Origin of Life

Origin of Life

Learning Objectives

• Understand the timeline of the universe and life.

  • Universe formed: approximately 14 billion years ago (bya)

  • Earth formed: approximately 4.5 bya

  • First prokaryotic cells appeared: approximately 3.5 bya

  • First eukaryotic cells appeared: approximately 2.0 bya

  • Modern humans, Homo sapiens, emerged: approximately 200,000 years ago

• Describe the steps involved in the abiotic formation of life from inorganic materials on early Earth.

• Articulate the RNA world hypothesis.

• Explain the endosymbiotic theory pertaining to the origin of eukaryotic cells.

• Discuss how the emergence of photosynthetic organisms transformed the Earth's conditions.

How Did Life Originate?

• The origin of life is theorized to have occurred through a four-stage process involving chemical and physical phenomena:

  1. Abiotic synthesis of small organic molecules - from inorganic substrates:

  • Small organic molecules include: simple sugars, fatty acids, amino acids, nucleotides.

  1. Joining of these small molecules into macromolecules:

  • Macromolecules include: polysaccharides, lipids, proteins, nucleic acids (especially RNA).

  1. Packaging of molecules into protocells:

  • Protocells are hypothesized to have lipid membranes, possibly containing RNA.

  1. Origin of self-replicating molecules:

  • RNA is believed to have arisen before DNA. The RNA world hypothesis suggests that primordial life was RNA-based prior to the emergence of DNA-based life.

Where Did Life Originate?

• Various hypotheses suggest potential origins for life:

  1. Small, warm ponds.

  2. Hydrothermal vents in the ocean.

  3. Clay beds in estuaries or bays.

  4. Tidal pools.

  5. Bubbles of foam formed by ocean waves.

  6. Meteorites possibly delivering organic compounds.

Early Earth's Atmospheric Composition

• Early Earth's atmosphere consisted of:

  • Methane (CH₄)

  • Ammonia (NH₃)

  • Nitrogen (N₂)

  • Carbon Dioxide (CO₂)

  • Water vapor (H₂O)

  • Hydrogen (H₂)

  • Hydrogen sulfide (H₂S)

  • Note: No free oxygen (O₂) existed at this time.

• Present-day atmosphere composition:

  • Nitrogen (78%)

  • Oxygen (21%)

  • Carbon Dioxide (~0.03%)

  • Remaining gases: Argon, water vapor, ozone (O₃).

Stanley Miller and Harold Urey's Experiment (1953)

• Demonstrated the plausibility of abiotic synthesis of organic molecules:

  • Early research from the 1930s by Oparin (Russia) and Haldane (England).

  • In 1953, Miller and Urey conducted an experiment that demonstrated that organic compounds could be formed under conditions mimicking early Earth.

  • Resulted in the production of amino acids and other organic molecules within a week.

  • Later experiments produced nitrogenous bases, critical components of RNA and DNA.

Timeline of Life's Origin

• Key dates in the development of life:

  • Universe: 14 billion years ago (bya)

  • Earth: 4.5 bya

  • First prokaryotes: 3.5 bya

  • First eukaryotes: 2.0 bya

  • Modern humans (Homo sapiens): approximately 200,000 years ago.

• Emergence of photosynthetic organisms significantly increased the availability of oxygen on Earth.

Photosynthesis and the Oxygen Revolution

• Concepts of the Oxygen Revolution:

  • Most atmospheric oxygen (O₂) has a biological origin, produced through oxygenic photosynthesis.

  • Initially, O₂ reacted with dissolved iron, leading to the formation of banded iron formations.

  • By around 2.7 billion years ago, O₂ began to accumulate in the atmosphere, altering the conditions on Earth.

  • The oxygen revolution occurred between 2.7 to 2.3 billion years ago and resulted in the extinction of many prokaryotic groups.

  • Surviving groups adapted by utilizing cellular respiration for energy extraction.

Key Events in the Timeline of Human Evolution (For Additional Context)

• Significant milestones in human evolution include:

  • 8–7 million years ago: Divergence of human and chimpanzee lineages.

