Origin and Evolution of Life: From First Cells to Biodiversity

Posted on Dec 12, 2024 in Biology

Theories on the Origin of Life

First Theories

  • Creation: This myth is the basis of most religions. In them, some sort of creative force acts on matter, such as land or rocks. For a long time in the West, the Genesis account was accepted literally.
  • Spontaneous Generation: In ancient Greece, it was believed that life appeared spontaneously whenever adequate conditions were met. Aristotle synthesized the ideas about spontaneous generation. The theory of spontaneous generation prevailed for over two thousand years until Francesco Redi struck the first blow to supporters of this dogma. Lazzaro Spallanzani also came close with experiments to demonstrate that this theory was false.
  • Louis Pasteur: Irrefutably demonstrated that spontaneous generation did not exist; living beings only come from other predecessors.
  • Panspermia: The cosmic origin of life. The panspermia theory posits that life came to Earth in the form of bacterial spores from outer space, driven by radiation pressure from stars. For supporters of panspermia, life is eternal. At present, the hypothesis of the extraterrestrial origin of life is again in vogue.

Modern Theories

  • Chemical Evolution of Life: The most accepted hypothesis is that living matter would be the result of a chemical evolution that preceded biological evolution. In 1924, Oparin proposed that chemical compounds that existed in the primitive atmosphere were used as raw material for the synthesis of simple organic compounds of living beings. The first living systems appeared after a long prebiotic evolution.
  • Primitive Conditions of the Earth: The early atmosphere possessed a more or less reducing environment since oxygen was not free and was probably made up of gases such as methane, ammonia, water vapor, carbon dioxide, and hydrogen sulfide.
  • Prebiotic Synthesis of Organic Molecules: It is possible that gases in the atmosphere reacted spontaneously when bombarded by intense radiation, resulting in the formation of small organic molecules. Rain washed chemical compounds from the atmosphere into the oceans, where they dissolved, resulting in the “primordial soup.” This set of organic molecules reacted with water, and biological transformation began with building blocks such as amino acids. Living matter appeared, and polymers of biological interest interacted. In the primordial soup, different microstructures emerged, grouping polymers, and complex processes developed that led to the formation of self-reproducing systems. These primitive living systems extended, resulting in biological evolution.
  • Clay Genesis: It is probable that biological polymers were synthesized and accumulated in primordial soup on clay surfaces. Clays act as biocatalysts, attracting and concentrating simple molecules on their surface and facilitating their polymerization to form the first polymers of biological interest. Recently, a new hypothesis on the origin of life suggests that confined spaces could have provided the right conditions to facilitate prebiotic chemistry.
  • Hydrothermal Vents: For some scientists, life originated in the depths of the oceans. The point of departure for chemical evolution could have been hydrothermal vents located in the submarine salts near the primitive atmosphere. The presence of pyrite would have provided the appropriate scaffolding for the formation of organic polymers.
  • RNA World: The next step in prebiotic evolution was the first synthesis of polymers of biological relevance. It is believed that RNA was the first biomolecule with storage capacity and catalytic activity to appear. RNA formed in the primitive seas. Subsequent evolution determined that the functional RNA molecule was replaced by DNA, which is a more stable molecule.

The First Cell: Cellular Evolution

All organisms are related because they come from ancestral cells. It is believed that the decisive moment in the origin of cells was the appearance of a biological membrane. The membrane separated the external environment from the internal environment, favoring the existence of a primitive metabolism in the ancestral cell that could obtain energy and use it to reproduce and respond to environmental changes. The first cells were anaerobic, fermentative, heterotrophic bacteria able to obtain food and energy directly from their environments. After the exhaustion of food stocks, the first cells developed mechanisms to obtain energy, such as photosynthesis in cyanobacteria. The release of oxygen as a consequence of photosynthesis resulted in an oxidizing atmosphere very similar to today’s. This oxygen became a deadly poison to anaerobic organisms. Many of the existing cells adapted and learned to use it in their metabolic reactions. Thus, primitive heterotrophic bacteria carried out aerobic cellular respiration. Eukaryotic cells later evolved from a symbiotic association, according to the endosymbiotic theory.

Evolution of Living Beings

Ancient Theories on the Origin of Species

  • Fixism or Creationism: Fixism is a theory that proposes that species do not change; they remain basically unchanged over time from creation.
  • Karl Linnaeus – Catastrophism: According to catastrophism, cataclysms destroy every existing species, so a creation of new species is subsequently produced. (Georges-Louis Leclerc, Comte de Buffon, Georges Cuvier)

Evolutionism

Evolutionary Hypothesis

  • Evolutionary Theories of Lamarck: Lamarck was the first naturalist who opposed the immutability of species and formulated a scientific hypothesis to explain the evolution of living beings. Lamarck’s evolutionary hypothesis is called transformism, and it states that species evolve to become other species gradually.

