Origin of Life and Biological Evolution
The Origin of Life
Spontaneous Generation: It was previously thought that living things arose from decaying organic matter, and any small demonstrations against that idea were rejected.
Pasteur’s Demonstration: Louis Pasteur demonstrated the falsehood of the traditional beliefs. His experiments showed that air contains organisms that break down organic matter, concluding: Every living thing comes from another living being.
Biological Evolution: The Origin of Biodiversity
Biological evolution is the process of transformation of species over time. The long process of evolution led to an enormous diversity of species. There are two main theories about the origin of species:
- Fixity: Species have remained immutable since their creation. This is currently not accepted.
- Evolutionism: Species can change and generate other species. Evolutionary theories include Lamarckism, Darwinism, and the Synthetic Theory.
Lamarck’s Theory
The first known theory of evolution, known as Lamarckism, was proposed by the naturalist Jean-Baptiste Lamarck. Lamarck’s theory is based on the following principles:
- Organisms show a trend towards complexity, evolving from simple forms to complex forms.
- The repeated use of an organ produces its development; “function creates the organ,” and its disuse produces its degeneration.
- Acquired characteristics are heritable. This is why Lamarck’s theory is known as the theory of acquired characteristics.
The Theory of Evolution by Darwin and Wallace
Evolution by Natural Selection: Darwinism proposes that natural selection is the mechanism by which species change over time. This theory can be summarized in three points:
- There is a struggle for survival between organisms. Environmental resources, such as food and space, are limited, so if more individuals are born than can survive, a struggle for survival between individuals is produced.
- Variability exists within a population. Not all individuals of the same species are exactly alike.
- The environment selects the fittest. Individuals with advantageous variations will have a better chance of surviving.
The Origin of Variability
It is now known that many of the differences between individuals are due to genetic changes, and these are generated by two processes: mutation and sexual reproduction.
Mutations as a Source of Variability: Mutations are alterations that occur randomly in genes. Those that affect gametes are transmitted to the descendants. Mutations can be harmful, beneficial, or neutral. Harmful mutations confer a disadvantage to survival and tend to be eliminated. Beneficial mutations provide some advantage. Neutral mutations are neither advantageous nor harmful; therefore, natural selection neither eliminates them nor favors them. Mutations are the basis of hereditary variations upon which natural selection acts.
Sexual Reproduction as a Source of Variability: Sexual reproduction creates variability due to gene recombination, which occurs during meiosis, and the random union of gametes in fertilization. The parental gene mix generates new genetic combinations in individuals, making them unique.
Neo-Darwinism or the Synthetic Theory of Evolution
New knowledge in the field of genetics led a group of scientists to formulate a new theory of evolution that proposed mutations, gene recombination, and natural selection as the main engines of evolutionary change. This theory is called Synthetic Theory or Neo-Darwinism, and it unifies different areas of biology. Its main characteristics can be summarized as follows:
- It rejects Lamarckism.
- Genetic variability is due to two processes: mutation and recombination.
- Natural selection acts on genetic variability.
- Natural selection leads to changes in the overall allele set of a population.
- The population evolves, not individuals.
- Evolution occurs gradually.
Hominization
Hominization is the evolutionary process that has led our species to its current state. It is characterized by the acquisition of the following:
- Erect posture: Elongation of the leg bones, shortening of the arms, S-shaped curvature of the backbone, shortening of the bones of the pelvis, curved birth canal, and attachment of the skull to the spine by its base.
- Reduction of the tail.
- Change in the function of the forelimbs: Hands evolved to be more precise, with the opposable thumb.
- Progressive development of the brain.
- Appearance of language.
- Changes in dentition.
- Reduction in the amount of body hair.