History of Life on Earth

Life on Earth

  • Earth formed from planetesimal aggregation ~4.55 BYA.

  • By 4 BYA, Earth's surface cooled enough for solidification and liquid water accumulation.

  • Life emerged between 4 and 3.5 BYA.

 

Formation of Organic Molecules

  • Organic molecules and macromolecules formed spontaneously and accumulated in early oceans (prebiotic soup).

  • Three main hypotheses for the origin of this soup:

    1. Reducing Atmosphere Hypothesis

    2. Extraterrestrial Hypothesis

    3. Deep-Sea Vent Hypothesis

1. Reducing Atmosphere Hypothesis

  • Early Earth's atmosphere was rich in water vapor and supported redox reactions for complex molecule formation.

  • Miller & Urey (1953) Experiment:

    • Simulated early Earth conditions.

    • Produced amino acids, sugars, and nitrogenous bases.

    • Other gas mixtures yielded similar results.

2. Extraterrestrial Hypothesis

  • Meteorites brought organic carbon to Earth.

  • Carbonaceous chondrites contain amino acids and nucleic acid bases.

  • Opposing argument: Intense heating upon impact may have destroyed most organic material.

3. Deep-Sea Vent Hypothesis

  • Biologically important molecules formed in temperature gradients at deep-sea vents.

  • Supported by experiments showing ammonia (NH₃) formation, a key component of amino/nucleic acids.

 

Formation of Organic Polymers

  • Polymer formation in water is unlikely due to hydrolysis competing with polymerization.

  • Two possible solutions:

    1. Clay Hypothesis:

      • Clay surfaces facilitated bond formation between nucleotides.

      • Experiments show polypeptides and nucleic acid polymers form on clay.

    2. Carbonyl Sulfide Hypothesis:

      • Carbonyl sulfide (CO₂ + sulfur) may have enabled polymer formation in water.

 

Formation of Cell-Like Structures

  • Protobionts: Aggregates of molecules with a boundary (e.g., lipid bilayer).

  • Characteristics of protobionts:

    1. Boundary separating internal & external environments.

    2. Information storage (polymers inside).

    3. Enzymatic function (polymeric reactions).

    4. Self-replication capability.

Possible Precursors to Cells

Coacervates

  • Spontaneous droplets formed from charged polymers (proteins, carbs, nucleic acids).

  • Selective absorption of molecules from surroundings.

  • Trapped enzymes enable primitive metabolism.

Liposomes

  • Vesicles with phospholipid bilayers.

  • Clay catalysts promote liposome growth and division.

  • RNA on clay surfaces → Liposomes enclose RNA, aiding replication.

 

Acquisition of Cellular Characteristics

  • RNA World Hypothesis: RNA was likely the first genetic material.

  • RNA had three key functions:

    1. Stored information.

    2. Self-replicated.

    3. Acted as an enzyme (ribozyme).

  • DNA & proteins do not perform all three functions.

Chemical Selection & Evolution of RNA

  • Chemical Selection: Molecules with advantageous properties increase in number.

  • Hypothetical RNA evolution:

    1. RNA mutates → gains ability to self-replicate.

    2. Second mutation enhances ribonucleotide synthesis → reduces reliance on prebiotic formation.

 

Advantages of DNA/RNA/Protein World

1. DNA for Information Storage

  • DNA took over RNA’s information storage role.

  • More stable than RNA, preventing mutations.

  • RNA may have served as a template for DNA synthesis.

2. Proteins for Metabolism & Cellular Functions

  • Proteins provide greater catalytic efficiency.

  • Enable structural roles, transport functions, and metabolic diversity.

  • Early RNA likely contributed to polypeptide formation.

  • RNA still plays a role in protein synthesis today (e.g., rRNA, tRNA).

 

Fossils

  • Preserved remains of past life on Earth.

  • Usually formed in sedimentary rock:

    • Organisms are quickly buried in gravel, sand, or mud.

    • Over time, layers build up, and sediments turn into rock.

    • Over millions of years, hard parts are replaced by minerals, creating a fossil representation of the original organism.

 

Radioisotope Dating

  • Fossil age is estimated by radiometric dating:

    • Measures the amount of a radioisotope and its decay product in the surrounding rock.

    • Each radioisotope has a unique half-life:

      • Half-life = Time required for half of the original isotope to decay.

  • Dating method:

    • Igneous rock near sedimentary fossils is often used for dating.

    • Igneous rock:

      • Forms from lava.

      • Initially contains uranium-235 but no lead-207 (its decay product).

 

History of Life on Earth

  • Geological Timescale: Timeline of Earth's history from origin (~4.55 BYA) to present.

  • Four eons (subdivided into eras):

    1. Hadean

    2. Archaean

    3. Proterozoic

    4. Phanerozoic

  • The first three eons are collectively called the "Precambrian".

  • Changes in living organisms result from two key processes:

    1. Genetic Changes:

      • Alter an organism’s characteristics.

      • Affect survival and reproduction.

    2. Environmental Changes:

      • Major changes over 4 billion years.

      • Can promote evolution of new organisms.

      • Can lead to extinction:

        • Mass extinction occurs if many species disappear simultaneously.

 

Major Environmental Changes

  1. Temperature:

    • 2.5 billion years of cooling followed by 2 billion years of fluctuations.

