Origins of Life: From Spontaneous Generation to Neo-Darwinism
The Theory of Spontaneous Generation
It was believed that living organisms could arise spontaneously from decaying matter, and even inorganic matter. Louis Pasteur demonstrated irrefutably in 1862 that spontaneous generation did not exist.
Panspermia Theory
This theory states that life has an extraterrestrial origin. Life came to Earth in the form of bacterial spores from outer space (defended by Arrhenius).
Difficulties of this theory:
- It does not explain how life arose in its place of origin.
- Any living specimen had to overcome the impact of the asteroid against the celestial body where life existed.
- Interplanetary travel.
- The sudden arrival on Earth.
Creationism
Each species is the result of a particular creative act (Linnaeus).
Oparin’s Theory of Primeval Soup
Proposed in 1920-30. The components of the primitive atmosphere were the initial nebula plus magma degassing. The first oceans were formed after the formation of simple organic compounds from gases of the primitive atmosphere. After the oceans formed, polymerization occurred, forming the primeval soup – aggregates of polymers, initially protobionts. Protobiont evolution towards definitive life.
Miller’s Experiment
In 1953, Miller synthesized carbohydrates, amino acids, and fatty acids from gases present in the primitive atmosphere.
The First Cell
It was very simple, possessing a membrane (for exchanging matter with the medium, a nutrient broth of organic primitive biomolecules), DNA (self-replicating genetic material), and ribosomes (to make proteins, including enzymes).
LUCA
A group of protobionts that could all coexist.
Cyanobacteria
The first photosynthetic beings: CO2 + water + light energy –> glucose and O2. This allowed an increase of O2 in the atmosphere.
Lamarckism
In early 1809, Lamarck published his “Zoological Philosophy.” He proposed that God created nature, which has a natural tendency towards complexity. This, and the adjustments caused by environmental variation, is the cause of the diversity of species.
Species: A set of individuals that can hybridize, resulting in fertile offspring.
Fixism
Species remain unchanged over time.
Catastrophism
Vanished species became extinct due to some sort of geological, climatic, or environmental catastrophe.
Malthus
Human population growth is much higher than the increase in food production, leading to a struggle for existence.
Darwin and Wallace
In 1859, Darwin published *The Origin of Species*. His ideas rested on those of Malthus and Lyell (currently, geological processes of the past are similar to current ones, without major catastrophes and without major extinctions).
Darwinism
- Species do not increase in number indefinitely because food resources are limited.
- Variability in offspring enables them to adapt to environmental changes.
- Natural selection: Organisms struggle to survive, and only the fittest succeed.
- There is a gradual, slow change in species.
Mutations (Phenotypic Variability)
The existence of dominant and recessive alleles and mutations are the cause of the variability of individuals. Different phenotypes appear suddenly. Natural selection would act on that individual variability, and the process would occur abruptly, in jumps, not slowly and gradually as Darwin defended.
Neo-Darwinism
- Variability of offspring is due to mutations, the existence of dominant and recessive alleles, multiple alleles, genetic recombination, and genomics, leading to new genotypes and increased phenotypic variability.
- Natural selection.
- Variation of gene frequencies: Populations, not individuals, evolve, seeing their gene frequencies altered by migration, mutation, genetic drift, and natural selection.
- Reproductive isolation: For a population to lead to a new species, it must be isolated from other populations so that the variation in gene frequencies is not diluted.