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Aristotle's theory of spontaneous generation is known as abiogenesis. He believed that simple life forms, such as insects, mice and fish, could arise from decaying organic matter, such as mud or rotting food. This led him to the conclusion that living organisms could arise through natural processes, without the need for pre-existing life. He observed, for example, that insects seemed to arise spontaneously from decaying flesh or that fish could apparently arise from turbid water.

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The theory of spontaneous generation by Theophrastus was the belief that living organisms could arise from non-living matter under certain conditions.

According to Theophrastus, small organisms such as insects and worms could arise spontaneously from decaying organic matter, such as rotting food or mud. Also, Theophrastus had detected the process of germination.

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Paracelsus contributed significantly to the development of early modern medicine. However, he is known for his concept of "Archeus" or "spiritus vitae," which he believed to be a vital force present in all living organisms. He theorized that diseases were caused by disturbances of this vital force and that the balance of this force could be restored through the use of herbal remedies and alchemical preparations. He had a vitalism approach and his emphasis on the importance of the living forces in the functioning of organisms are more in line with the ideas of vitalism (Paracelsus did not propose a detailed theory of spontaneous generation like some of his contemporaries or predecessors).

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Experiments and discoveries refuting the theory of spontaneous generation:

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Experiments and discoveries refuting the theory of spontaneous generation:

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Redi conducted a series of experiments to test the idea that maggots were spontaneously generated from decaying meat. In one experiment, he placed meat in three jars: one open to the air, one covered with gauze, and one completely sealed. He observed that maggots only appeared in the meat of the open jar, where flies could gain access to lay their eggs. The flesh of the covered jar and the flesh of the closed jar remained free of maggots. This experiment showed that maggots did not arise spontaneously from the meat, but came from fly eggs.

Also, he conducted experiments with sealed jars containing seeds to test the idea of spontaneous generation of plants. He observed that the seeds in the sealed jars did not sprout, while those in the open jars did. This further supported the idea that living organisms were not spontaneously generated from non-living matter.

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Spallanzani's experiments included meticulously designed setups to avoid introducing microorganisms into his samples. One of his experiments involved boiling broth in sealed jars to sterilize it and observing whether microorganisms appeared spontaneously. He found that no microorganisms appeared in the broth as long as the flasks remained sealed, demonstrating that the microorganisms present in the broth before boiling were killed by the heat and that no new microorganisms were spontaneously generated. He also conducted experiments in which he boiled the broth in flasks with long, curved necks that allowed air to enter but prevented particles suspended in the air from reaching the broth. In these cases, the broth remained free of microorganisms, which further supported the idea that the presence of microorganisms in the broth was due to contamination from the air rather than spontaneous generation.

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One of Pasteur's experiments involved using gooseneck flasks. He filled the flasks with nutrient broth, boiled them to sterilize the contents, and then bent the necks of the flasks into an S-shape. In this way, air could enter the flask but dust and microorganisms were prevented from reaching the nutrient broth. The broth remained sterile despite exposure to air, disproving spontaneous generation of microorganisms. He also performed an experiment using straight-necked flasks filled with nutrient broth, which he boiled to sterilize. He then left some flasks open to the air while sealing others. The open vials quickly became contaminated with microorganisms, while the sealed ones remained sterile. Finally, in one experiment he used a microscope to examine the microorganisms present in his experiments.

He observed that the microorganisms present in the contaminated samples were not spontaneously generated, but were similar to those found in the environment.

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Four hypotheses of the origin of the first cell from non-living matter

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Four hypotheses of the origin of the first cell from non-living matter

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They hypothesized that the early Earth's atmosphere was reducing, meaning it lacked free oxygen. Scientists passed vapor through a mix of methane (CH4), water vapor (H20), hydrogen (H2) and ammonia (NH3), to represent atmospheric gases from the beginnings of the earth. They used electric discharges as a source of energy. Amino acids were formed and other carbon compounds necessary for life.

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Source of Energy - Assembly of caron compounds into polymers (1950s)

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Source of Energy - Assembly of caron compounds into polymers (1950s)

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A possible site for the origin of the first carbon compounds is around deep-sea vents. There are cracks in the Earth's surface, characterized by gshing hot water carrying reduced inorganic chemicals. These chemicals represent readily accessibe supplies of energy.

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If phospholipids or other amphipathic carbon compunds were among the first carbon compounds, they would have naturally assembled into bilayers. Experiments have shown that these bilayers readily form vesicles resembling the plasma membrane of a small cell.

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RNA is capable of both storing genetic information and catalyzing chemical reactions, functions similar to those of DNA and proteins, respectively. It has been proposed that self-replicating RNA molecules, or ribozymes, may have arisen spontaneously and served as the first genetic material; it could act as an enzyme (catalyzer). Over time, these RNA molecules could have evolved into more complex biological systems, eventually leading to the emergence of cells.

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The endosymbiotic theory is a scientific hypothesis that explains the origin of eukaryotic cells and suggests that eukaryotic cells evolved from the symbiotic association between different types of prokaryotic organisms (cells without a defined nucleus), specifically between a larger prokaryotic cell (called a host) and a smaller bacterium (called a symbiont).

According to this theory, mitochondria and chloroplasts have a bacterial origin. The main idea is that a primitive prokaryotic cell “invited” another smaller prokaryotic cell (such as a bacterium) to live inside it in a symbiotic relationship. Over time, this relationship became mutually beneficial, and the host cell and the symbiont bacterium developed a stable symbiotic relationship.

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- Mitochondria, the energy-producing organelles of eukaryotic cells, and chloroplasts, the sites of photosynthesis in plant cells, have their own DNA, ribosomes and membranes. Their structure and size resemble those of some prokaryotes, circular and nonlinear.

-Their structure and size resemble those of some prokaryotes

-Mitochondria and chloroplasts replicate independently within eukaryotic cells, similar to how bacteria reproduce as in binary fission.

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Cooper, G. M. (2000). The Origin and Evolution of Cells. The Cell - NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK9841/

IB HL 1.5.U2. (n.d). Welcome To IB Biology HL. https://mysciencesquad.weebly.com/ib-hl-15u2.html

Gentile, A. (2021). Message in a bottle: revisiting the origin of life. Nature Italy. https://doi.org/10.1038/d43978-021-00144-0

Kazilek. (n.d). Cells Living in Cells. https://askabiologist.asu.edu/explore/cells-living-in-cells

Libretexts. (2024 ,april 21). 3.1: Spontaneous generation. Biology LibreTexts. https://bio.libretexts.org/Bookshelves/Microbiology/Microbiology_(OpenStax)/03%3A_The_Cell/3.01%3A_Spontaneous_Generation#:~:text=The%20Greek%20philosopher%20Aristotle%20

Philippus Aureolus Paracelsus | Encyclopedia.com. (n. d.). https://www.encyclopedia.com/people/medicine/medicine-biographies/philippus-aureolus-paracelsus

Spontaneous generation. Nau Edu. (n.d). https://www2.nau.edu/gaud/bio301/content/spngen.htm

Theophrastus (Stanford Encyclopedia of Philosophy). (2020, september 24). https://plato.stanford.edu/entries/theophrastus/#:~:text=In%20his%20treatise%20On%20Piety,and%20by%20committing%20an%20injustice.

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