  2. Atmosphere:

    • Oxygen increase began ~2.4 BYA.

  3. Landmasses:

    • Continents formed and shifted over time.

  4. Floods & Glaciations:

    • Catastrophic floods and glaciers periodically reshaped the planet.

  5. Volcanic Eruptions:

    • Killed organisms, formed islands, and altered atmospheric conditions.

  6. Meteor Impacts:

    • Frequent in Earth’s history, affecting life and climate.

 

Archaean Eon (~3.8–2.5 BYA)

  • Primordial oceans supported diverse microbial life.

  • All life was prokaryotic and anaerobic (low oxygen atmosphere).

  • Two domains of life emerged:

    1. Archaea

    2. Bacteria

Heterotrophs vs. Autotrophs

  • Two ways organisms obtain energy:

    1. Heterotrophs:

      • Consume organic molecules for energy.

    2. Autotrophs:

      • Generate energy from inorganic molecules or light.

  • Early life was likely heterotrophic, relying on organic molecules in the prebiotic soup.

  • As organic molecules dwindled, autotrophs evolved.

 

Stromatolites & the Rise of Oxygen

  • Earliest fossils: Cyanobacteria (autotrophs).

    • Cyanobacteria form stromatolites, layered structures of calcium carbonate.

    • Photosynthesis:

      • Produced organic molecules from CO₂.

      • Released oxygen (O₂) as a waste product.

  • Consequences of rising oxygen:

    • Anaerobic species declined or adapted to anoxic environments.

    • Aerobic species evolved.

    • Paved the way for eukaryotes.

 

Proterozoic Eon (~2.5 BYA – 541 MYA)

Endosymbiotic Origin of Eukaryotes

  • Endosymbiosis: One organism lives inside another in a mutualistic relationship.

  • Example:

    • Paramecium and Chlorella (algae):

      • Paramecium protects algae.

      • Algae provide nutrients.

      • Paramecium can digest algae if necessary.

 

Lynn Margulis’ Endosymbiotic Theory (1970)

  • Proposed that eukaryotic organelles (mitochondria & chloroplasts) originated from symbiotic prokaryotes.

  • Process:

    1. Endosymbiont (prokaryote) provided energy or nutrients.

    2. Host cell provided protection.

    1. Over time, the relationship became obligate (essential for survival).

 

Multicellular Eukaryotes

  • Origin: Arose around 1.5 BYA during the Proterozoic eon (oldest fossil is 1.2 billion years old, resembling red algae).

  • Two possible origins:

    1. Cells aggregate to form a colony.

    2. Cells stick together after dividing.

 

Three Major Lineages of Cells

  • All cells descended from a common ancestral cell (LUCA).

  • Three major lineages (domains): Bacteria, Archaea, Eukarya.

  • Archaea are more closely related to Eukarya than to Bacteria.

 

Proterozoic Eon: Multicellular Animals

  • Timeframe: Emerged around 632 mya.

  • First animals: Invertebrates (no backbone).

  • Bilateral symmetry ancestor: Vernanimalcula guizhouena (580-600 mya).

 

Phanerozoic Eon (543 mya to present)

  • Proliferation of multicellular eukaryotic life.

 

Cambrian Period (543-490 mya)

  • Cambrian Explosion: Sudden increase in animal diversity.

  • First vertebrates: Around 520 mya.

Causes of Cambrian Explosion:

  • Evolution of shells.

  • Increased atmospheric oxygen.

  • Evolutionary arms race between predators and prey.

 

Ordovician Period (490-443 mya)

  • First land plants and arthropods.

  • Mass extinction: 60% of marine invertebrates wiped out by climate change.

 

Silurian Period (443-417 mya)

  • Land colonization: Major colonization of land by plants and animals.

  • First vascular plants.

 

Devonian Period (417-353 mya)

  • Terrestrial species increase: Ferns, first trees and forests.

  • Extinctions: Prolonged extinctions near the end.

 

Carboniferous Period (354-290 mya)

  • Coal deposits formed.

  • First flying insects.

  • Emergence of reptiles.

 

Permian Period (290-248 mya)

  • Pangea: Supercontinent formed.

  • Mass extinction: 90-95% of species wiped out.

 

Transition to Mesozoic Era

  • Age of Dinosaurs.

  • Hot, dry climate.

 

Triassic Period (248-206 mya)

  • New reptiles: Crocodiles, turtles.

  • First dinosaurs and mammals.

  • Gymnosperms dominated.

 

Jurassic Period (206-144 mya)

  • Reptile dominance: Enormous dinosaurs.

  • First bird.

 

Cretaceous Period (144-65 mya)

  • Flowering plants: First appearance of angiosperms.

  • Mass extinction: Dinosaurs and many other species died out.

 

Transition to Cenozoic Era (65 mya to present)

  • Age of Mammals: Mammals became the largest terrestrial animals.

 

Tertiary Period (65-1.8 mya)

  • Angiosperms: Became dominant land plants.

  • Mammals and fish: Diversified.

  • Hominoids: Appear around 7 mya (includes humans, chimpanzees, gorillas).

 

Quaternary Period (1.8 mya - present)

  • Ice Ages: Covered Europe and North America.

  • Homo sapiens: Appear around 170,000 years ago.

 

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