Life's Journey: From Origin Theories to Evolutionary Mechanisms

The Origin of Life

The Concept of Life

In biology, life refers to the ability of living organisms to perform vital functions: nutrition, interaction, and reproduction.

These vital functions are common and essential to all living organisms, regardless of their level of complexity or organization. This is known as the constancy principle of vital functions.

Significance of Vital Functions

All living organisms interact with their environment.

The biological significance of vital functions lies in their ability to enable continuous interaction between living organisms and their environment.

The reproduction function allows new organisms to form, ensuring the species' longevity and evolution over time.
The interaction function enables organisms to respond to stimuli and adapt to environmental conditions, which is crucial for their survival.
Through the nutrition function, living organisms exchange matter and energy with their surroundings.

Primordial Conditions

Geophysical Evolution of Earth

The first traces of life on Earth are about 3,800 million years old. Our planet's geophysical evolution occurred during this 800-million-year period.

Chemical Evolution

This process gave rise to the biogenic elements, along with the other chemical elements and compounds that make up matter on Earth.

Theories of the Origin of Life

A scientific theory is a coherent set of ideas and scientific laws that define the relationships between those ideas. It is expressed as a principle or set of principles, such as cell theory.

The Theory of Spontaneous Generation

Proposed by Aristotle (384-322 BC), this theory assumed that the spontaneous generation of simple life forms resulted from the interaction between inert matter and a vital force or divine intervention, called entelechy.

Scientists such as Francesco Redi, Anton van Leeuwenhoek, and Lazzaro Spallanzani began to question this theory in the 17th and 18th centuries, supported by new technologies like microscopes.
Then, in the 19th century, Louis Pasteur definitively refuted this theory, providing conclusive evidence to disprove it.

Pasteur's experiments led to the widespread acceptance of the biogenesis principle: 'life comes from life.' This aligns with one of the assumptions of cell theory: cells are the anatomical, functional, and reproductive units of living organisms.

Panspermia Theory

The Panspermia theory argues for a cosmic origin of life, stating that life originates in outer space and travels from one location to another.

This theory was first proposed in the 5th century BC by Anaxagoras. However, it gained popularity in the 20th century when organic matter, including fossilized bacteria and primitive DNA molecules, was observed in the analysis of certain meteorites.

One of its greatest supporters was Swedish chemist Svante Arrhenius. He proposed that life, in the form of spores or bacteria, travels within cosmic dust or rock fragments, propelled by cosmic radiation. However, despite the high resistance of bacterial life, it has not yet been conclusively proven that it could survive the extreme conditions of a journey through space.

Panspermia is considered a hypothesis rather than a scientific theory because, although plausible, it currently lacks conclusive arguments to fully explain the origin of life on Earth.

Oparin-Haldane Theory

The main proponents of the Oparin-Haldane Theory were Russian biochemist Aleksandr Oparin and British geneticist John Haldane. According to their hypothesis, the primitive oceans contained a large amount of dissolved organic compounds, forming what is known as the 'primordial soup'.

Earth's primitive atmosphere was dense and rich in water vapor, ammonia, methane, and carbon dioxide, notably lacking free oxygen. Constant and powerful electric discharges provided enough energy to give rise to the first organic compounds. These compounds then dissolved in the primitive oceans to form the primordial soup, where life is thought to have begun.

The Oparin-Haldane hypothesis is based on three key assumptions: the specific conditions of primitive Earth, the capacity of chemical elements to interact and form compounds, and the generation of complex molecules from simpler ones.

This theory suggests that life developed over several stages.

The Evolution of Living Organisms

Fossils provide a window into Earth's past, revealing species that once inhabited our planet. Many are now extinct, replaced by others better adapted to current environmental conditions.

The existence of fossils is one of the main pieces of evidence supporting the evolution of living organisms.

Fixism Versus Biological Evolution

Up to the 19th century, a prevalent belief was that current species had remained almost exactly the same throughout Earth's history. This is the central idea of fixism. However, as early as the 4th century BC, the discovery of fossils allowed thinkers such as Anaximander to intuitively develop the idea of species evolution.

Biological evolution is the continuous process by which species transform and new ones emerge, driven by changes that develop from one generation to the next.

Biological evolution is now considered a real, natural, and scientifically proven fact. It is therefore a scientific theory.

Evidence of Evolution in Living Organisms

The theory of evolution is supported by evidence from various scientific disciplines. Evidence suggesting that today's living organisms share a common ancestor is categorized into the following areas:

Paleontological Evidence

This includes evidence from the fossil record, which reveals differences between organisms corresponding to different geological periods.

Fossils of organisms that represent intermediates between two major groups of living things confirm the evolutionary link between them. For example, the Tiktaalik is a fossil exhibiting characteristics of both fish and amphibians. The modern platypus also displays intermediate characteristics between birds and mammals.

Comparative Anatomy Evidence

Comparative anatomy reveals changes occurring in a species due to adaptation to new environmental conditions or the emergence of new species from existing ones. It is based on the comparative study of organ morphology in living organisms and fossils. There are three types of organs: homologous organs, which share a common evolutionary origin but have adapted to different functions; analogous organs, which have similar functions but different evolutionary origins; and vestigial organs, which have lost their original function during evolution.

