Chapter 25: The Origin and Diversification of Eukaryotes

Introduction

This chapter explores the evolutionary history and diversity of eukaryotic cells, focusing on their origins, the process of endosymbiosis, and the rise of multicellularity. Understanding these concepts is fundamental to grasping the complexity and diversity of life on Earth.

Prokaryotic and Eukaryotic Cells

Comparing Cell Types

Cells are classified as either prokaryotic or eukaryotic based on their structural and functional characteristics.

• Prokaryotic Cells: Small, simple cells lacking a nucleus and membrane-bound organelles. Examples include bacteria and archaea.

• Eukaryotic Cells: Larger, more complex cells with a nucleus and various membrane-bound organelles. Found in animals, plants, fungi, and protists.

• Similarities: Both types have a plasma membrane, cytoplasm, ribosomes, and DNA.

Protists: The Diverse Eukaryotes

Definition and Examples

Protists are a diverse group of mostly unicellular eukaryotes. They are not classified as plants, animals, or fungi, but share characteristics with each group. Examples include Paramecium, Euplotes, Didinium, Stentor, and Vorticella.

• Protists exhibit a wide range of forms and functions.

• Many are aquatic, but some inhabit moist terrestrial environments.

• Some protists, like ciliates, can dramatically change shape due to their complex cytoskeleton.

Evolutionary Origins of Eukaryotes

Key Features and Their Origins

The eukaryotic cell is a mosaic of features derived from both archaeal and bacterial ancestors.

Endosymbiosis and Organelle Evolution

The endosymbiotic theory proposes that mitochondria and plastids (such as chloroplasts) originated as free-living bacteria that were engulfed by ancestral eukaryotic cells.

• Serial endosymbiosis hypothesis: Mitochondria evolved before plastids through a sequence of endosymbiotic events.

• Evidence includes similarities in DNA, double membranes, and reproduction by binary fission.

Diversity of Plastids

Plastids, such as chloroplasts, show diversity due to primary and secondary endosymbiosis events.

• Primary endosymbiosis: A eukaryote engulfs a cyanobacterium, leading to the first plastids.

• Secondary endosymbiosis: A eukaryote engulfs another eukaryotic alga, resulting in more complex plastids.

Evolution of Multicellularity

From Colonies to Complex Organisms

The first multicellular forms were colonies—groups of connected cells with little or no differentiation. Over time, true multicellularity evolved independently in several eukaryotic lineages.

• Colonies can be simple filaments, balls, or sheets of cells.

• Cells in colonies are attached by shared cell walls or connecting proteins.

• Example: Volvox (a green alga) forms colonies and shares proteins with its unicellular relatives like Chlamydomonas.

Supergroups of Eukaryotes

Major Clades and Examples

Eukaryotes are classified into several supergroups based on genetic and morphological evidence.

Ecological Roles and Symbiosis

Protists in Ecosystems

Protists play vital roles in ecological systems, acting as producers, consumers, and symbionts.

• Photosynthetic protists (e.g., dinoflagellates) are major producers in aquatic environments.

• Some form mutualistic relationships, such as dinoflagellates providing food for coral polyps.

• Others are symbionts in animal guts, aiding in digestion (e.g., wood-digesting protists in termites).

Parasitic Protists and Human Health

Some protists are parasites that cause diseases in plants, animals, and humans.

• Phytophthora ramorum: Causes sudden oak death in trees.

• Phytophthora infestans: Causes potato late blight.

• Plasmodium: The apicomplexan parasite responsible for malaria, requiring both mosquitoes and humans to complete its life cycle.

Malaria remains a major global health issue, infecting about 200 million people and causing approximately 600,000 deaths annually.

Summary

• Eukaryotes originated through a combination of archaeal and bacterial features, with key innovations such as the nucleus and organelles arising via endosymbiosis.

• Protists are a diverse group of eukaryotes with significant ecological and medical importance.

• Multicellularity evolved multiple times, leading to the complex life forms seen today.