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:
Reducing Atmosphere Hypothesis
Extraterrestrial Hypothesis
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:
Clay Hypothesis:
Clay surfaces facilitated bond formation between nucleotides.
Experiments show polypeptides and nucleic acid polymers form on clay.
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:
Boundary separating internal & external environments.
Information storage (polymers inside).
Enzymatic function (polymeric reactions).
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:
Stored information.
Self-replicated.
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:
RNA mutates → gains ability to self-replicate.
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):
Hadean
Archaean
Proterozoic
Phanerozoic
The first three eons are collectively called the "Precambrian".
Changes in living organisms result from two key processes:
Genetic Changes:
Alter an organism’s characteristics.
Affect survival and reproduction.
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
Temperature:
2.5 billion years of cooling followed by 2 billion years of fluctuations.
Atmosphere:
Oxygen increase began ~2.4 BYA.
Landmasses:
Continents formed and shifted over time.
Floods & Glaciations:
Catastrophic floods and glaciers periodically reshaped the planet.
Volcanic Eruptions:
Killed organisms, formed islands, and altered atmospheric conditions.
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:
Archaea
Bacteria
Heterotrophs vs. Autotrophs
Two ways organisms obtain energy:
Heterotrophs:
Consume organic molecules for energy.
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:
Endosymbiont (prokaryote) provided energy or nutrients.
Host cell provided protection.
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:
Cells aggregate to form a colony.
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.