Macroevolution Lecture Flashcards

Conditions on Early Earth and the Origin of Life

• Formation and Environmental Conditions of Early Earth:

  • Earth formed approximately $4.6\text{ billion}$ years ago.

  • Between $4.6\text{ billion}$ and $3.9\text{ billion}$ years ago, the planet underwent severe meteor bombardment. As cooling occurred and bombardment slowed around $3.9\text{ billion}$ years ago, environmental conditions differed radically from present-day Earth.

  • Earth's early atmosphere was thick with water vapor ($H_2O$) and chemical compounds released by intense volcanic eruptions, including:

  • Nitrogen ($N_2$) and nitrogen oxides

  • Carbon dioxide ($CO_2$)

  • Methane ($CH_4$)

  • Ammonia ($NH_3$)

  • Hydrogen ($H_2$)

  • Hydrogen sulfide ($H_2S$)

  • Energy inputs were significantly higher than today, characterized by continuous lightning, extreme volcanic activity, and intense solar ultraviolet (UV) radiation due to the lack of an ozone layer.

• Fossil Evidence of Early Life:

  • The earliest direct fossil evidence for life dates back $3.5\text{ billion}$ years to fossilized stromatolites.

  • Stromatolites are layered rocks formed by the binding of sediment sediments by ancient photosynthetic prokaryotes.

  • Because these $3.5\text{-billion}$ year-old prokaryotes were already complex enough to perform photosynthesis, life must have originated even earlier, potentially as early as $3.9\text{ billion}$ years ago.

• Four-Stage Hypothesis for the Origin of Life:

  1. Stage 1: Abiotic Synthesis of Organic Monomers: Nonliving synthesis of small organic molecules, such as amino acids and nitrogenous bases.

  2. Stage 2: Polymerization: Polymerization of monomers into biological macromolecules, including proteins and nucleic acids.

  3. Stage 3: Formation of Protocells: Packaging of macromolecules into "protocells"—abiotic membrane-enclosed vesicles that maintained an internal chemical environment distinct from their surroundings. basically that oil surrounding protocell

  4. Stage 4: Origin of Self-Replicating Molecules: Development of self-replicating inheritance systems, primarily involving RNA, making biological evolution possible.

Experimental Evidence for Abiotic Synthesis

• The Oparin-Haldane Hypothesis:

  • In the 1920s, Russian scientist A. I. Oparin and British scientist J. B. S. Haldane independently proposed that organic molecules could form spontaneously on early Earth.

  • They noted that modern atmospheres are rich in molecular oxygen ($O_2$), which oxidizes and breaks down complex chemical bonds.

  • Early Earth's atmosphere was a reducing atmosphere (electron-adding), rich in electron donors and devoid of free $O_2$, facilitating the formation of organic compounds given an adequate energy source.

• The Miller-Urey Experiment (1953):

  • Graduate student Stanley Miller, working under advisor Harold Urey, constructed an airtight glass apparatus to empirically test the Oparin-Haldane hypothesis.

• Experimental Setup and Procedure:

  1. The "Sea": Water ($H_2O$) in a lower flask was heated to simulate the early ocean and generate water vapor.

  2. The "Atmosphere": Water vapor moved into an upper reaction chamber containing a gas mixture simulating early atmosphere ($CH_4$, $NH_3$, and $H_2$).

  3. Energy Input: Electrodes discharged electrical sparks into the gas mixture to simulate atmospheric lightning.

  4. Condensation: A water condenser cooled the gas mixture, causing synthesized molecules to rain down into a lower collection trap.

  5. Control Experiment: A parallel setup without electrical discharges was run as a negative control.

  6. Sampling: Chemical samples were drawn from the trap for analysis after continuous operation.

• Experimental Results:

  • Within one week, the Miller-Urey setup synthesized abundant amino acids (the building blocks of proteins) and other complex organic compounds.

