Animal Evolution: Phylogeny, Diversification, and Major Events

Animal Evolution

Overview and Learning Objectives

This section introduces the study of animal evolution, focusing on phylogenetic relationships, the timeline of diversification, factors influencing evolutionary change, and major extinction events.

• Understand general phylogenetic relationships among major animal groups.

• Describe the timeline of animal evolution and diversification.

• Explain factors that promoted diversification in different lineages.

• Identify major extinction events in animal history.

Basic Animal Needs

Essential Functions of Animals

All animals must perform certain basic functions to survive, grow, and reproduce. These functions are fundamental to understanding animal physiology and evolution.

• Obtain, extract, and absorb nutrients: Animals must acquire food, break it down, and absorb nutrients for energy and growth.

• Obtain oxygen and deliver to tissues: Oxygen is required for cellular respiration, which produces energy.

• Remove waste generated by metabolism: Metabolic processes produce waste products (e.g., carbon dioxide, ammonia) that must be excreted.

• Move themselves: Most animals have some form of locomotion to find food, escape predators, or reproduce.

What is an Animal?

Defining Characteristics

Animals are a diverse group of multicellular, eukaryotic organisms with several defining features.

• Multicellular: Composed of multiple cells with specialized functions.

• Heterotrophic: Obtain energy by consuming other organisms (unlike plants, which are autotrophic). Additional info: Fungi are also heterotrophic, but their digestion is external, while animals digest food internally.

• Internal digestion: Food is ingested and broken down inside the body.

• Muscle and nervous tissue: Most animals have specialized tissues for movement and response to stimuli.

• Phylogenetic relationships: Animals are classified based on evolutionary relationships and shared ancestry.

Animal Phylogeny

Major Animal Groups and Relationships

Phylogenetic trees illustrate the evolutionary relationships among animal groups, tracing back to a common ancestor.

• Common ancestor: All animals share a common ancestor, likely a colonial flagellate protist.

• Major branches: Key splits in the animal tree include the development of tissues, symmetry, and body cavities.

• Protostomes vs. Deuterostomes: Two major clades distinguished by embryonic development patterns.

Example: The phylogenetic tree shows relationships among sponges, cnidarians, arthropods, mollusks, echinoderms, and chordates.

Origins of Animals

Early Evolution and the Cambrian Explosion

The origin of animals is traced to single-celled ancestors, with a major diversification event known as the Cambrian Explosion.

• Closest living relatives: Choanoflagellates are protists most closely related to animals.

• Animal ancestor: Likely arose ~900 million years ago (mya).

• Early diversification: Initially slow, with few major groups.

• Cambrian Explosion (~540 mya): Rapid increase in animal diversity, appearance of most major animal phyla, and development of complex body forms.

Example: Fossil records from the Cambrian period show a dramatic increase in animal forms, including early arthropods, mollusks, and chordates.

Factors Promoting Diversification

Environmental and Genetic Drivers

Several factors contributed to the rapid diversification of animals during evolutionary history.

• Increase in atmospheric oxygen: Supported higher metabolic rates and larger body sizes.

• Development of new habitats: Expansion of shallow seas and continental shelves provided new ecological niches.

• Evolution of Hox genes: Regulatory genes that control body plan development, allowing for greater morphological diversity.

• Predator-prey interactions: Evolutionary arms races drove the development of new adaptations.

Major Extinction Events

Mass Extinctions and Their Impact

Throughout Earth's history, several mass extinction events have dramatically reshaped animal diversity.

Event	Approximate Time (mya)	Main Effects
Ordovician-Silurian	~445	Marine species loss due to glaciation and sea level fall
Devonian	~360	Extinction of many marine groups, possibly due to anoxia
Permian-Triassic	~252	Largest extinction; ~96% of marine species lost, linked to volcanic activity and climate change
Triassic-Jurassic	~200	Loss of many reptiles, paving way for dinosaur dominance
Cretaceous-Paleogene (K-Pg)	~66	Extinction of non-avian dinosaurs, rise of mammals and birds

Example: The Chicxulub impact is linked to the K-Pg extinction, which eliminated non-avian dinosaurs and allowed mammals to diversify.

Summary Table: Major Events in Animal Evolution

Time Period	Key Events
~900 mya	Origin of animal ancestor (choanoflagellate-like protist)
~540 mya	Cambrian Explosion; rapid diversification of animal phyla
~470-440 mya	Colonization of land by plants and animals
~360 mya	Evolution of tetrapods (four-limbed vertebrates)
~252 mya	Permian-Triassic extinction; major loss of species
~66 mya	K-Pg extinction; end of non-avian dinosaurs, rise of mammals

Key Terms and Concepts

• Phylogeny: The evolutionary history and relationships among species or groups.

• Cambrian Explosion: A period of rapid evolutionary diversification of animals about 540 million years ago.

• Hox genes: Genes that control the body plan of an embryo along the head-tail axis.

• Extinction event: A widespread and rapid decrease in the biodiversity on Earth.

• Choanoflagellates: Single-celled or colonial protists considered the closest living relatives of animals.