one was around over a billion years ago to observe the events of endosymbiosis. Why should we think that a mitochondrion used to be a free-living organism in its own right? It turns out that many lines of evidence support this idea. When you look at it this way, mitochondria really resemble tiny bacteria making their livings inside eukaryotic cells! Chloroplasts are like tiny green factories within plant cells that help convert energy from sunlight into sugars, and they have many similarities to mitochondria. The evidence suggests that these chloroplast organelles were also once free-living bacteria.