The Origins and Evolution of Life

Posted on Jul 31, 2024 in Biology

1. What is Life?

We can define life through the lens of genetic code, which is used to construct proteins from a DNA sequence. The so-called border cases fall within the limits of living things.

1.2. Characteristics of Living Things

  • Molecular Complexity: The composition of living organisms is highly intricate.
  • Auto Maintenance: Living things incorporate foreign matter and energy, utilizing them to build their own components.
  • Sensitivity: The ability to perceive changes in their environment and react accordingly.
  • Reproduction: The capacity to reproduce, meaning to generate copies of themselves.

2. The Origin of Life

2.1. Early Theories on the Origin of Life

The Theory of Spontaneous Generation: This theory posited that living things arose spontaneously from decaying organic matter, potentially even inorganic matter.

Francesco Redi’s Experiment: Redi placed pieces of meat into three flasks, sealing two and leaving the third open. After a few days, maggots appeared in the open flask but not the sealed ones. Redi concluded that the maggots originated from eggs deposited on the meat by flies.

Louis Pasteur’s Experiment: Pasteur placed broth in flasks with bent necks, allowing air to enter while trapping potential airborne microorganisms in the curvature. He boiled the broth and then broke the neck of one flask. Decomposition, caused by microorganisms, was observed only in the flask with the broken neck. This experiment established that all living beings originate from pre-existing living beings.

2.2. Prebiotic Synthesis

The most widely accepted hypothesis among scientists today is that life emerged as a result of chemical evolution.

Oparin and Haldane’s Hypothesis:

  1. Formation of Simple Organic Molecules: In a primitive atmosphere composed of methane, ammonia, hydrogen, and water vapor, these gases, exposed to intense radiation and electrical discharges from storms, reacted to form simple organic molecules.
  2. Formation of Complex Molecules: Rain carried atmospheric organic compounds into the oceans, where they dissolved and interacted, leading to the accumulation of more complex organic compounds. This accumulation in early oceans is known as the primordial soup.
  3. Formation of Coacervates: Some of these molecules organized to form macromolecules, and lipid binding could have led to the formation of coacervates.

Miller-Urey Experiment: Miller recreated the primitive Earth conditions proposed by Oparin in a laboratory setting and successfully produced organic compounds. While significant, the Miller-Urey experiment holds primarily historical value, as it doesn’t fully explain the origin of life.

2.3. Conditions of the Early Earth

Characteristics of the Early Earth:

  • High levels of ultraviolet radiation reached the surface.
  • The atmosphere was vastly different from today’s, resembling the gaseous envelopes of giant planets, rich in methane, ammonia, and hydrogen. These are considered primitive atmospheres, distinct from the secondary atmospheres of the present and the Archean eon.
  • The Earth was subjected to frequent asteroid bombardments.

2.4. Possible Scenarios for Life: Panspermia and Hydrothermal Vents

Three main scenarios are proposed for the origin of life:

  1. Tidal Pools: Life could have arisen in a coastal area where evaporation favored the concentration of the products of early synthesis. However, this scenario faces objections due to the high exposure to harsh conditions.
  2. Undersea Hydrothermal Vents: These are locations where volcanic gases and water emerge at high temperatures. Arguments in favor of this scenario include protection from asteroid impacts, a reducing environment, and the fact that it’s the only ecosystem independent of terrestrial solar energy. Primitive organisms thrive in these environments.
  3. Panspermia: This theory suggests that the first living organisms originated elsewhere in the universe and were transported to Earth on asteroids or comets. Two discoveries support this hypothesis: the 1969 discovery of a meteorite in Australia containing amino acids, suggesting their formation in space, and the 1996 discovery of structures resembling terrestrial bacteria in a Martian meteorite found in Antarctica.

2.5. The First Cells

The first cells are believed to have emerged around 3.8 to 4 billion years ago. The appearance of a membrane separating the internal environment from the external environment is considered the main driving force behind this development. These early organisms were very simple. The first photosynthetic autotrophs, organisms capable of synthesizing carbohydrates using sunlight as an energy source and releasing sulfur in the process, appeared around 3.5 billion years ago. Eukaryotic cells are thought to have arisen from the symbiosis of two or more prokaryotic cells.

