Evolution and Diversity of Eukaryotes

Organisms and Eukaryotic Evolution

How to Move an Organism

  • Context of the question emphasizes understanding organismal biology and movement.

  • Relates to the broader theme of Eukaryotes.

Eukaryotes Overview

  • Eukaryote defined: Organisms whose cells contain a nucleus and other organelles enclosed within membranes.

  • Covers their complexity compared to prokaryotes.

Unit 3: Evolution and History of Life

Major Topics Covered

  • Origin and Diversification of Eukaryotes

    • Evolution of Eukaryotes

    • Evolution of Multicellularity

    • Protistan Characteristics

    • Protistan Diversity

    • Protistan Evolutionary Relationships

Early Life

Key Learning Objectives

  • What do I need to know?

    • Evolutionary history of major characteristics defining single-celled and multicellular eukaryotes.

    • Characteristics of protistans, including adaptations and major groups.

    • The factors contributing to protistan diversity.

    • Significance of protistans as ancestral groups contributing to eukaryotic taxa.

Critical Thinking Questions

  • Diversity of Protistans: Why are protistans so diverse?

  • Ancestry: Understand their role in eukaryotic evolution.

Timeline of Major Events in the History of Life

  • Origin of Earth: Approximately 4.6 billion years ago (bya).

  • Earliest Cells (Prokaryote Microfossils): 3.5 bya; indicates the first life forms.

  • Origin of Eukaryotes: Between 2.5 and 4.5 bya.

    • Earliest fossil cells date back to 1.8 bya.

    • Chemical evidence suggests evolution as early as 2.7 bya.

  • Origin of Photosynthesis: Between 3.5 and 2.8 bya.

  • Oldest Eukaryote Fossils: 1.7 bya.

  • First Multicellular Organisms: 1.2 bya.

  • Formation of the Pangaea Supercontinent: 250 million years ago (mya).

  • Extinction of Dinosaurs: 65 mya.

  • Earliest Humans: 6 mya.

Phylogeny

Major Groups

  • Plants: Includes flowering plants, conifers, ginkgos, cycads, horsetails, ferns, lycophytes, bryophytes.

  • Fungi: Various forms including sac fungi, club fungi, zygospore-forming fungi.

  • Animals: Arthropods, chordates, flatworms, annelids, mollusks, echinoderms, and others.

  • Protists: Said to have rapid divergences expressed in choanoflagellates, ciliates, apicomplexans, and protist diversity.

  • Taxonomic Hierarchy: Demonstrates the interconnectedness of eukaryotic life and shows how major groups evolved together.

Evolution of Early Eukaryotes

Characteristics of Early Eukaryotes

  • Defined as unicellular organisms with complex structures:

    • Membrane-bound nucleus

    • Presence of organelles

    • A cytoskeleton that facilitates movements and functions.

  • Fossil records indicate first eukaryotes emerged around 1.8 billion years ago.

Evolution Through Endosymbiosis

  • Key Mechanism: Endosymbiosis is pivotal in the origin of mitochondria and chloroplasts.

    • Process: Engulfing of aerobic bacteria leading to mitochondrion formation.

    • Formation of plastids from photosynthetic bacteria.

Evidence Supporting Endosymbiosis

  • Membrane Structure: Inner membranes of mitochondria and chloroplasts resemble prokaryotic plasma membranes.

  • Division Mechanism: Mitochondria and chloroplasts divide similarly to some prokaryotes.

  • DNA Structure: Their DNA is similar to prokaryotic DNA, supporting a common ancestry.

  • Ribosomal Activity: Mitochondria and chloroplasts have their own ribosomes which resemble those of prokaryotes more than eukaryotes.

Origin of Multicellularity

Framework for Understanding

  • The transition to multicellularity represents a significant evolutionary leap.

    • Definition of Multicellularity: Organisms composed of multiple cells that can specialize into different types.

    • Convergent Evolution: Multicellularity evolved independently in various groups, e.g., red, green, brown algae, plants, fungi, and animals.

