Evolution and Characteristics of Animals

Defining Animals

  • Characteristics of Animals:

    • Heterotrophs: Organisms that obtain their food by consuming other organisms.

    • Multicellular: Composed of multiple cells.

    • No cell walls: Unlike plants, animals do not have rigid cell walls.

    • Movement: All animals have some form of movement at some point in their life cycle.

    • Diversity in morphology: Variation in shape and structure among different species, influenced by HOX genes.

    • Diversity in habitats: Animals can live in various environments, including terrestrial, aquatic, and aerial.

    • Sexual Reproduction: Most animals reproduce sexually, involving the combination of genetic material from two parents.

    • Tissues: Animals are composed of specialized cells forming tissues.

Evolution of the Animal Body Plan

  • Five Key Innovations in the evolution of animal body plans:

    1. Tissues

    2. Symmetry

    3. Body cavity

    4. Development

    5. Segmentation

Evolution of the Animal Body Plan

  1. Evolution of Tissues:

    • Parazoa: Organisms that lack defined tissues and organs. Example: Phylum Porifera (sponges).

    • Eumetazoa: Organisms with distinct tissues and irreversible differentiation for most cell types, encompassing all other animals.

  2. Evolution of Symmetry:

    • Parazoa: Lack symmetry.

    • Eumetazoa: Display symmetry along an axis through the body.

      • Types of Symmetry:

      • Radial Symmetry: Organisms are arranged around a central axis, allowing two equal halves in any 2-D plane.

      • Bilateral Symmetry: Body can be divided into right and left halves that are mirror images. The sagittal plane bisects the body into two equal halves.

    • Significance: Symmetry enhances complex movement capabilities.

  3. Advantages of Bilateral Symmetry:

    • 1. Cephalization: Development of a cranial central nervous system (CNS).

    • 2. Greater Mobility: Beyond linear movement.

  4. Evolution of Coelom (Body Cavity):

    • Eumetazoa: Possess three germ layers:

      • Ectoderm: Forms body coverings and nervous system.

      • Mesoderm: Develops into the skeleton and muscles.

      • Endoderm: Forms digestive organs and intestines.

    • Coelom: A space surrounded by mesodermal tissue formed during development.

  5. Types of Body Cavities:

    • Acoelomates: Organisms lacking a body cavity.

    • Pseudocoelomates: Body cavity located between mesoderm and endoderm.

    • Coelomates: Body cavity completely enclosed in mesoderm.

  6. Patterns of Development in Bilaterians: Two groups:

    • Protostomes: Develop mouth first from the blastopore, anus (if present) develops either from the blastopore or another region of the embryo.

    • Deuterostomes: Develop anus first from the blastopore, mouth later from another region of the embryo.

  7. Segmentation:

    • Definition: The division of the body into regions, allowing for more efficient and flexible movement.

    • Functions: Development of body regions for different functions and regeneration of body parts in some taxa.

    • Evolutionary Appearance: Segmentation has appeared several times in animal evolution.

The Earliest Animals

  • Dickinsonia: An organism from the Ediacaran biota (570 - 550 million years ago), exhibited bilateral symmetry and suggested evidence of cholesterol.

  • Charniodiscus: Another significant organism from the same era.

Animal History

  • Cambrian Period: By 500 million years ago, all major animal phyla were present.

  • Cambrian Explosion: A major adaptive radiation event that occurred over 30 million years, significantly evolving basic shapes and sizes.

    • Burgess Shale: Key fossil site for studying this evolutionary change.

    • Homeobox (Hox) gene complex: This genetic framework evolved during this period, leading to rapid changes in animal body plans.