The Origin and Diversification of Eukaryotes

Timeline and Defining Characteristics of Eukaryotic Evolution

Origins of Eukaryotes: * Fossil Record: The first eukaryotic organisms appear in the fossil record approximately $1.8$ billion years ago. * Chemical Evidence: Chemical markers suggest eukaryotes may have evolved as early as $2.7$ billion years ago. * Structural Features: Eukaryotes are characterized by having a nucleus, membrane-bound organelles, and a complex cytoskeleton. * Cytoskeletal Function: The complex cytoskeleton enables eukaryotes to maintain unique and asymmetrical cell shapes.
Major Evolutionary Milestones: * Novel Features ( $1.2$ Billion Years Ago): Significant developments during this period included multicellularity, sexual life cycles, and photosynthesis. * Ediacaran Period ( $635$ Million Years Ago): Large, macroscopic eukaryotes developed and became the dominant life forms. * Cambrian Explosion ( $535$ Million Years Ago): This event marked the end of Ediacaran dominance and a massive diversification of life forms.

Endosymbiosis and the Hybrid Nature of Eukaryotes

Combination Organisms: * DNA sequence data indicate that eukaryotes are "combination" organisms. * Genetic traits and cell characteristics are derived from both Archaea and Bacteria.
Endosymbiont Theory: * Definition: Endosymbiosis is a symbiotic relationship where one organism (the endosymbiont) lives inside the body or cell of another organism (the host). * Mechanism: Bacterial ancestors of mitochondria and plastids likely entered host cells as parasites or undigested prey. * Environmental Utility: The relationship became beneficial as the atmosphere changed; for example, anaerobic host cells benefited from the presence of aerobic endosymbionts as oxygen levels increased. * Integration: Over time, the host and endosymbiont became a single, inseparable organism.
Evolution of Mitochondria and Plastids: * Serial Endosymbiosis Hypothesis: Suggests mitochondria and plastids evolved sequentially rather than simultaneously. * Mitochondria: Arose during the first endosymbiotic event in all eukaryotes from a single alpha proteobacterium ancestor. * Plastids: Evolved later, occurring only in specific lineages from a cyanobacterium ancestor engulfed by a eukaryote. * Host Cell Origin: Phylogenomic analysis suggests the ancestral host cell belonged to an Archaea lineage or a closely related group.
Evidence for Endosymbiotic Origin: * Replication: Mitochondria and plastids utilize a splitting process for replication similar to bacteria. * DNA Structure: DNA is organized into a circular chromosome and lacks histones. * Biochemistry: Inner membrane enzymes and transport systems are homologous to those found in bacterial plasma membranes. * Transcription/Translation: They possess the ability to transcribe and translate their own DNA into proteins. * Ribosomes: They have ribosome structures and antibiotic sensitivities similar to bacteria.

Evolutionary Path of Plastids and Secondary Endosymbiosis

Primary Endosymbiosis: * A heterotrophic eukaryote engulfed a photosynthetic cyanobacterium. * This led to two photosynthetic protist lineages: red algae and green algae. * Membranes and transport proteins in red and green algae are homologous to those of cyanobacteria.
Secondary Endosymbiosis: * Occurs when a heterotrophic eukaryote ingests eukaryotic algal cells (red or green algae). * Evidence of secondary endosymbiosis includes plastids that possess $3$ or $4$ membranes.

The Evolution of Multicellularity

Independent Origins: * Multicellularity evolved several times independently across different lineages, including red, green, and brown algae, plants, fungi, and animals.
Colonial Beginnings: * The first multicellular forms were colonies: collections of connected cells with minimal or no differentiation. * Forms included simple filaments, balls, or cell sheets. * Cells attached via shared cell walls or connecting proteins.
Complex Multicellularity Case Study: Volvox: * Volvox (multicellular green alga) forms a monophyletic group with Chlamydomonas (single-celled alga). * Homologous proteins found in the cell wall of Chlamydomonas constitute the extracellular matrix (ECM) of Volvox. * Transition involved changes in the expression of existing cellular genes.
Origin of Multicellular Animals: * Choanoflagellates: These are the closest living relatives to animals. The common ancestor was likely a unicellular suspension feeder. * Molecular Requirements: Multicellularity required mechanisms for cell adhesion and signaling. * Cadherins: Animal cadherin attachment proteins share many functional domains with choanoflagellates; only one domain is unique to animals.

