The Origin and Diversification of Eukaryotes

Timeline and Defining Features of Eukaryotes\n- Eukaryotes first appear in the fossil record approximately $1.8$ billion years ago, with chemical evidence suggesting evolution as early as $2.7$ billion years ago.\n- Defined by the presence of a nucleus, membrane-bound organelles, and a complex cytoskeleton that permits unique, asymmetrical cell shapes.\n- Multicellularity and sexual life cycles emerged around $1.2$ billion years ago; macroscopic eukaryotes appeared $635$ million years ago during the Ediacaran period.\n- The Cambrian Explosion occurred approximately $535$ million years ago, ending the dominance of Ediacaran lifeforms.\n\n# Endosymbiosis in Eukaryotic Evolution\n- Endosymbiont theory proposes that mitochondria and plastids originated from bacteria living within larger host cells.\n- Serial endosymbiosis hypothesis: Mitochondria evolved first from a single alpha proteobacterium ancestor, followed by plastids in specific lineages.\n- Evidence for bacterial origins includes circular D N A without histones, similar ribosome structures, splitting processes for replication, and homologous transport system enzymes.\n- Plastids originated from a cyanobacterium engulfed by a heterotrophic eukaryote, leading to red and green algae lineages.\n- Secondary endosymbiosis: Heterotrophic eukaryotes ingested algal cells, resulting in plastids with $3$ or $4$ membranes.\n\n# The Origin of Multicellularity\n- Multicellularity evolved independently multiple times, giving rise to algae, plants, fungi, and animals.\n- The first forms were colonies with little differentiation, held together by shared cell walls or connecting proteins.\n- In Volvox, complex multicellularity likely evolved from single-celled ancestors like Chlamydomonas through changes in gene expression and extracellular matrix proteins.\n- Animals' closest relatives, Choanoflagellates, share homologous protein domains like cadherin with animals, essential for cell adhesion and signaling.\n\n# Major Eukaryotic Supergroups\n- Excavata: Known for an "excavated" feeding groove; includes Diplomonads (with reduced mitochondria called mitosomes), Parabasalids (with hydrogenosomes), and Euglenozoans (featuring a crystalline rod in flagella).\n- SAR: A monophyletic supergroup including Stramenopiles (Diatoms with silicon dioxide walls and brown algae), Alveolates (Dinoflagellates and Ciliates with membrane-enclosed sacs), and Rhizarians (Amoebas with threadlike pseudopodia, such as Forams and Cercozoans).\n- Archaeplastida: Includes red algae (containing phycoerythrin), green algae (charophytes and chlorophytes), and land plants.\n- Unikonta: Includes Amoebozoans (with lobe-shaped pseudopodia like slime molds) and Opisthokonts (animals, fungi, and relatives like Nucleariids and Choanoflagellates).\n\n# Ecological Importance and Human Health\n- Photosynthetic protists act as producers, performing approximately $30\%$ of the world’s photosynthesis using light to convert $CO_2$ to organic compounds.\n- Protist blooms, triggered by nutrients like nitrogen or phosphorus, can cause toxic "red tides" or trap carbon as dead diatoms sink.\n- Symbiotic relationships: Dinoflagellates nourish coral polyps, while wood-digesting protists assist termites.\n- Parasitic impacts: Phytophthora infestens caused the Irish Potato Famine ($1845$–$1852$); Trypanosoma species cause sleeping sickness and Chagas disease; Plasmodium causes malaria, a leading cause of infectious death.