The Origin and Evolution of Life and Humans

Posted on Nov 18, 2024 in Biology

The Origin and Evolution of Life

Early Hypotheses

The earliest explanation for the origin of life attributed it to a higher being creating Earth and all life. The panspermia hypothesis proposed that meteorites containing organic molecules seeded life on Earth, but it didn’t explain the origin of these molecules or how they survived space. Arrhenius, a proponent of this idea, believed that organic molecules were essential for life, but later discoveries showed that organic molecules don’t necessarily imply life.

Oparin’s Hypothesis

Oparin suggested that primitive life forms appeared on Earth after hundreds of millions of years. He proposed two processes for the formation of organic molecules:

  • Chemical Processes: In the absence of oxygen, which destroys organic molecules, increasingly complex molecular aggregates formed.
  • Energy Sources: As Earth’s surface cooled, water vapor condensed to form primitive oceans, where these compounds accumulated, creating the “primordial soup.” Changes in initial conditions and gas mixtures could have led to the formation of nucleotides (DNA and RNA).

Prebiotic Synthesis

Oparin believed that monomers, the first organic molecules, combined to form polymers in the primordial soup, creating coacervates, which he considered precursors to living organisms. While this has been replicated in laboratories, it doesn’t explain how coacervates developed the genetic information and vital functions necessary for life. This raises the question of how the first living being arose if, as Pasteur stated, all life comes from pre-existing life.

Current Hypotheses

  • Updated Oparin’s Hypothesis: This revises Oparin’s theory to include a less simplistic atmosphere, potentially containing some oxygen. The presence of ancient iron oxides and carbonates, and less neon than expected in the current atmosphere, suggests that meteorites impacting Earth might have brought the first organic compounds. This theory posits that membranes formed first, followed by proteins, and then DNA or RNA.
  • RNA-World Hypothesis: This hypothesis, proposed about 25 years ago, suggests that self-replicating RNA molecules were the precursors to life, later evolving into the more stable DNA.
  • Clay-Crystal Hypothesis: This theory proposes that crystals in clay, formed by ions, ordered amino acids to create the first proteins. These protein-crystal structures were then enveloped by a membrane, and genetic information was transferred from minerals to RNA during evolution.

Early Life and the Rise of Oxygen

The first living cells were heterotrophic, meaning they consumed organic matter from the primordial soup. They were also anaerobic prokaryotes, lacking a nuclear membrane and using fermentation for energy. As organic matter dwindled, some organisms adapted to produce their own organic matter, becoming autotrophic.

Around 2.4 billion years ago, an oxidizing atmosphere emerged, leading to the development of respiration and aerobic organisms, which grew and reproduced faster than anaerobic organisms. Some oxygen (O2) transformed into ozone (O3) due to ultraviolet radiation, creating a protective layer that shielded cells from harmful UV rays. This allowed organisms to leave the water and colonize land.

Alternative Theories

  • Creationism (Fixism): This belief system attributes the creation of Earth and all life to a divine being.
  • Other Fixist Theories: Proponents like Carl von Linné and Georges Cuvier believed species remained unchanged over time, explaining fossil records through successive creations or cataclysms.

Biological Evolution

Lamarck’s Theory

Lamarck challenged fixism, proposing that living things have an inherent tendency towards perfection, allowing them to adapt to their environment. He believed that function creates the organ, meaning new organs could arise in response to environmental changes. While influential, this theory is incorrect as acquired characteristics are not inherited.

Darwin’s Theory

Darwin’s theory of evolution rests on these principles:

  • Change over Time: Life forms are not static but constantly evolving, with some species going extinct and new ones emerging.
  • Gradualism: Evolution is a slow and continuous process, not marked by sudden leaps or changes.
  • Common Descent: Similar organisms are related and share a common ancestor, tracing all life back to a single origin.
  • Natural Selection: This two-phase process drives evolution. First, spontaneous variations arise in individuals. Second, in the struggle for survival, individuals with advantageous variations are more likely to survive, reproduce, and pass on their traits.

Neo-Darwinism

Neo-Darwinism builds on Darwin’s theory, identifying spontaneous mutations and recombination as the sources of hereditary variation. It emphasizes gradual evolution and the two-stage mechanism of natural selection: the production of chromosomal mutations and the selection of individuals with advantageous genes.

The Origin of Humans

Around 30 million years ago, Old World monkeys (higher primates) began the hominization process. This process can be divided into four phases:

  1. Pre-human Phase: Australopithecines and Homo habilis.
  2. Archaic Human Phase: Archanthropines like Homo erectus.
  3. Primitive Human Phase: Paleoanthropines like Homo neanderthalensis.
  4. Modern Human Phase: Neoanthropines like Homo sapiens and Homo sapiens sapiens.

Around 50,000 years ago, as the last ice age ended, humans migrated from Africa. One branch reached Australia, another reached Central Asia, splitting into groups that migrated to Europe and across the Bering Strait to America. The Polynesian islands were colonized last, during the first millennium AD.

Hominid Species

  • Australopithecus: Characterized by bipedalism, they lived in Africa between 4 and 1.6 million years ago. They had small skulls (around 400 cm3). The oldest known species is Australopithecus anamensis.
  • Homo habilis: Sharing a common ancestor with Australopithecus, they lived in Africa between 2.3 and 1.4 million years ago. Their skull size ranged from 520 to 750 cm3, and they stood between 0.9 and 1.3 meters tall.
  • Homo erectus: Lived in Africa, Asia, and Europe between 1.8 million and 27,000 years ago. Their skull size ranged from 800 to 1300 cm3, and they stood between 1.6 and 1.8 meters tall.
  • Homo sapiens neanderthalensis: Fossils found in Europe, the Middle East, and Central Asia date back to 35,000 to 20,000 years ago. They had a skull size of around 1500 cm3 and were slightly shorter than modern humans. They were hunter-gatherers, used fire, and buried their dead.
  • Homo sapiens sapiens: Originating in Africa, they colonized Asia, Europe, and America around 130,000 years ago. They have a skull size of 1300-1500 cm3 and a prominent chin. They are the modern humans and created cave paintings and other forms of art.

The Hominization Process

  • Bipedalism: Walking upright expanded the visual field and freed hands for carrying offspring, food, or tools.
  • Strengthened Social Bonds: Human females can mate year-round, unlike other primates. This led to males hunting while females cared for offspring, fostering altruistic behavior and a sense of possession. Social groups increased survival chances, and parents provided more care for children.
  • Dietary Changes: Adapting to the savanna environment led to a diet richer in animal protein, reducing the size of the digestive system and allowing for brain development.
  • Language Acquisition: Associating objects with words eventually led to the expression of ideas through language.

Human Classification

  • Kingdom: Animalia
  • Class: Mammalia
  • Order: Primates