Principles of cell evolution, types and development of cells
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| Principles of cell evolution, types and development of cells |
Theories of cellular evolution are explanations that seek to understand when and how cells emerged. Usually they refer to eukaryotic cells, that is, they have a nucleus that is separated by a cellular membrane where they contain the genetic material. Unlike prokaryotic cells, which were simple and appeared on Earth about 3,700 million years ago, eukaryotic cells are more complex, larger and appear more recently.
Because eukaryotic cells are the basis of most living things, such as plants and animals, many theories have been developed regarding their origin and why they appear.
Development of first cells
The first cells appeared at least 3,700 million years ago, about 750 million years after the Earth was formed. Although we do not know for sure how the first cells appeared, we do know very well 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 primitive Earth, a discharge of energy could spontaneously produce organic molecules to form. This was demonstrated in the 50s by the experiments of Stanley Miller, in which he managed to form 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 person was able to use material from their environment. He was then born, a first cell.
These were the first cells to reproduce independently for the first time, given the lack of competition for the fuel they used. However, because their numbers greatly increased (precisely because of the lack of competition), cells soon had to become more sophisticated to reproduce. So started the process of development.
Types of cells and their development
For many years, it was believed that there were only two types of cells, prokaryotes (literally "without a nucleus") and eukaryotes, with more complex and later beginnings. However, over the past two centuries they have been identifying another cell type that does not fit with either of the other two characteristics.
These cells have been known since the 90s as "archaea", literally meaning "old ones". In this way, a classification system of three domains is used today: Archaea, Bacteria and Eukarya.
archaea cells
Archaea (also known as archaea) are cells without a nucleus, similar to bacteria, but due to some characteristics that they are considered independent organisms.
Like the rest of the cells, they are microbes. Its cell wall is very resistant, which allows them to live in extreme environments (as in asteroids in space, without protection of any kind).
Their diet is also very different, as 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 is inside the cell. Inside we can find genetic material suspended inside the cytoplasm, as well as some ribosomes (the energy-generating organelles inside the cell).
Prokaryotic cells, despite having many different types, are all classified as bacteria. To be able to adapt to the environment more effectively, many of them have other additions, such as flagella to move freely or a sticky wall, capsule, that allows them to adhere to other organisms. Is..
Eukaryotic cells
Eukaryotic cells are the most complex and the largest of the three types. They differ from prokaryotes and archaea primarily in that they have a nucleus, where they store DNA. In addition, they have a variety of cellular organs, which allow them to perform a variety of functions.
Eukaryotic cells are the basis of all complex life on Earth. Because of this, scientists have been studying its origin for several decades, and have developed what is called the endosymbiotic theory of cell development.
Endosymbiotic theory of cell development
Eukaryotic cells are much more evolved 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 similarity between the mitochondria and the chloroplasts of eukaryotic cells with those of bacteria, both in their form and in their functioning.
Therefore, the scientists defending it propose that at some point in evolution, a larger cell absorbed a bacterium and began to use it to extract the energy needed to survive and reproduce.
Absorbed bacteria, on the other hand, have more chances to leave the lineage, as well as gain more protection from living inside a larger cell. Therefore, there was a symbiotic relationship; Hence the name of the theory.
After millions of years of evolution, mitochondria and chloroplasts, which used to be independent bacteria, have found particular use. Therefore, they can no longer survive outside the cell.
Evidence for the Endosymbiotic Theory
In everyday language we use the word "theory" to describe an opinion that is not based on facts. However, a theory in the world of science is an explanation of a phenomenon confirmed by experiments and observation.
Endosymbiotic theory is not an exception. Several clues lead us to wonder how it came about in animal and vegetable cells. The following are some of these evidences:
• Mitochondria and chloroplasts have their own DNA → These two types of organelles are the same that have DNA inside their cytoplasm, which is different from the main DNA of the cell.
• Both organelles reproduce on their own → since they have their own DNA, chloroplasts and mitochondria can replicate independently of the cell, and direct their own division.
• They have a single 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.
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