3. The Evolution of Living Beings

3.1. The Long Road to Evolution

Fixism: This theory proposed that species were immutable, remaining essentially unchanged over time. It was characterized by an anthropocentric worldview, a belief in a young Earth (around 6,000 years old), and reliance on common sense as evidence. Georges Cuvier, a proponent of fixism, presented the theory of catastrophism, which suggested that periodic catastrophic events led to the extinction of most living things. Creationism, another form of fixism, proposed that species were created by a supreme being to occupy specific niches in nature.

Evolutionism: This theory asserts that biological species are not fixed but transform over time. It is based on the following ideas:

  • Entities naturally change over time.
  • Environmental conditions are not static.
  • Environmental changes create new needs.
  • Inheritance of acquired characteristics.

3.2. The Darwinian Revolution

The Theory of Evolution by Natural Selection: This theory posits that more individuals are born than can survive, leading to competition for resources.

  • Variation: Small differences, or variations, exist between individuals of the same species.
  • Natural Selection: Some variations provide advantages in the struggle for survival.
  • Speciation: Over time, these advantageous variations become more common, leading to changes in the species.

3.3. Evolution After Darwin: The Synthetic Theory of Evolution

The synthetic theory of evolution combined Darwin’s ideas with modern genetics. Its most important contributions include:

  • The evolutionary unit is not the individual but the population.
  • The source of variability is mutation, which is a random and sudden change in DNA.

3.4. The Formation of Species

Speciation: The process by which one species gives rise to two or more new species. It occurs in several stages:

  1. Two populations of the same species become isolated.
  2. Both populations evolve independently in their respective environments.
  3. The accumulation of differences between the populations, driven by natural selection, eventually leads to the formation of two distinct species.

3.5. Evidence for Evolution

  • The Fossil Record: Fossils provide evidence that species have appeared and become extinct throughout Earth’s history.
  • Embryonic Development: In the early stages of development, embryos of different vertebrates share striking similarities, which gradually disappear as they mature. These similarities point to a common ancestry.
  • Geographical Distribution of Living Things: Continents, once united, shared the same fauna. The present-day distribution of species, with related yet distinct species on different continents, supports the idea of common ancestry and subsequent evolution.
  • Molecular Biology: Comparing nucleotide sequences in the DNA of different species can provide insights into their evolutionary relationships. Similarities in DNA sequences indicate closer relationships.

4. Origin and Evolution of Man

4.1. Man’s Position in the Zoological Scale

Humans belong to the order Primates. The main characteristics of primates include:

  • Highly mobile limbs with prehensile fingers and opposable thumbs.
  • Flat nails on fingers and toes.
  • Forward-facing eyes with stereoscopic vision.
  • Unspecialized digestive system.
  • Small litter size and high dependence of offspring on the mother.

4.2. What Changes Made Us Human?

  • Bipedalism: Walking upright on two legs is the primary distinguishing feature between hominids and apes.
  • Position of the Foramen Magnum: The foramen magnum, the hole in the skull where the spinal cord connects, is positioned more centrally and underneath the skull in hominids, reflecting an upright posture.
  • Hip Structure: Hominin femurs are angled inward from the hips, converging at the knees, while ape femurs are oriented more vertically.
  • Foot Morphology: Chimpanzee feet resemble human hands. In contrast, hominid feet evolved into flattened platforms for walking.
  • Hand Anatomy: Hominid hands transitioned from power grips to precise grips, facilitating tool use.
  • Encephalization and Life Cycle: The human brain, with a volume of 1300-1400 cm³, is about three times larger than that of a chimpanzee. This development is linked to a more diverse diet, earlier births, prolonged infancy and childhood, and social living, which enhances survival.

4.3. The Evolution of Hominids

The human evolutionary tree is still being pieced together, with several alternative models proposed. Some of the most representative species in the hominid lineage include:

  • Orrorin tugenensis
  • Australopithecus
  • Homo habilis
  • Homo ergaster
  • Homo erectus
  • Homo antecessor
  • Homo neanderthalensis
  • Homo sapiens