biol 1p92 (lec three)

History of Life on Earth

• Earth formed from planetesimals approximately 4.55 billion years ago (BYA).
• By 4 billion years ago, the outer layers cooled enough for liquid water to accumulate.
• Fossil prokaryotes and modern cyanobacteria are shown, with fossil prokaryote image labeled (a) & modern cyanobacteria labeled (b)
• The central dogma of molecular biology is shown as a diagram with arrows pointing from DNA to RNA to protein, with labels for replication, transcription, and translation.
• Four stage process of life's development on Earth:
  • Nucleotides and amino acids were produced before cells.
  • Nucleotides polymerized into RNA and/or DNA, and amino acids into proteins.
  • Polymers became enclosed in membranes.
  • Polymers enclosed in membranes acquired cellular properties.
• Prebiotic soup formed from organic molecules accumulating in early oceans.
• Possible origins of the prebiotic soup:
  • Reducing atmosphere hypothesis (tested by Miller-Urey experiment 1953).
    • Early Earth's atmosphere contained water vapor (H₂O), H₂, CH₄, NH₃, with little O₂.
    • Redox reactions led to complex organic molecules.
  • Extraterrestrial hypothesis: Organic carbon brought to Earth by meteorites.
    • Carbonaceous chondrites contain significant organic carbon, including amino acids and nucleic acid bases.
    • Opponents argue much would be destroyed in intense heat and collision.
  • Deep-sea vent hypothesis: Biologically important molecules formed in the temperature gradient of hot vent water and cold ocean water.
    • Experiments supported this, showing NH₃ formation in these conditions.

Formation of Organic Polymers

• Prebiotic synthesis of polymers is unlikely in aqueous solutions.
• Hydrolysis (breakdown by water) competes with polymerization.
• Likely occurred on clay.
• Interactions between cations (e.g., Mg²⁺) on clay surfaces and nucleotides promote bond formation.
• Experiments show polypeptide and nucleic acid polymer formation on clay surfaces.
• Recent work suggests carbonyl sulfide can produce aqueous conditions for polymer formation in water.

Formation of Cell-like Structures

• Protobiont:

  • Aggregate of prebiotically produced molecules and macromolecules.
  • Acquired a boundary (like a lipid bilayer).
  • Mantained an internal chemical environment distinct from surroundings.
  • Four characteristics:
    • Boundary separating internal from external environment.
    • Polymers inside with information.
    • Polymers with enzymatic function.
    • Capable of self-replication.

• Coacervates:

  • Formed spontaneously from association of charged polymers.
  • Enzymes trapped inside can perform primitive metabolic functions.

• Liposomes:

  • Vesicles surrounded by a phospholipid bilayer.
  • Clay catalyzes liposome formation, growth, and division.

Acquisition of Cellular Characteristics

• Majority of scientists favor RNA as the first macromolecule in protobionts.
• Three key RNA functions:
  • Ability to store information.
  • Capacity for self-replication.
  • Enzymatic function (ribozymes).
• DNA and proteins likely evolved later.

How did First RNA Molecules Produce Cell-like Characteristics?

• Chemical selection: Chemical within a mixture with special properties increasing in number over others.
• Hypothetical two-step scenario:
  • RNA molecule mutation with self-replication ability using pre-existing RNA as templates.
  • Second mutation providing ability to catalyze a step in ribonucleotide synthesis.

RNA World

• Hypothetical period on early Earth when information necessary for life was entirely RNA.
• Over time, mutations and chemical selection led to increased complexity.
• Replaced by the modern DNA/RNA/protein world.

Advantages of DNA/RNA/Protein World

• Information storage: DNA takes over information storage from RNA, making it more stable.
• RNA still capable of complex catalytic functions.
• Metabolism and cellular functions: Proteins perform tasks like structural, transport, and catalytic functions; ancestral RNA still plays a crucial role in protein synthesis.

Lines of Evidence: Fossils

• Preserved remains of past life on Earth
• Studied by paleontologists
• Usually formed within sedimentary rock
• Organisms are buried quickly in gravel, sand, or mud.
• Over time, more layers pile up and compact to form rock.
• Hard parts are replaced by minerals forming a representation of the original organism.

Layers of Sedimentary Rock

• Eroded sediments settle in water, forming layers (sedimentation).
• Compaction of layers, or presses, the lower layers.
• Salt crystals glue the layers and form mass to become sedimentary rock.
• Lower layers are older than upper layers.

Radioisotope Dating

• Fossil age is estimated by radiometric dating.
• Measuring the amount of a radioisotope and its decay product in the rock surrounding it will help date the fossil.
• Each radioisotope has a unique half-life.
• The half-life = time required for exactly one-half of the original isotope to decay.
• Usually igneous rock is dated in sedimentary rock vicinity.

Factors Affecting the Fossil Record

• Anatomy: organisms with hard parts are more likely to be preserved.
• Size: large organisms are more likely to fossilize than small ones.
• Number: species that existed in larger numbers or larger areas are more likely to fossilize.
• Environment: species in marine environments are more likely to fossilize.
• Time: species that lived longer or more recently are more likely to be found.
• Geological processes and Paleontology.

History of Life on Earth: Geological Timescale

• Includes a timeline of Earth's history, categorized into eons.
• Eons include Hadean, Archaean, Proterozoic, and Phanerozoic (and their periods).

History of Life on Earth: Changes in Living Organisms

• Changes in living organisms result from the interaction between genetic and environmental changes.
• Genetic changes affect characteristics influencing survival and reproduction.
• Environmental changes over billions of years can allow new organisms to flourish and lead to extinctions.

Archaean Eon

• Period from 3.8 to 2.5 BYA
• Diverse microbial life flourished in primordial oceans.
• All life forms were prokaryotic and anaerobic
• Atmosphere contained little free oxygen.
• Two domains of prokaryotic life diverged.

