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.
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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.
The organelles that seemed to have been their own cells include the mitochondria and, in photosynthetic cells, the chloroplast. Both of these organelles have their own DNA and their own ribosomes that do not match the rest of the cell. This indicates that they could survive and reproduce on their own. In fact, the DNA in the chloroplast is very similar to photosynthetic bacteria called cyanobacteria. The DNA in the mitochondria is most like that of the bacteria that causes typhus.
Before these prokaryotes were able to undergo endosymbiosis, they first most likely had to become colonial organisms. Colonial organisms are groups of prokaryotic, single-celled organisms that live in close proximity to other single-celled prokaryotes. Advantage to Colony Even though the individual single-celled organisms remained separate and could survive independently, there was some sort of advantage to living close to other prokaryotes.
Whether this was a function of protection or a way to get more energy, colonialism has to be beneficial in some manner for all of the prokaryotes involved in the colony. Once these single-celled living things were within close enough proximity to one another, they took their symbiotic relationship one step further.
The larger unicellular organism engulfed other, smaller, single-celled organisms. At that point, they were no longer independent colonial organisms but instead were one large cell. When the larger cell that had engulfed the smaller cells started to divide, copies of the smaller prokaryotes inside were made and passed down to the daughter cells.
Eventually, the smaller prokaryotes that had been engulfed adapted and evolved into some of the organelles we know of today in eukaryotic cells such as the mitochondria and chloroplasts. Other Organelles Other organelles eventually arose from these first organelles, including the nucleus where the DNA in a eukaryote is housed, the endoplasmic reticulum and the Golgi apparatus. In the modern eukaryotic cell, these parts are known as membrane-bound organelles.
They still do not appear in prokaryotic cells like bacteria and archaea but are present in all organisms classified under the Eukarya domain.
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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.
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?