  • Subsequent variations of the experiment using alternative gas mixtures and energy sources (e.g., UV radiation, volcanic heat) produced all $20$ standard amino acids, sugars, lipids, and nitrogenous bases present in nucleic acids, confirming that Stage 1 abiotic synthesis was chemically feasible under early Earth conditions.

Stages in the Origin of the First Cells

• Stage 2: Polymerization of Monomers into Polymers:

  • Abiotic polymerization occurs without enzymatic activity.

  • Monomers splashed onto hot sand, clay, or volcanic rock surface can concentrate and spontaneously bond into polymers.

  • Ocean waves washing organic monomers onto fresh lava or hot rocks polymerized them into polypeptides and polynucleotides, which were then washed back into the sea.

• Stage 3: Packaging of Polymers into Protocells:

  • Lipids added to water spontaneously assemble into bilayered membrane sacs or vesicles.

  • These abiotically produced vesicles display cell-like properties:

  • Maintenance of an internal chemical milieu distinct from surrounding fluid.

  • Ability to absorb material, grow in size, and divide (reproduce) when reaching a critical volume threshold.

• Stage 4: Origin of Self-Replicating Molecules and the RNA World:

  • Contemporary organisms transfer genetic information along the pathway: $\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein assembly}$.

  • Primitive inheritance likely operated on an RNA World model:

  • Short RNA monomers (nucleotides) spontaneously align alongside pre-existing RNA strands through complementary base pairing ($A$ with $U$, $G$ with $C$).

  • Ribozymes (catalytic RNA molecules) catalyzed their own replication without requiring protein enzymes.

  • Natural selection acted on protocells containing self-replicating RNA, favoring those with superior metabolic and replication efficiency.

Major Events in the History of Life

• Geologic Eons:

  • Macroevolution represents the broad pattern of evolutionary changes over geological time.

  • Earth's history ($4.6\text{ billion}$ years) is divided into three primary Eons:

  • Archaean Eon: From Earth's origin to $2.5\text{ billion}$ years ago.

  • Proterozoic Eon: From $2.5\text{ billion}$ to $0.542\text{ billion}$ ($542\text{ million}$) years ago.

  • Phanerozoic Eon: Spans the last $542\text{ million}$ years (encompassing the Paleozoic, Mesozoic, and Cenozoic eras).

  • Together, the Archaean and Proterozoic eons spanned approximately $4\text{ billion}$ years.

• Chronological Milestones of Life:

  • Prokaryotic Dominance ($3.5$ to $2.0\text{ billion}$ years ago): Prokaryotes were the sole inhabitants of Earth for $1.5\text{ billion}$ years.

  • Prokaryotic oxygenic photosynthesis (cyanobacteria) produced $O_2$ gas, saturating surrounding oceans and accumulating in the atmosphere starting around $2.7\text{ billion}$ years ago.

  • Oxygen accumulation triggered metabolic evolution, allowing anaerobic and aerobic cellular respiration pathways to flourish.

  • Single-Celled Eukaryotes ($2.1\text{ billion}$ years ago): Oldest eukaryotic cell fossils appear.

  • Multicellular Ancestry ($1.5\text{ billion}$ years ago): Common ancestor of all multicellular eukaryotes evolved.

  • Multicellular Eukaryote Fossils ($1.2\text{ billion}$ years ago): Oldest physical fossils of multicellular organisms.

  • Colonization of Land ($500\text{ million}$ years ago): Multicellular plants, fungi, and animals transitioned to terrestrial environments.

  • Hominid Lineage Divergence ($6$ to $7\text{ million}$ years ago): Human ancestors diverged from other primate lineages.

  • Origin of Homo sapiens ($195,000$ years ago): Emergence of modern human species.

  • Relative Time Scale: If Earth's $4.6\text{-billion}$-year history were condensed into a single hour, modern humans (Homo sapiens) appeared less than $0.2\text{ seconds}$ ago.

Geologic Dating Methods and the Geologic Record

• Radiometric Dating:

  • Absolute ages of rocks and fossils are determined by measuring the decay of radioactive isotopes.

