THEORIES OF CELL EVOLUTION - BIOLOGY - 2024

The theories of evolution cell are explanations trying to understand when and how cells emerged. Normally they refer to eukaryotic cells, that is, those that have a nucleus separated by a cell membrane where they contain the genetic material.

Unlike prokaryotic cells, which are simpler and appeared about 3.7 billion years ago on Earth, eukaryotic cells are much more complex, larger, and more recently appearing.

High definition image of a single-celled organism. Via Youtube.

Since eukaryotic cells are the basis of most living things, such as plants and animals, several theories have been developed about their origin and why they appeared.

Development of the first cells

The first cells appeared at least 3.7 billion years ago, about 750 million years after Earth was formed. Although we do not know for sure how the first cells appeared, we do know quite precisely how they developed.

However, one of the most accepted theories about the formation of the first cells is the following: given the atmospheric conditions of the early Earth, a discharge of energy can cause organic molecules to form spontaneously.

This was demonstrated by Stanley Miller's experiments in the 1950s, in which he succeeded in creating organic molecules from hydrogen, methane, and ammonia.

Later, the first complex organic molecules (also called macromolecules) were formed. At some point in the evolution of these molecules, the first one capable of replicating itself emerged using materials from its environment. Then, for the first time, a cell was born.

These first cells could have reproduced freely at first, given the lack of competition for the fuel they used. However, because their numbers increased considerably (precisely because of this lack of competition), cells soon had to become more sophisticated in order to continue reproducing. Thus began the process of evolution.

Cell types and their development

For many years, it was believed that only two types of cells existed, the prokaryotic (which literally means "without a nucleus") and the more complex and later onset eukaryotic. However, in the last two centuries, other types of cells have been identified that do not fit the characteristics of either of the other two.

These cells have been known since the 90's as "archaea", which literally means "ancient ones." In this way, today a classification system of three domains is used: Archaea, Bacteria and Eucaria.

Archaea cells

Archaea (also known as Archaea) are cells without a nucleus, very similar to bacteria but with certain characteristics that have led them to be considered as independent organisms.

Like all other cells, they are microscopic organisms. Their cell wall is very resistant, which allows them to live in extreme environments (even on asteroids in space, without the protection of any type of atmosphere).

Their diet is also very different, since they take advantage of inorganic compounds such as hydrogen, carbon dioxide or sulfur instead of oxygen.

Prokaryotic cells (bacteria)

Prokaryotic cells are the simplest of the three types. They have only one cell membrane, which surrounds the interior of the cell. Inside we can find the genetic material suspended within the cytoplasm, as well as some ribosomes (the organelles that generate energy within the cell).

Prokaryotic cells, despite being of many different types, are all classified as bacteria. In order to adapt to the environment more efficiently, many of them have other additions, such as flagella to move freely or a sticky wall, the capsule, which allows them to adhere to other organisms.

Eukaryotic cells

Eukaryotic cells are the most complex and largest of the three types. They differ from prokaryotes and archaea mainly in that they have a nucleus, where they store DNA. In addition, they have several types of cellular organelles, which allow them to perform different types of functions.

Eukaryotic cells are the basis of all complex life that exists on Earth. Because of this, scientists have been studying its origin for many decades, and have developed the so-called Endosymbiotic Theory of cell development.

Endosymbiotic Theory of Cell Evolution

Eukaryotic cells are much more developed than archaea or bacteria. Only a few decades ago a satisfactory explanation for its emergence was found: the endosymbiotic theory.

This theory is based on the similarities that the mitochondria and chloroplasts of eukaryotic cells have with bacteria, both in their form and in their function.

Therefore, the scientists who defend it propose that at some point in evolution, a large cell absorbed a bacterium and began to use it to extract the energy necessary to survive and reproduce.

The absorbed bacteria, meanwhile, gained greater probabilities of leaving offspring, as well as greater security by being inside a larger cell. Therefore, a symbiotic relationship occurred; hence the name of the theory.

After millions of years of evolution, mitochondria and chloroplasts, which were previously independent bacteria, have specialized. Therefore, they can no longer survive outside the cell.

Evidences for the endosymbiotic theory

In everyday language we use the word "theory" to describe an opinion that is not based on fact. However, in the world of science a theory is an explanation of a phenomenon confirmed by experiments and observation.

The endosymbiotic theory is no exception. Several clues lead us to think that this is how animal and plant cells arose. Some of these evidences are the following:

• Mitochondria and chloroplasts have their own DNA → These two types of organelles are the only ones that have DNA within their cytoplasm, being separated from the main DNA of the cell.
• Both organelles reproduce on their own → Because they have their own DNA, chloroplasts and mitochondria can replicate independently of the cell, and direct their own division.
• They have a cell membrane → Unlike the rest of the cell's organelles, both mitochondria and chloroplasts have a double cell membrane that separates them from the rest. This type of membrane is also present in bacteria.

References

1. "Prokaryotic cells" in: Khan Academy. Retrieved on: January 17, 2018 from Khan Academy: es.khanacademy.org.
2. "Differences between eukaryotic and prokaryotic cell" in: Difference Between. Retrieved on: January 17, 2018 from Difference Between: difference-entre.com.
3. "From prokaryotes to eukaryotes" in: Understanding Evolution. Retrieved on: January 17, 2018 from Understanding Evolution: evolution.berkeley.edu.
4. "The Origin and Evolution of Cells" in: NCBI. Retrieved on: January 17, 2018 from NCBI: ncbi.nlm.nih.gov.
5. "The Evolution of the Cell" in: Learn Genetics. Retrieved on: January 17, 2018 from Learn Genetics: learn.genetics.utah.edu.