Chemical Evolution and Biological Evolution: The Miller-Urey Experiment and Endosymbiotic Theory

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Chemical evolution refers to the processes that created the molecules that make up living things (biomolecules) and then formed structures called protocells, which exhibited a certain organization and were separated from the environment by membranes.


  1. They introduced the gases Oparin believed existed in the primitive atmosphere into a chamber.

  2. Then they ran electric currents through it to simulate the energy from lightning.

  3. The products created by the chemical reactions that took place in the chamber collected in a container that imitated the 'primitive ocean'.

  4. When they extracted the liquid of the 'primitive ocean' and analyzed it, they observed biomolecules that had formed from the simple inorganic products.


Biological evolution refers to the processes that led protocells to become different types of cells and resulted in all the different organisms ever to have inhabited the Earth.


  1. A primitive prokaryotic cell folds its membrane and creates the cell nucleus.

  2. The cell engulfs other prokaryotic cells.

These cells are not digested by the first cell but live in symbiosis with it. They form the mitochondria, the flagella, and the chloroplasts.

2A. A prokaryotic cell capable of respiration creates the mitochondria.

2B. A long, very mobile prokaryotic cell creates the flagellum.

2C. A prokaryotic cell capable of photosynthesis creates the chloroplast.

What is catastrophism?

Cuvier proposed the theory of catastrophism. According to this theory, Earth had experienced many catastrophes in its history, for example, the Great Flood, and these had led to the disappearance of many species. He maintained that fossils were the remains of extinct species that had died out as a result of these catastrophes.

FIXIST THEORIES: uphold the idea that the species that exist on Earth were created by God in the form that we see them today and that they have not changed in any way. In other words, they maintain these species have remained unchanged throughout the history of the Earth.

ACTUALISM: Developed by Lyell, upheld that the events of the Earth's distant past were caused by the action of forces that are identical to those that are in operation today; that changes in nature take place slowly, gradually, and continually and are not caused by catastrophes.

EVOLUTIONISM: Maintains that species undergo a process of transformation which results in the appearance of new species.


  • Spontaneous generation is the mechanism that caused the appearance of the simplest life forms, which then evolved into more complex organisms.

  • An internal impulse exists within organisms that drives them to improve themselves and become more complex. Environmental changes have an impact on this process, causing the organisms to use some organs more than others and resulting in the organs either becoming more developed or atrophying.

  • The characteristics acquired through their use or disuse are transferred to the next generation; they are inheritable traits.


  • The world is not static but in a process of transformation.

  • The process of changes is gradual and continuous.

  • Similar organisms are related and have a common ancestor.

  • Evolutionary change is the result of natural selection.

Natural selection is a process that favors the individuals in populations that are best adapted to the environment and which, over time, will lead to the appearance of new species.

Neo-Darwinism, also known as synthetic theory, is a revised version of the theory of natural selection, enriched by contributions from studies made in fields such as genetics, paleontology, and taxonomy after Darwin's time.

Punctualism, or the theory of punctuated equilibrium, proposed by N. Eldredge and S. J. Gould in 1972, further developed Neo-Darwinist theory to explain some sudden evolutionary leaps that were observed when studying fossils and which could not be explained by gradual changes. This theory suggests that there are certain periods of intense speciation, which interrupt or punctuate the equilibrium of the normal process of adaptation through natural selection.

It suggests the evolutionary process takes in leaps and bounds.

ADAPTATION: Adaptation is the accumulation of changes in the characteristics of organisms and populations that can be inherited in order to adapt to the changes in the environment and allow them to survive.

STRUCTURAL ADAPTATION: Structural adaptations affect certain organs. For example, the extremities of vertebrates have adapted in different ways to allow animals to swim, fly, run, etc.

PHYSIOLOGICAL ADAPTATIONS: Physiological adaptations affect the function of an organism. For example, the metabolism of dromedaries turns accumulated fat in their hump into water.

BEHAVIORAL ADAPTATION: Behavioral adaptations are influenced by several factors such as reproduction, food, etc. For example, many birds migrate to warmer climates during cold seasons.

HOMOLOGOUS ORGANS: Homologous organs have the same origin and the same basic structure, although their shape is different as a result of adaptation to different functions.

ANALOGOUS ORGANS: Analogous organs have a different origin but a similar shape, as they have adapted to the environment to perform the same function.

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