  • Half-life: The fixed duration of time required for $50\%$ of a parent radioactive isotope sample to decay into its daughter product.

• Carbon-14 Decay Metrics ($t_{1/2} = 5,700\text{ years}$ or $5.7\text{ thousand years}$):

  • $0\text{ years} \rightarrow 1\text{ (100\% parent isotope remaining)}$

  • $5.7\text{ thousand years} \rightarrow \frac{1}{2}\text{ (50\% remaining)}$

  • $11.4\text{ thousand years} \rightarrow \frac{1}{4}\text{ (25\% remaining)}$

  • $17.1\text{ thousand years} \rightarrow \frac{1}{8}\text{ (12.5\% remaining)}$

  • $22.8\text{ thousand years} \rightarrow \frac{1}{16}\text{ (6.25\% remaining)}$

  • $28.5\text{ thousand years} \rightarrow \frac{1}{32}\text{ (3.125\% remaining)}$

  • Carbon-14 is useful for dating organic materials up to $\sim 75,000\text{ years}$ old. Isotopes with half-lives in the hundreds of millions of years (e.g., Potassium-40, Uranium-238) are used for older geological strata.

  • Relative Layer Dating: Fossil ages can also be inferred relative to the sedimentary rock layers (strata) positioned directly above and below where the fossil was discovered.

• Structure of the Geologic Record (Phanerozoic Eon):

• Paleozoic Era ($542$ to $251\text{ million}$ years ago):

  • Ediacaran Period ($635$ to $542\text{ million}$ years ago): Diverse algae and soft-bodied invertebrate animals appear.

  • Cambrian Period ($542$ to $488\text{ million}$ years ago): Cambrian Explosion; rapid diversification of major animal phyla.

  • Ordovician Period ($488$ to $444\text{ million}$ years ago): Marine algae abundant; colonization of land by plants, fungi, and animals.

  • Silurian Period ($444$ to $416\text{ million}$ years ago): Diversification of early vascular plants.

  • Devonian Period ($416$ to $359\text{ million}$ years ago): Diversification of bony fishes; first tetrapods and insects appear.

  • Carboniferous Period ($359$ to $251\text{ million}$ years ago): Extensive forests of seedless vascular plants form coal beds; first seed plants appear; origin of reptiles; amphibians dominant.

  • Permian Period ($251\text{ million}$ years ago boundary): Radiation of reptiles; origin of modern insect groups; mass extinction of marine and terrestrial organisms at period end.

• Mesozoic Era ($251$ to $65.5\text{ million}$ years ago):

  • Triassic Period ($251$ to $199.6\text{ million}$ years ago): Gymnosperms dominate land; dinosaurs evolve and radiate; origin of mammals.

  • Jurassic Period ($199.6$ to $145.5\text{ million}$ years ago): Gymnosperms continue as dominant plants; dinosaurs abundant and diverse.

  • Cretaceous Period ($145.5$ to $65.5\text{ million}$ years ago): Flowering plants (angiosperms) appear and diversify; mass extinction at end of period claims most dinosaurs.

• Cenozoic Era ($65.5\text{ million}$ years ago to Present):

  • Paleogene / Neogene (Tertiary) Epochs:

  • Paleocene ($65.5\text{ million}$ years ago): Major radiation of mammals, birds, and pollinating insects.

  • Eocene ($55.8\text{ million}$ years ago): Angiosperm dominance increases; radiation of modern mammalian orders.

  • Oligocene ($33.9\text{ million}$ years ago): Origins of many primate groups.

  • Miocene ($23\text{ million}$ years ago): Continued radiation of mammals and angiosperms; earliest direct human ancestors.

  • Pliocene ($5.3\text{ million}$ years ago): Appearance of bipedal human ancestors.

  • Quaternary Period ($2.6\text{ million}$ years ago to Present):

  • Pleistocene ($2.6\text{ million}$ to $0.01\text{ million}$ years ago): Ice ages; origin of the genus Homo.