This hypothesis is based on two points:

  1. The function creates the organ. Organisms adapt as the organs most commonly used develop, while there is a continuous atrophy of less utilized organs.
  2. Acquired characters are inherited. The series of amendments and characters acquired by an individual throughout his life are inherited by their descendants.

Theory of Evolution of Darwin-Wallace

In the 19th century, further studies on the evolution of species were announced by two British naturalists, Darwin and Wallace. Both shared their findings and presented them to the Linnean Society. One year later, Darwin published his “On the Origin of Species by Means of Natural Selection,” in which he formulated the theory of natural selection to explain the evolutionary mechanism. The theory of Darwinian evolution is based on the following principles:

  • High reproductive capacity of living beings
  • Variability of populations
  • Natural selection
  • Species evolve

The theory of evolution of Darwin-Wallace creates a bond of kinship among all living beings. Today’s species are the result of the progressive and continuous adaptive divergence of preceding species.

Neo-Darwinism: Synthetic Theory of Evolution

Darwin did not know how hereditary traits are passed from generation to generation or what causes the variability of populations on which natural selection acts. The theory of evolution was unemployed for a long time until a synthesis was carried out between the theory of Darwinian selection and Mendelian genetics. Thus arose Neo-Darwinism, also known as the synthetic theory of evolution. According to the theory proposed by Neo-Darwinism, evolution is due to two factors:

  • Existence of Genetic Variability in a Population: Although most of the genes are common, there is a large number of different genotypes. This variability is caused by mutations and recombination that takes place during sexual reproduction.
  • Performance of Natural Selection and Evolutionary Basis: Natural selection varies the proportions of genes in each population, increasing the frequencies of advantageous combinations. Therefore, the overall population slowly but continuously evolves to be increasingly better adapted to its environment.

New Theories of Evolution

Neutralism or Neutralist Theory

This theory states that it is not natural selection but pure chance that varies populations in which a particular mutated gene can be dispersed without any selective advantage.

Punctuated Equilibrium Theory or Saltationism

Punctuated equilibrium poses that the absence of intermediate steps in the fossil record should not be attributed to it being incomplete but rather to evolution occurring in leaps.

Scientific Evidence: Evidence of Evolution

Classic Evidence

  • Morphological: Based on the study of comparative anatomy, one can learn about the similarities and differences of the same organ in different species. Homologous organs are the result of divergent evolution, and analogous organs are an example of convergent evolution.
  • Paleontological: Paleontology studies fossils, which are mineralized remains of organisms that have been preserved in sedimentary rocks. Fossils allow us to know the lifestyles of the past and study their evolution.
  • Embryonic Relationship: Embryology studies the early development of organisms. Ernst Haeckel proposed the principle that the embryonic development of an animal summarizes the development of its species.
  • Taxonomic: The system of classification of living things groups organisms that have similar characteristics.

Recent Evidence

  • Comparative Biochemistry and Cell Biology: Living things have a chemical unity. Data confirm that:
    • The genetic code establishes the relationship between the sequence of nucleic acids and proteins.
    • ATP is the molecule that provides energy for cell metabolism.
    • Metabolic and physiological processes are common to most important groups of living beings, very different from one another.
  • Protein and DNA Sequences: A comparison of the nucleotide sequences of the same gene or amino acid sequences of the same protein in different species reveals the existence of some similarities. The number of differences is directly related to the evolutionary distance between the species compared.
  • DNA Hybridization: It relies on the ability of denatured DNA to reconstruct hydrogen bridges, the basis of their chains, after being subjected to a denaturing process.
  • Chromosome Banding Study: Evolutionary cytogenetics studies and compares the characteristic chromosome shapes of each species. Chromosome banding studies consist of longitudinal differentiation and characterization of each pair of homologous chromosomes.
  • Immunological or Serological Analysis: The introduction of an antigen into an organism results in the production of antibodies that can bind to antigens in an agglutination reaction. Thus, antibodies against human antigens give a very strong response in humans.

The Result of Evolution: Biodiversity

A species is a set of organisms that have morphological similarities, can reproduce, give rise to fertile offspring, and have reproductive isolation from other species. Reproductive isolation is the inability of some organisms to reproduce with others, and when it appears between different populations of one species, it opens the door to the process of speciation.

The Process of Speciation

The appearance of two or more different species from an initial one. The most important factor in this process is geographic isolation, which is the separation of populations by physical barriers such as mountains, rivers, or oceans.

The Classification of Living Things

According to the binomial system, a species name consists of two Latin words written in italics, the first of which corresponds to the genus. For example, the human species is called Homo sapiens and therefore belongs to the genus Homo. Taxonomy is the branch of biology responsible for the classification and nomenclature of living beings.