Biogeographical Evidence

Biogeographical evidence examines the distribution of current species, noting that they are not found on every continent, even where suitable habitats exist. This uneven distribution can be explained as a consequence of organisms spreading out from their place of origin.

Embryological Evidence

The comparative study of embryological evidence in living organisms reveals morphological similarities among all vertebrate groups during the early stages of embryonic development.

This significant similarity among the embryos of fish, amphibians, reptiles, birds, and mammals in their early developmental stages supports the theory of evolution, suggesting a common origin for all vertebrates. Furthermore, a greater level of similarity between two species correlates with a longer developmental period during which their embryos exhibit morphological resemblances.

Biochemical and Molecular Evidence

All living organisms are composed of the same fundamental biological elements and biomolecules. These observations, provided by molecular biology, serve as strong evidence supporting the theory of evolution from a common ancestor.

Additionally, the genetic code is universal. This is another key fact from molecular biology supporting the idea of evolution in living organisms. Furthermore, the molecules of closely related species are more similar than those of distantly related species, and vice versa.

Lamarckism and Darwinism

Jean-Baptiste de Monet, Chevalier de Lamarck (1744-1829), established Lamarckism, which posits that the diversity among living organisms on Earth results from their adaptation to different environments.

Charles Darwin (1809-1882) and Alfred Russel Wallace (1823-1913) independently proposed the theory of evolution by natural selection.

These theories are based on the following principles:

• Lamarckism:
  • Environmental changes create new needs.
  • These needs determine the use or disuse of certain organs, which develop or deteriorate accordingly: the principle of use and disuse.
  • Acquired traits are kept and passed down to offspring.
• Darwinism:
  • Organisms have a high capacity for reproduction, so species tend to produce a large number of offspring.
  • The variability of descendants, as offspring from a single lineage are similar but not identical to one another.
  • The struggle for existence in adverse conditions. The best-adapted organisms survive and can reproduce. This is how they are selected.
  • The natural selection of the best-adapted organisms, because the selected organisms reproduce and pass favorable traits down to their descendants.
  • After many generations, the traits that most favor survival in a specific environment will have been naturally selected.

Neo-Darwinism or Synthetic Theory

Neo-Darwinism, also known as the synthetic theory of evolution, was proposed in the mid-20th century by Huxley, Dobzhansky, and Mayr. It combines principles of genetics and molecular biology with Darwin's theory of evolution.

The key principles of synthetic theory are:

• Variability among the organisms in a population is a result of genetic recombination that occurs in sexual reproduction and genetic mutations.
• Mutations occur randomly. Many of them are negative and even lethal, so they disappear. Others that may seem innocuous turn out to be favorable when environmental conditions change.
• Nature selects the mutations that favor survival in the new environmental conditions. They are called adaptive mutations.
• In this way, populations gradually adapt to their environment.

The Mechanisms of Evolution

Genetic Variability

Genetic variability refers to the range of different genotypes within a species, leading to individual organisms with diverse traits.

There are two causes or sources of genetic variability in living organisms:

• Mutations: These are changes that occur in organisms' genetic material. They may affect DNA nucleotides, genes, chromosomes, or mitochondrial DNA. The changes are expressed in the organism's phenotype or traits. Mutations occur randomly and do not usually depend on the environment.
• Genetic recombination: This is the mixing of genes produced by gametes during sexual reproduction. This process leads to variability because it allows maternal and paternal genes to combine, producing a large number of individuals with different traits.

Natural Selection

Natural selection is the process by which the environment favors organisms best adapted to its conditions. These organisms are more likely to reproduce and pass their favorable traits to their offspring.

Consequences of Evolution

The key consequences of the evolutionary process are:

• Adaptation of Organisms to Their Environment

  Living organisms adapt to new environmental conditions when their surroundings change. These adaptations determine how many descendants of a population will make it to the next generation. If the changes are sudden, some species are not able to adapt and they become extinct.

• Speciation: The Appearance of New Species

  Speciation is the process by which new species evolve from a common ancestor.

• Diversification and Increased Biodiversity

  Another consequence is the increase in the number and diversity of species, resulting in the biodiversity on our planet today.

  Adaptive convergence occurs when different species inhabit similar environments, resulting in similarities in their form even though they are not related. Processes of convergent evolution or adaptive convergence lead to the development of analogous organs. These organs are adapted to the same function but their evolutionary origins are different.

  Evolutionary divergence occurs when populations inhabit different environments and so develop different adaptations. Processes of divergent evolution or evolutionary divergence lead to the development of homologous organs. These organs, which have the same origin, are not obviously similar and are adapted to the species' environment.

Evolutionary Controversies

As with other complex theories, evolution allows for different points of view:

• Selectionism Versus Neutralism

  The controversy between selectionists and neutralists is based on the mechanisms of evolution. Selectionists argue that the key process is natural selection, while neutralists argue that it is chance.

• Gradualism Versus Punctuated Equilibrium

  In this case, the controversy lies in the way new species appear. Gradualists argue that it is gradual, while punctuated equilibrium (or saltation) states that they appear suddenly and then remain stable for long periods of time.

Phylogenetic Trees

Phylogenetic trees are diagrams that visually represent the evolutionary relationships between species, tracing them back to a common ancestor.