  • Holocene ($0.01\text{ million}$ years ago / $10,000\text{ years}$ to Present): Historical human time.

Mechanisms of Macroevolution and Plate Tectonics

• Plate Tectonics and Continental Drift:

  • According to plate tectonics theory, Earth's crust is fragmented into rigid, irregularly shaped plates floating atop the semi-fluid underlying mantle.

  • Continental Drift: Convection currents within the mantle cause tectonic plates to shift positions over time.

  • Since the origin of multicellular life ($1.5\text{ billion}$ years ago), landmasses gathered into a single supercontinent on three separate occasions.

• Pangaea and Supercontinental Dynamics ($250\text{ million}$ years ago):

  • During the late Paleozoic era, landmasses converged to form Pangaea.

  • Ecological Impacts:

  • Marine: Fusing of continents reduced total coastline, drained shallow coastal marine habitats, and deepened ocean basins, causing severe marine die-offs.

  • Terrestrial: Formation of massive continental interiors led to harsh, cold, and arid inland climates.

  • Breakup of Pangaea:

  • Mesozoic Era ($135\text{ million}$ years ago): Split into northern landmass (Laurasia) and southern landmass (Gondwana).

  • Cretaceous ($65.5\text{ million}$ years ago): Modern continents split further; isolated Australian marsupials evolved separately from placental mammals on other continents.

  • Collision of India ($55\text{ million}$ years ago): India collided with the Eurasian plate, giving rise to the Himalayan mountain range.

• Biogeographical Evidence - Lungfish Distribution:

  • Fossilized lungfish are present across every modern continent except Antarctica.

  • Living lungfishes exist exclusively in freshwater habitats of South America, Africa, and Australia.

  • This pattern confirms lungfish evolved prior to the breakup of Pangaea when all continental landmasses were contiguous.

• Plate Boundaries and Geologic Hazards:

  • Plate boundary movements generate volcanic eruptions and earthquakes.

  • Fault lines mark transform boundaries where plates slide past each other, such as the San Andreas Fault in California (Pacific Plate sliding north past the North American Plate).

Mass Extinctions and Adaptive Radiations

• Mass Extinction Events:

  • Over the past $500\text{ million}$ years, five major mass extinctions occurred, each eliminating more than $50\%$ of Earth's species.

  • Permian Mass Extinction ($251\text{ million}$ years ago):

  • Marks the boundary between Paleozoic and Mesozoic eras.

  • Eliminated $96\%$ of marine animal species and devastated terrestrial life.

  • Driven by massive volcanic activity, mantle plumes, and Pangaea formation.

  • Cretaceous Mass Extinction ($65.5\text{ million}$ years ago):

  • Marks the boundary between Mesozoic and Cenozoic eras.

  • Extinguished all non-avian dinosaurs.

  • Caused by a massive asteroid impact at Chicxulub on the Yucatán Peninsula, generating debris clouds that blocked sunlight and severely altered global climate.

• Consequences and Recovery:

  • Marine family diversity required $100\text{ million}$ years to recover back to baseline levels after the Permian extinction.

  • Current anthropogenic extinction rates are $100\text{ to } 1,000\text{ times}$ higher than background extinction rates, signaling a potential sixth mass extinction.

• Adaptive Radiations:

  • Rapid evolutionary bursts where many new species form from a single common ancestor to fill open ecological niches.

  • Typically follow mass extinctions or major evolutionary innovations.

  • Example: Mammalian adaptive radiation expanded exponentially following the extinction of non-avian dinosaurs ($65\text{ million}$ years ago), leading to modern lineages: Monotremes ($5\text{ species}$), Marsupials ($324\text{ species}$), and Eutherians/Placentals ($5,010\text{ species}$).

Evolutionary Novelties and Structural Adaptation

• Gradual Refinement of Complex Structures:

  • Complex biological structures evolve incrementally from simpler functional ancestral precursors.

  • At every intermediate stage, the evolving trait must confer a selective advantage and increase organismal fitness.