Eukaryotic Classification: The Four Supergroups

General Classification Trends: * Protists are no longer classified as a single kingdom because the group is polyphyletic (some protists are more closely related to plants, animals, or fungi than to other protists). * Current models divide eukaryotes into four "supergroups," though the root of the tree and relationships among taxa remain unresolved.
Supergroup 1: Excavata: * Named for the "excavated" feeding groove on one side of the cell body. * Diplomonads: * Reduced mitochondria called mitosomes. * Two haploid nuclei. * Anaerobic biochemical pathways. * Move using multiple flagella. * Example: Giardia intestinalis (parasite). * Parabasalids: * Reduced mitochondria called hydrogenosomes. * Generate energy anaerobically, releasing $H_2$ gas as a by-product. * Examples: Trichomonas vaginalis (sexually transmitted parasite); Hypermastigids (live in termite/cockroach guts to break down cellulose via bacterial symbionts). * Euglenozoans: * Distinguishing feature: A rod with a spiral or crystalline structure inside each flagellum. * Includes predatory heterotrophs, photosynthetic autotrophs, and parasites. * Examples: Trypanosoma cruzi (Chagas disease) and T. brucei (sleeping sickness).
Supergroup 2: SAR (Stramenopiles, Alveolates, and Rhizarians): * Stramenopiles: Includes essential photosynthetic organisms. * Diatoms: Unicellular algae with unique two-part, glass-like walls made of silicon dioxide ( $SiO_2$ ). Highly abundant; influence global $CO_2$ levels. * Brown Algae: Largest and most complex algae; all are multicellular and mostly marine (seaweeds). Features include the holdfast (rootlike), stipe (stemlike), and blades (leaflike). Gas-filled floats provide buoyancy. Structure is analogous to plants. * Alveolates: Possess membrane-enclosed sacs (alveoli) under the plasma membrane. * Dinoflagellates: Cells reinforced by cellulose plates; two flagella in grooves. Can be phototrophs, heterotrophs, or mixotrophs (both). Cause "red tides" (explosive growth) that produce toxins and kill marine life. * Ciliates: Use cilia for movement and feeding. Predators of bacteria/protists. Examples: Paramecium, Vorticella, Stentor, and Colpodea. * Rhizarians: Many are amoebas with threadlike pseudopodia. * Forams (Foraminiferans): Named for porous shells called tests. Receive nourishment from symbiotic algae. Found in marine and freshwater. * Cercozoans: Amoeboid and flagellated protists. Most are heterotrophs (parasites/predators). Paulinella chromatophora is an autotroph with a unique chromatophore derived from a different cyanobacterium than other plastids.
Supergroup 3: Archaeplastida: * Includes red algae, green algae, and land plants. * Red Algae: Reddish due to the accessory pigment phycoerythrin. Color darkens with depth (greenish-red to black). Reproduce sexually; unflagellated gametes depend on water currents. * Green Algae: Most closely related to land plants (similar chloroplast structure and pigments). * Charophytes: The lineage most closely related to land plants. * Chlorophytes: Mostly freshwater; reproduce via biflagellated gametes. Includes unicellular (free-living or symbiotic) and multicellular (Volvox, Ulva) forms.
Supergroup 4: Unikonta: * Amoebozoans: Amoebas with lobe- or tube-shaped pseudopodia. * Tubulinids: Common in soil and aquatic environments; feed on bacteria/detritus. * Slime Molds: Produce fruiting bodies for spore dispersal (evolutionary convergence with fungi). Dictyostelium life cycle involves independent cells aggregating into a slug-like formation when food is scarce. * Opisthokonts: Diverse group including animals, fungi, and protist relatives. * Nucleariids: Close relatives of fungi. * Choanoflagellates: Close relatives of animals.

Ecological Importance and Impact on Human Health

Protists as Producers: * Photosynthetic protists (diatoms, dinoflagellates, multicellular algae) form the base of aquatic food webs. * They perform approximately $30\%$ of the world’s photosynthesis. * Production is limited by availability of Nitrogen ( $N$ ), Phosphorus ( $P$ ), or Iron ( $Fe$ ).
Ecological Consequences of Blooms: * Human-induced agricultural runoff increases nutrient availability, causing population explosions (blooms). * Carbon Sequestration: Dead diatoms sink to the ocean floor, trapping carbon for centuries. * Negative Impact: Blooms can deplete oxygen and release toxins, causing mass mortality of aquatic organisms.
Symbiotic and Parasitic Roles: * Mutualism: Dinoflagellates provide food for coral polyps; wood-digesting protists (and their bacterial symbionts) break down cellulose for termites. * Plant Pathology: Phytophthora infestans causes late blight in potatoes/tomatoes; caused the Irish Potato Famine ( $1845$ - $1852$ ), resulting in over $1$ million deaths. * Human Health: * Trypanosoma brucei (sleeping sickness) and T. cruzi (Chagas disease) use insect vectors and evade immunity by switching surface proteins. * Apicomplexans: Specialized parasites of animals. Plasmodium causes malaria and requires both mosquitoes and humans to complete its life cycle. Malaria is a leading cause of death by infectious disease, rivaling tuberculosis.