  • 7–6 million years ago: Earliest evidence of bipedalism and the reduction of large canine teeth.

  • 4 million years ago: Emergence of the genus Australopithecus.

  • 2.7 million years ago: Appearance of the genus Paranthropus.

  • 2.4–1.6 million years ago: Existence of Homo habilis, who utilized stone tools and had brain sizes similar to chimpanzees.

  • 1.8 million to 100,000 years ago: Existence of Homo erectus, possibly the first species to control fire and employ complex tools.

  • 800,000 to 200,000 years ago: Existence of Homo heidelbergensis.

  • 200,000 years ago: Anatomically modern humans emerged in Africa.

  • 50,000 years ago: Acquisition of symbolic culture and complex language began.

How Did Eukaryotic Cells Originate?

• Key Points Regarding Eukaryotic Origins:

  1. Timeline of Prokaryotes vs Eukaryotes:

  • First prokaryotes appeared around 3.5 billion years ago.

  • First eukaryotes emerged around 2.0 billion years ago.

  1. Structural Similarities:

  • Chloroplasts and mitochondria in eukaryotic cells resemble ancient photosynthetic bacteria.

  • Mitochondria:

    • Composed of an outer membrane and an inner membrane.

  • Chloroplasts:

    • Composed of an outer membrane, a second outer membrane, and internal membranes.

Endosymbiotic Theory

• Definition of Endosymbiotic Theory:

  • Suggests that eukaryotic cells originated from a symbiotic relationship (mutualism) between prokaryotic organisms.

  • Some organelles like mitochondria and chloroplasts were once free-living prokaryotes that became incorporated into larger eukaryotic cells, creating a mutually beneficial relationship.

Evidence Supporting the Endosymbiotic Theory

• Endosymbiotic theory proposed for the first time in 1905:

  • Evidence includes:

  • Mitochondria and chloroplasts grow and reproduce independently, akin to bacteria; they divide similarly.

  • They synthesize their own proteins and share similarities with bacteria regarding protein synthesis and response to antibiotics.

  • Each organelle has a distinct genome resembling that of bacteria:

    • Often in circular chromosomes (discovered in the 1960s).

  • Organelles contain their own ribosomes, which are smaller and more akin to bacterial ribosomes than those found in the cell.

  • Infoldings of membranes in prokaryotes have similar functionalities to those found in mitochondria (aerobic respiration) and chloroplasts (photosynthesis).

The Origin of Multicellularity

• Significance of Multicellularity Evolution:

  • The evolution of eukaryotic cells allowed for diversification into numerous unicellular forms.

  • A subsequent wave of diversification occurred with the evolution of multicellularity, leading to the emergence of algae, plants, fungi, and animals.

  • DNA sequence comparisons suggest that multicellular eukaryotes shared a common ancestor approximately 1.5 billion years ago.

  • The oldest known fossils of multicellular eukaryotes are small algae that existed around 1.2 billion years ago.

The Colonization of Land

• Key Developments in Land Colonization:

  • Fungi, plants, and animals began colonizing land approximately 500 million years ago.

  • Vascular tissue in plants, essential for internal material transportation, emerged around 420 million years ago.

  • Plants and fungi likely engaged in a mutually beneficial symbiotic relationship while colonizing land.

  • Among land animals, arthropods and tetrapods are the most diverse.

  • Tetrapods evolved from lobe-finned fishes approximately 365 million years ago.

Classification of Life Forms

• Three Domains of Life:

  • Eubacteria

  • Archaea

  • Eukarya

• Six Kingdoms of Life:

  • Bacteria

  • Archaea

  • Protista

  • Plant

  • Fungi

  • Animal

• Origin of the Three Domain Classification by Carl Woese.

Biodiversity on Earth

• Species Estimates:

  • According to the International Union for Conservation of Nature (IUCN), there are about 8.7 million species on Earth.

  • Only approximately 1.2 million species have been scientifically described and cataloged.

  • Among the described species, around 41,000 are categorized as threatened with extinction.