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500 BCE
Spontaneous Generation
This theory describes that Living Things came from non-living matter with a special force called vital force. For example, in ancient Egypt they believed that heat of the Sun on the sediment of the Nile produced Snakes and Crocodiles. This thesis was the most common until the 17th century when Scientists began to perform controlled experiments, the two most important are Redi's experiment (17th century) and Pasteur's experiment (19th century). -
Redi's experiment
Francesco Redi created an experiment to show that larvae did not appear spontaneously in decomposing meat, but rather came from fly eggs.
He put meat in three different containers, at the same time, but differently covered. The first one he left open. The second he covered with gauze. And the last one sealed it airtight.
After days, he saw that the larvae only appear in the first container, which shows that life is not produced by non-living beings. -
Pasteur's experiment
- He put meat broth into two flasks with S-shaped necks to keep out contaminants.
- He sterilized the liquid by heating it. After weeks, the broth had not broken down.
- He cut the neck of one of the flasks. After a few days she observed that the broth in this flask had decomposed and the broth in the other flask remained unchanged.
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Conclusion of pasteur's experiment
In conclusion, the S-shaped neck allowed air to pass through, but not microorganisms. After cutting the neck, microorganisms were able to enter and cause decomposition. This finally served to definitively reject spontaneous generation showing that microorganisms were not created by spontaneous generation either. -
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Spontaneous generation vs Abiogenic hypothesis:
• Both have the question of how life originated from non-living matter.
• Spontaneous generation had been refuted, and abiogenic remains as plausible hypothesis.
• Spontaneous generation explains the spontaneous appearance of complex life from non-living matter, while abiogenic hypothesis describes a gradual formation of simple organic molecules leading to the emergence of life. -
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Abiogenic hypothesis
This hypothesis states that the first appearance of life occurred when meteorite bombardment stopped and conditions allowed the production of the first simple organic compounds through physicochemical processes. This led to more complex structures that the ancestors of the first cells created. And after 300 million years the first atoms were formed. -
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Abiogenic hypothesis vs Panspermia hypothesis
• They make different assumptions about the conditions and processes involved in the emergence of life.
• Abiogenesis focuses on the emergence of life from non-living matter on Earth through natural processes, whereas panspermia suggests that life arrived on Earth from extraterrestrial sources. -
Panspermia hypothesis
This theory explains that the first living beings were the microorganisms on Earth transported by materials from outer space. Some examples are asteroids, comets, meteorites...
Some objections to this are that they do not survive radiation in space, shock forces, or high temperatures when they collide with Earth. -
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Panspermia hypothesis vs molecular panspermia hypothesis
Both propose the idea that life could have originated elsewhere in the universe and then reached Earth.
Panspermia suggests the transport of living organisms from space to Earth, while molecular panspermia focuses specifically on the transport of organic molecules.
Panspermia is proposed as a mechanism to explain the origins of life on Earth, while molecular panspermia provides insights into the origins of organic molecules and the prebiotic chemistry that led to the development of life. -
Molecular Panspermia Hypothesis
This theory is very similar to the panspermia hypothesis, but instead of microorganisms arriving on Earth, they were organic molecules that arrived from space and continued the formation of life on Earth.
It has some evidence such as the discovery of organic molecules in star dust and in comets and meteorites. -
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Molecular panspermia Hypothesis vs Oparin's and Haldane's Hypothesis
• Both hypotheses suggest that these organic molecules provided the building blocks for life on Earth, contributing to the development of early life forms.
• Molecular panspermia specifically focuses on the transport of organic molecules through space from extraterrestrial sources to Earth, whereas Oparin's and Haldane's hypothesis focuses on the synthesis of organic molecules on Earth through prebiotic chemistry. -
Oparin’s and Haldane’s hypothesis: prebiotic synthesis and chemical evolution.
This thesis explains that the first living beings were created thanks to inorganic matter through various physicochemical processes promoted by the conditions of the primitive Earth 3800 mya. -
His proposal for the evolution of molecules from inorganic (simpler) to organic (complex) (I):
- The components of the early atmosphere were methane (CH4), ammonia (NH3), hydrogen (H₂) and water vapor (H₂O).
- The cooling of the Earth produced the first rains with the atmosphere and the molecules dissolved in the first seas. Later on, they mixed with the energy of electric storms, volcanism and solar radiation forming simple organic molecules.
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His proposal for the evolution of molecules from inorganic (simpler) to organic (complex)(II):
- The interaction between simple organic molecules in the seas (primordial soup) created complex organic molecules, called assembly, and then the two mixed and formed coacervates.
- Coacervates with molecules capable of self-replication transformed over the years into protocells and then into living beings.
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Miller-Urey experiment
Stanley Miller mixed some gases, which he thought were present in the atmosphere, in a container and applied high-voltage electrical discharges, producing various organic compounds (including amino acids). Today we know that his descriptions of the primitive atmosphere were wrong and therefore also the explanation of the origin of life. -
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Oparin's and Haldane's Hypothesis vs current hypothesis
• Both hypotheses propose mechanisms for the formation of organic molecules from inorganic precursors, emphasizing the role of chemical evolution in the transition from non-living to living entities.
• Both hypotheses consider the importance of environmental conditions and geochemical processes in shaping the early Earth and facilitating the emergence of life. -
Current hypothesis
Today we know that the primitive atmosphere was made up of carbon dioxide (CO2), nitrogen (N2) and water vapor (H₂O). The ozone layer did not exist due to the absence of oxygen and this causes organic molecules to become useless due to the intense ultraviolet radiation.
It is thought that prebiotic synthesis and chemical evolution may have taken place in deep-sea hydrothermal vents, where some reducing compounds, such as methane (CH4), hydrogen (H₂), and hydrogen sulfide (H₂S), were expelled. -
Discussion now focuses on which came 1st: replication or metabolism.
- 1st Metabolism: chemical reactions used for getting energy and molecules for building new structures, were the firs tones to be organized, creating organic systems.
- 1st Replication: nucleic acids (capable of self-replicating), were show in the organic system before metabolism, and they produced the proteins needed for metabolism.