Chapter 30: An Introduction to Animals – Study Notes

Introduction to Animals

Overview and Importance of Animals

Animals are a diverse group of multicellular, eukaryotic organisms that play essential roles in ecosystems and human society. Understanding their characteristics, evolutionary history, and diversity is fundamental to the study of biology.

• Humans depend on animals for food, materials (such as fibers and leather), pollination of crops, transportation, and power.

• Domesticated animals have been crucial in agriculture and society.

• Animals are also important for scientific research and as sources of useful genetic material.

Distinguishing Characteristics of Animals

Key Traits

Animals are defined by a set of shared characteristics that distinguish them from other life forms.

• Multicellularity: Animals are composed of multiple cells lacking cell walls, with cells organized into tissues (except sponges).

• Heterotrophy: Animals obtain energy and carbon by ingesting other organisms or organic material.

• Motility: Most animals are capable of movement at some stage in their life cycle.

• Nervous and Muscle Tissue: Most animals (except sponges) have specialized cells for conducting electrical signals (neurons) and for movement (muscle cells).

• Monophyletic Clade: Animals form a single evolutionary lineage within the domain Eukarya.

Origin and Early Evolution of Animals

Pre-Cambrian and Cambrian Animal Life

The evolutionary history of animals is marked by several key periods, especially the Ediacaran and Cambrian periods.

• Ediacaran Period (635–543 million years ago): The earliest known animal fossils (Ediacaran biota) likely evolved from protists. Choanoflagellates, a group of protists, are considered the closest living relatives of animals.

• Cambrian Period (541–488 million years ago): Known for the "Cambrian Explosion," a rapid diversification of animal life. Most major animal phyla first appear in the fossil record during this time.

Significance of the Cambrian Explosion:

• Rapid appearance of new body plans and phyla.

• Evolution of key features such as hard parts, complex eyes, and predation.

• Possible causes include increased oxygen levels, genetic innovations (e.g., Hox genes), and ecological interactions.

Mass Extinctions and Animal Diversification

Animal evolution has been shaped by several mass extinction events, which have led to major shifts in diversity and the emergence of new groups.

• Permian-Triassic Extinction (~252 mya): The largest extinction event, eliminating ~95% of marine species and many terrestrial groups.

• Cretaceous-Paleogene Extinction (~66 mya): Caused by a meteorite impact and volcanic activity, leading to the extinction of non-avian dinosaurs and allowing mammals and birds to diversify.

• Five major mass extinctions have occurred since the Cambrian period; a potential sixth, human-driven extinction is currently underway.

Phylogeny and Classification of Animals

Major Animal Lineages

Animals are classified into approximately 30–35 phyla, with ongoing debate as new data emerge. The three major non-bilaterian phyla are:

• Porifera (sponges): Sessile, mostly marine suspension feeders; lack true tissues but have specialized cells.

• Ctenophora (comb jellies): Marine predators with sticky cells (colloblasts) for capturing prey; swim using cilia.

• Cnidaria (jellyfish, corals, sea anemones): Possess specialized stinging cells (cnidocytes) for prey capture and defense.

Animal Phylogeny and the Opisthokonta

Animals belong to the eukaryotic supergroup Opisthokonta, which also includes fungi and choanoflagellates. Molecular and morphological evidence supports a close relationship between animals and choanoflagellates.

Key Innovations in Animal Evolution

Embryonic Tissue Layers (Germ Layers)

The development of distinct tissue layers was a major evolutionary step.

• Diploblasts: Animals with two germ layers (ectoderm and endoderm), e.g., cnidarians and ctenophores.

• Triploblasts: Animals with three germ layers (ectoderm, mesoderm, endoderm), including most animal phyla.

Fate of Germ Layers:

• Ectoderm: Forms skin and nervous system.

• Endoderm: Forms the lining of the digestive tract.

• Mesoderm: Forms muscles, circulatory system, and internal organs.

Body Symmetry and Cephalization

Symmetry refers to the arrangement of body structures. Cephalization is the development of a head region with concentrated sensory organs and nervous tissue.

• Radial Symmetry: Body parts arranged around a central axis (e.g., cnidarians).

• Bilateral Symmetry: Single plane divides body into left and right halves; associated with cephalization and more complex nervous systems.

Origin of the Nervous System

• Nerve Net: Diffuse network of neurons (e.g., in cnidarians).

• Central Nervous System (CNS): Clusters of neurons (ganglia, brain) and nerve cords; found in bilaterians.

Body Cavities: The Coelom

The coelom is a fluid-filled body cavity that provides space for organ development and movement.

• Coelomates: Animals with a body cavity completely lined by mesoderm.

• Acoelomates: Animals lacking a coelom (e.g., flatworms).

• Pseudocoelomates: Animals with a body cavity partially lined by mesoderm (e.g., roundworms).

Protostomes vs. Deuterostomes

Bilaterian animals are divided based on embryonic development:

• Protostomes: Mouth develops before anus from the blastopore (e.g., mollusks, annelids, arthropods).

• Deuterostomes: Anus develops before mouth (e.g., echinoderms, chordates).

Segmentation

Segmentation is the division of the body into repeated units, seen in annelids, arthropods, and vertebrates. It allows for specialization of body regions and more efficient movement.

Themes in Animal Diversification

Ecological and Evolutionary Drivers

• Increased oxygen levels enabled larger, more active animals.

• Evolution of predation led to new adaptations and body plans.

• Genetic innovations (e.g., Hox genes) allowed for greater morphological diversity.

• New ecological niches promoted further diversification.

Sensory and Feeding Adaptations

• Animals evolved diverse sensory organs (sight, hearing, taste, smell, touch, temperature, magnetic and electric field detection).

• Feeding strategies include predation, herbivory, parasitism, and detritivory.

• Four general feeding types: suspension feeders, deposit feeders, fluid feeders, and mass feeders.

Movement and Skeletal Systems

• Movement is powered by muscle and is essential for finding food, mates, and escaping predators.

• Three types of skeletons: hydrostatic (fluid-filled), endoskeleton (internal), and exoskeleton (external).

Reproduction and Development

• Most animals reproduce sexually, but asexual reproduction (e.g., parthenogenesis) also occurs.

• Fertilization can be internal or external.

• Developmental strategies include oviparity (egg-laying), viviparity (live birth), and ovoviviparity (eggs retained inside the body).

• Many animals undergo metamorphosis, with distinct larval and adult stages.

Table: Comparison of Major Non-Bilaterian Animal Phyla

Phylum	Key Features	Habitat	Feeding
Porifera (Sponges)	No true tissues, sessile adults, specialized cells	Mostly marine, some freshwater	Suspension feeders
Ctenophora (Comb jellies)	Diploblastic, cilia for movement, colloblasts for prey capture	Marine	Predators
Cnidaria (Jellyfish, corals, sea anemones)	Diploblastic, radial symmetry, cnidocytes (stinging cells)	Marine and freshwater	Predators, some suspension feeders

Summary

• Animals are a monophyletic group of multicellular, heterotrophic eukaryotes with diverse forms and functions.

• Their evolutionary history is marked by key innovations such as tissues, symmetry, body cavities, and segmentation.

• Major diversification events, including the Cambrian explosion and mass extinctions, have shaped animal diversity.

• Understanding animal biology is essential for appreciating their ecological roles and evolutionary significance.