Heterotrophs vs Autotrophs

• Heterotrophs derive energy from chemical bonds in consumed organic molecules.
• Autotrophs derive energy from inorganic molecules or light.
• Likely heterotrophs first, followed by autotrophs as prebiotic soup dwindled.
• Earliest fossils are cyanobacteria.

Stromatolites

• Certain autotrophic cyanobacteria form stromatolites (layered structure of calcium carbonate).
• Cyanobacteria produce organic molecules from CO₂.
• Produce oxygen as a waste product.
• Spelled doom for anaerobic prokaryotic groups.
• Allowed the evolution of aerobic species.

Rising Oxygen

• Proliferation of ancient cyanobacteria produced oxygen as a byproduct of photosynthesis.
• Anaerobic species were killed off or became restricted to anoxic environments.
• Paved way for aerobic respiration and the emergence of eukaryotes.

Proterozoic Eon: Endosymbiotic Origin of Eukaryotes

• Endosymbiosis: one organism living within another.
• Evidence suggests that some organelles (e.g., mitochondria, chloroplasts) evolved from symbiotic prokaryotic cells.
• Endosymbiont provided resources, and the host provided protection.
• Developed into a mutually beneficial relationship over time.

Proterozoic Eon: Multicellular Animals

• Emerged toward the end of the Proterozoic Eon (632 million years ago).
• First animals were invertebrates.
• Earliest known ancestor of animals with bilateral symmetry (Vernanimalcula guizhouena).

Phanerozoic Eon

• 543 million years ago to the present.
• Proliferation of multicellular eukaryotic life extensive.
• Three eras: Paleozoic, Mesozoic, and Cenozoic (and their periods)

Cambrian Period

• 543-490 million years ago
• Warm & wet climate, no ice caps.
• Cambrian explosion: abrupt increase in animal species diversity (echinoderms, arthropods, mollusks, & chordates).
• First vertebrates (~520 million years ago).
• Burgess Shale: rock bed in Canadian Rockies with an abundance of fossils, including soft tissues.

Ordovician Period

• 490-443 million years ago.
• Trilobites and brachiopods were abundant.
• Hard-shelled marine invertebrates.
• First invasion of early land plants and arthropods.
• Dramatically changing climate led to a mass extinction.

Silurian Period

• 443-417 million years ago.
• Relatively stable climate.
• Melting glaciers increased ocean levels.
• New fish, appearance of coral reefs.
• Significant development of vertebrates and plants.
• Major colonization of land by terrestrial plants and animals.
• Adaptations preventing drying out.

Devonian Period

• 417-353 million years ago.
• Increased terrestrial species (ferns, horsetails, gymnosperms).
• Trees and forests emerged.
• Insects and tetrapods appeared.
• Marine invertebrates and fish flourished.
• Series of extinctions eliminated many marine species during later part of the period.

Tetrapod Evolution

• Tiktaalik: "fishapod," shared characteristics with primitive fish and tetrapods.
• Ichthyostega: four limbs, adapted for survival on land; dragged itself forward using strong front limbs.

Carboniferous Period

• 354-290 million years ago.
• Cooler land, covered by forested swamps.
• Rich coal deposits formed.
• Plants and animals diversified, including very large plants and trees.
• First flying insects.
• Amphibians were prevalent.
• Reptiles emerged.

Permian Period

• 290-248 million years ago.
• Continental drift formed Pangea.
• Interior regions became dry with seasonal fluctuations.
• Forest shift to gymnosperms, emergence of conifers and reptile dominance.
• First mammals appeared.
• Largest known mass extinction event (90-95% of marine, large proportion of terrestrial species).

Transition to Mesozoic Era

• Permian extinction marked boundary between Paleozoic & Mesozoic Eras.
• Mesozoic Era ("middle animals") - the age of dinosaurs.

Triassic Period

• 248-206 million years ago.
• Reptiles plentiful, including crocodiles and turtles.
• Emergence of first dinosaurs and mammals.
• Gymnosperms dominated land plants.
• Volcanic eruptions led to global warming and mass extinctions.

Jurassic Period

• 206-144 million years ago.
• Gymnosperms remained dominant vegetation.
• Reptiles dominated land animals, including dinosaurs of enormous size.
• First known bird appeared.
• Mammals present, but not prevalent.

Cretaceous Period

• 144-65 million years ago.
• Earliest flowering plants (angiosperms) emerged.
• Diversification of angiosperms.
• Dinosaurs still dominant on land.
• Mass extinction at the end of the period (dinosaurs and many other species died).

Transition to Cenozoic Era

• Spans the most recent 65 million years of Earth's history.
• Tertiary and Quaternary periods.
• Tropical conditions replaced by a colder, drier climate.
• Mammals became largest terrestrial animals – the "Age of Mammals."
• Diversification of birds, fish, insects, and flowering plants.

Tertiary Period

• 65-1.8 million years ago.
• Angiosperms became dominant land plants.
• Insects became important pollinators.
• Surviving mammals expanded rapidly.
• Fish and sharks diversified.
• Hominoids appeared (~7 million years ago).
• Included humans, chimpanzees, gorillas, orangutans, and gibbons, as well as their ancestors.

Quaternary Period

• 1.8 million years ago to the present.
• Periodic ice ages covered much of Europe and North America.
• Widespread extinction of many mammal species.
• Certain hominins evolved to resemble living humans.
• Homo sapiens appeared (~170,000 years ago).

Test your understanding of chemical selection and the evolution of life on Earth with this quiz. Explore concepts regarding the characteristics of mutant RNA, atmospheric changes, and early prokaryotic life forms. Challenge yourself with matching terms and hypotheses related to early evolutionary processes.