Case Studies in Multicellularity

Example: Colonial Algae and Volvox Evolution

  • Multicellular Volvox: Indicates descent from colonial forms; comparison shows homology in proteins.

    • Few novel genes required for this evolution, indicating limited genetic change necessity.

Example: Choanoflagellates to Animals

  • Specific Evolutionary Steps: Direct lineage from choanoflagellates to sponges illustrates key evolutionary processes in multicellularity.

  • Adhesion and Communication: Essential for multicellular evolution; examples include cadherins that facilitate cell adherence.

    • Shared domains through evolution indicate genetic continuity but with novel adaptations in animals.

Supergroups of Eukaryotes

Classification Based on Morphology and Molecular Data

  1. Excavata: Characterized by unique cytoskeleton structures and some having feeding grooves.

  2. “SAR” Clade: Includes diverse groups like stramenopiles, alveolates, rhizarians.

  3. Archaeplastida: Includes red algae, green algae, and land plants.

  4. Unikonta: Comprises animals, fungi, and certain protists; two clades being amoebozoans and opisthokonts.

Deep Dive into Excavates

  • Notable groups include Diplomonads, Parabasalids, Euglenozoans.

    • Examples: Trichomonas vaginalis (parabasalid parasite) and Euglena.

“SAR” Clade Characteristics

  • Stramenopiles: Includes diatoms and brown algae; diatoms noted for glass walls made from silicon dioxide.

  • Alveolates: Diverse group that includes dinoflagellates and Toxoplasma.

  • Rhizarians: Foraminiferans, known for tests (porous shells) and often harboring endosymbiotic algae.

Archaeplastids

  • Group includes red algae, green algae, and land plants.

  • Palsids Origins: Evolved through symbiosis when a protist engulfed a cyanobacterial endosymbiont.

    • This evolutionary step resulted in diverse descendants among red and green algae.

Specifics of Red Algae

  • Example: Bonnemaisonia hamifera known for its red pigmentation and multicellular form. Reproduce sexually.

Specifics of Green Algae

  • Distinguished by the absence of pigments masking chlorophyll.

    • Consist of two main groups: Chlorophytes and Charophytes, the later being the direct ancestors of land plants (e.g., Ulva, Caulerpa).

Unikonts

Distinct Features and Classification

  • Include a variety of organisms such as animals, fungi, and some protists.

  • Subdivided into two clades: Amoebozoans and Opisthokonts, with significant diversity and functional adaptations.

Amoebozoans Overview

  • Known as true amoebas, predominantly free-living or parasitic.

  • Slime Molds: Once classified among fungi; notable for cellular individuality but also for their ability to aggregate into a colony under certain conditions.

Review of Protistan Diversity

Structural and Functional Aspects

  • Protists demonstrate complex biology; primarily unicellular but some multicellular forms exist.

  • Exhibit a variety of nutritional modes: photoautotrophs, heterotrophs, mixotrophs.

  • Capability for both sexual and asexual reproduction.

Ecological Roles of Protists

  • As key producers in ecosystems, accounting for 30% of global photosynthesis.

  • Photosynthetic protists are critical in aquatic environments, with implications for global nutrient cycles.

Impacts of Climate Change on Photosynthetic Protists

  • Rising sea temperatures due to climate change negatively impacts photosynthetic protists, potentially disrupting ecosystems and fisheries.

Effects of Protists on Human Health

Example: Malaria

  • Caused by Plasmodium, a member of SAR - Alveolate,

    • Results in approximately 400,000 deaths annually.

    • Lifecycle includes stages in both mosquitoes and humans, highlighting transmission pathways and reproductive processes.

Visual Representation:

  • Understanding the lifecycle of Plasmodium involves complex interactions between host organisms and varied lifecycle stages (sporozoites, merozoites).

Conclusion: Drawing the Tree of Life

  • Emphasizing the interconnectedness and branching patterns of evolutionary lineages across all classifications of living organisms.