• Mollusc Eye Structural Evolution:

  1. Patch of pigmented cells (photoreceptors): Present in simple limpets (Patella); detects presence/absence of light.

  2. Eyecup: Present in abalones; phototactic directionality.

  3. Simple pinhole eye: Present in Nautilus; fluid-filled cavity forms focused images without a lens.

  4. Eye with primitive lens: Present in marine snails; translucent protective layer (cornea) and internal lens concentrate light.

  5. Complex camera lens-type eye: Present in squid and octopuses; flexible lens and retina rivaling vertebrate eyes, evolved independently via convergent evolution.

• Exaptations:

  • Structures that originally evolved in one context but become co-opted for an entirely different function.

  • Examples:

  • Feathers evolved initially in non-avian dinosaurs for thermoregulation/insulation and were later co-opted for flight in birds.

  • Penguin forelimbs evolved originally for aerial flight in bird ancestors and were co-opted into flippers for underwater swimming.

  • Evolution is non-goal-directed; natural selection reflects immediate environmental interaction without long-term foresight.

Systematics and Phylogeny

• Homology vs. Analogy:

  • Phylogeny: The evolutionary history of a species or group of species.

  • Homologies: Similarities resulting from shared ancestry (e.g., forelimb bone structure in mammals).

  • Analogies: Similarities resulting from convergent evolution, where distinct lineages independently adapt to similar environments and selective pressures without sharing a recent common ancestor.

• Taxonomic Hierarchy (Linnaean Classification):

  • Carolus Linnaeus established binomial nomenclature, assigning organisms a two-part Latin name: Genus species (e.g., Felis catus).

• Taxonomic Levels (from broadest to most specific):

  • Domain: Eukarya

  • Kingdom: Animalia

  • Phylum: Chordata

  • Class: Mammalia

  • Order: Carnivora

  • Family: Felidae

  • Genus: Felis

  • Species: Felis catus

• Phylogenetic Trees:

  • Branching diagrams representing testable hypotheses of evolutionary descent.

  • Taxonomic groupings inside Order Carnivora:

  • Felidae: Felis catus (domestic cat)

  • Mustelidae: Mustela frenata (long-tailed weasel), Lutra lutra (European otter)

  • Canidae: Canis latrans (coyote), Canis lupus (wolf)

Molecular Systematics and the Tree of Life

• Molecular Systematics:

  • Uses nucleic acid (DNA/RNA) and amino acid sequence comparisons to determine evolutionary relationships.

  • Over $110\text{ billion}$ DNA base pairs sequenced across thousands of species fuel modern phylogenetic reconstruction.

  • Sequence Divergence Rate: Species with recent common ancestors display high DNA sequence identity, whereas species long separated on independent evolutionary paths display higher sequence divergence.

• Genomic Homologies:

  • Humans and chimpanzees share nearly identical genomes ($\sim 99\%$ sequence homology in shared genes).

  • $99\%$ of human genes have detectable homologs in mice (Mus musculus).

  • $50\%$ of human genes have recognizable homologs in single-celled baker's yeast (Saccharomyces cerevisiae).

• The Three-Domain System:

  • Modern molecular systematics classifies all life into three Domains:

  1. Domain Bacteria (Prokaryotic)

  2. Domain Archaea (Prokaryotic)

  3. Domain Eukarya (Eukaryotic: Protists, Plants, Fungi, Animals)

• Domain Interrelationships and Horizontal Gene Transfer:

  • Biochemical and genetic evidence shows Archaea diverged very early from Bacteria and shares a more recent common ancestor with Eukarya than with Bacteria.

  • Horizontal Gene Transfer (HGT): Gene movement between organisms across domain boundaries through mechanisms such as plasmid exchange, viral infection, and endosymbiosis (e.g., mitochondrial and chloroplast ancestors transferring genes to eukaryotic nuclear genomes).

  • Because early gene transfer was extensive, the base of the Tree of Life is better modeled as a interconnected Ring of Life rather than a single bifurcating trunk.