Chapter 27: The Rise of Animal Diversity

27.1: Origin of Animals and Early-Diverging Lineages

The origin of animals is traced back to unicellular eukaryotes, with evidence from fossils and molecular data. Early-diverging animal lineages provide insight into the evolution of complex multicellular organisms.

• Evidence for Animal Origins: Fossil records and molecular analyses suggest animals evolved from choanoflagellate-like ancestors over 700 million years ago.

• Key Traits of Early Animals:

  • Multicellularity

  • Heterotrophy (obtaining food by ingestion)

  • Cells lacking cell walls

  • Specialized tissues (e.g., nervous and muscle tissue in most animals)

• Early-Diverging Lineages: Sponges (phylum Porifera) are among the earliest animals, lacking true tissues. Cnidarians (e.g., jellyfish, corals) possess true tissues and radial symmetry.

Example: Sponges filter feed using specialized cells called choanocytes, reflecting their evolutionary link to choanoflagellates.

27.2: The Cambrian Explosion and Animal Evolution

The Cambrian explosion (about 535–525 million years ago) marks a period of rapid diversification, with the appearance of most major animal phyla.

• Major Milestones:

  • Emergence of bilateral symmetry

  • Development of complex body plans

  • Evolution of hard body parts (e.g., shells, exoskeletons)

• Possible Causes: Increased oxygen levels, evolution of predation, and changes in developmental genes (e.g., Hox genes).

Example: Fossils from the Burgess Shale show a diversity of body forms, including early arthropods and chordates.

27.3: Animal Body Plans, Invertebrates, and Chordates

Animal body plans are sets of morphological and developmental traits that define major groups. Invertebrates and chordates are two broad categories based on the presence or absence of a backbone.

• Body Plan Features:

  • Symmetry (radial vs. bilateral)

  • Number of germ layers (diploblastic vs. triploblastic)

  • Presence of a body cavity (coelom)

• Invertebrates: Animals without a backbone; comprise about 95% of all animal species. Examples include arthropods, mollusks, annelids, and cnidarians.

• Chordates: Defined by four key traits at some stage of development:

  • Notochord

  • Dorsal, hollow nerve cord

  • Pharyngeal slits or clefts

  • Muscular, post-anal tail

Example: Humans are chordates, possessing all four traits during embryonic development.

Comparison of Major Animal Groups

27.5: Colonization of Land by Arthropods and Tetrapods

The transition from aquatic to terrestrial life required significant adaptations. Arthropods and tetrapods were among the first animals to colonize land.

• Arthropods: First animals to colonize land (~450 million years ago). Adaptations include a waterproof exoskeleton, jointed appendages, and specialized respiratory structures (e.g., tracheae, book lungs).

• Tetrapods: Descended from lobe-finned fishes. Adaptations for land include limbs with digits, lungs, and changes in sensory systems.

Example: Insects (arthropods) and amphibians (early tetrapods) are key groups in the history of terrestrial colonization.

27.6: Amniotes and Adaptations for Terrestrial Life

Amniotes are a clade of tetrapods that evolved adaptations for life on land, most notably the amniotic egg.

• Amniotic Egg: Contains specialized membranes (amnion, chorion, yolk sac, allantois) that protect and nourish the embryo, allowing reproduction away from water.

• Major Amniote Groups: Reptiles (including birds) and mammals.

• Adaptations in Present-Day Amniotes:

  • Waterproof skin (scales, feathers, or hair)

  • Efficient lungs and circulatory systems

  • Internal fertilization

  • Parental care (especially in mammals)

Example: Mammals produce milk and have hair, while birds (reptilian amniotes) have feathers and lay shelled eggs.

Key Terms Table

Additional info: The Cambrian explosion is often linked to the evolution of Hox genes, which control body plan development. Amniotic eggs are a key innovation that allowed vertebrates to reproduce in dry environments.