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.