The Origin and Diversification of Eukaryotes (Campbell Biology in Focus, Chapter 25)

Chapter 25: The Origin and Diversification of Eukaryotes

Key Concepts Overview

This chapter explores the evolutionary history of eukaryotes, focusing on their origin through endosymbiosis, the emergence of multicellularity, the classification of major eukaryotic groups, and the ecological roles of single-celled eukaryotes.

• Concept 25.1: Eukaryotes arose by endosymbiosis more than 1.8 billion years ago.

• Concept 25.2: Multicellularity has originated several times in eukaryotes.

• Concept 25.3: Four “supergroups” of eukaryotes have been proposed based on morphological and molecular data.

• Concept 25.4: Single-celled eukaryotes play key roles in ecological communities and affect human health.

Origin of Eukaryotes

Endosymbiosis and the Emergence of Eukaryotes

The earliest eukaryotes were unicellular organisms with cells that were structurally more complex than those of prokaryotes. The origin of eukaryotes is closely linked to the process of endosymbiosis, where one cell lives inside another, leading to the development of key organelles.

• Membrane-bound organelles: Eukaryotic cells isolate cellular functions in organelles such as the nucleus and mitochondria.

• Cytoskeleton: Provides structural support and enables cells to change shape and move.

• Endosymbiosis: An ancestral cell engulfed an aerobic bacterium (related to proteobacteria), forming an early eukaryote (a heterotroph).

• Plastids: The first plastids (e.g., chloroplasts) arose when a eukaryotic cell engulfed a photosynthetic bacterium, resulting in an early photosynthetic eukaryote.

Example: The mitochondria in animal cells and the chloroplasts in plant cells are believed to have originated from free-living bacteria that were engulfed by ancestral eukaryotic cells.

The Fossil Record of Early Eukaryotes

Evidence and Diversification

Chemical and fossil evidence indicates that eukaryotes have existed for at least 1.8 billion years, with chemical markers dating back to 2.7 billion years ago. Early eukaryotes were mostly unicellular protists, but over time, complex multicellularity and sexual life cycles evolved.

• Oldest fossils: Widely accepted eukaryotic fossils are about 1.8 billion years old.

• Protists: Informal term for diverse, mostly unicellular eukaryotes.

• Novel features: Complex multicellularity, sexual reproduction, and photosynthesis appeared around 1.2 billion years ago.

• Ediacaran period: Large multicellular eukaryotes appear in the fossil record (635–541 million years ago), with a rapid increase in size and diversity.

Example: Fossils of red algae from 1.2 billion years ago are among the oldest eukaryotes that can be taxonomically resolved.

Endosymbiosis in Eukaryotic Evolution

Genetic and Cellular Evidence

Genomic data suggest that eukaryotes are "combination" organisms, with genes and cellular features derived from both archaea and bacteria. Endosymbiosis explains the mixed origins of eukaryotic cell components.

• Replication, transcription, and translation enzymes: Mostly archaeal in origin.

• Mitochondria and plastids: Bacterial in origin.

• Endoplasmic reticulum: Features from both archaeal and bacterial sources.

Feature	Inferred Origin
Replication enzymes	Archaeal
Transcription enzymes	Archaeal
Translation enzymes	Mostly archaeal
Cell division apparatus	Mostly archaeal
Endoplasmic reticulum	Archaeal and bacterial
Mitochondrion	Bacterial
Metabolic genes	Mostly bacterial

Origin of Mitochondria and Plastids

Endosymbiont Theory and Serial Endosymbiosis

The endosymbiont theory proposes that mitochondria and plastids (such as chloroplasts) originated from bacteria that were engulfed by larger host cells. This symbiotic relationship became permanent, resulting in the organelles found in modern eukaryotes.

• Mitochondria: Evolved from an aerobic alpha-proteobacterium engulfed by an ancestral archaeal host cell.

• Plastids: Evolved later from a cyanobacterium engulfed by a heterotrophic eukaryote.

• Serial endosymbiosis hypothesis: Mitochondria arose first, followed by plastids in certain lineages.

Example: All eukaryotes have mitochondria, but only some (e.g., plants and algae) have plastids.

Supporting Evidence for Endosymbiotic Origins

• Mitochondria and plastids replicate by splitting, similar to bacteria.

• They have circular DNA lacking histones, like bacterial chromosomes.

• Ribosomes in these organelles resemble bacterial ribosomes and are sensitive to antibiotics that affect bacteria.

• Inner membrane enzymes and transport systems are homologous to those in bacterial plasma membranes.

• Both organelles can transcribe and translate their own DNA.

Plastid Evolution: Primary and Secondary Endosymbiosis

Primary Endosymbiosis

Plastids in red and green algae originated from a cyanobacterium engulfed by a eukaryotic cell. These plastids have two membranes, and their transport proteins are homologous to those in cyanobacteria.

• Red algae and green algae: Both descended from a common ancestor with a cyanobacterial plastid.

Secondary Endosymbiosis

Secondary endosymbiosis occurred when eukaryotic algal cells (red or green algae) were themselves engulfed by other heterotrophic eukaryotes, leading to further diversification of plastid-bearing lineages.

• Chlorarachniophytes: Green algae that evolved through secondary endosymbiosis; their plastids contain a vestigial nucleus called a nucleomorph.

Type of Endosymbiosis	Example
Primary	Cyanobacterium engulfed by eukaryote (origin of red/green algae)
Secondary	Red or green algal cell engulfed by another eukaryote (e.g., stramenopiles, euglenids)

Classification: Eukaryotic Supergroups

Major Lineages

Based on morphological and molecular data, eukaryotes are classified into four major "supergroups":

• Excavata

• SAR (Stramenopiles, Alveolates, Rhizarians)

• Archaeplastida

• Unikonta

Additional info: The relationships among these supergroups are still being resolved, and the base of the eukaryotic phylogeny is a polytomy (an unresolved branching).

Summary of Key Features of Early Eukaryotes

• Emergence in the fossil record ~1.8 billion years ago, with chemical evidence from 2.7 billion years ago.

• Early eukaryotes had a nucleus, infolded membranes, and a cytoskeleton supporting variable size and shape.

• Complex multicellularity, sexual life cycles, and photosynthesis appeared by 1.2 billion years ago.

• Large multicellular eukaryotes are found in the Ediacaran period (635–541 million years ago).

• Eukaryotes arose through serial endosymbiosis: mitochondria first, plastids later.

Review Questions (for Self-Assessment)

• Which features might mitochondria and chloroplasts retain from their endosymbiotic origins? Plasma membrane, DNA, and ribosomes.

• What is the correct sequence for the evolution of photosynthetic eukaryotes? Ancestral anaerobic prokaryote engulfs heterotrophic aerobic prokaryote; descendant aerobic eukaryote then engulfs a photosynthetic prokaryote.

• Which organelles most likely arose from endosymbiosis? Chloroplasts and mitochondria.

• What evidence supports the endosymbiotic origin of mitochondria and plastids? Chloroplasts and mitochondria have their own DNA similar to bacterial chromosomes.

• What is the likely identity of the host cell that led to the origin of eukaryotes? Heterotrophic archaean.

• Which describes an example of secondary endosymbiosis? A red alga is ingested by a heterotrophic eukaryote.