Animal Evolution and Early Branching Phyla Notes

Unikont Clade and Animal Origins

  • Animals belong to the Unikont clade, which also includes:
    • Choanoflagellates
    • Nucleariids
    • Fungi
  • Unikonts diverged from other eukaryotes approximately 1 billion years ago.

First Animal Appearances

  • First animals appear in the fossil record around 560 million years ago, notably:
    • Dickinsonia costata (Diameter: about 8 cm)
    • Other early forms such as Chordates, Arthropods, Anomalocaris, Hallucigenia.
  • The Cambrian Explosion resulted in a large number of new animal species evolving during the Cambrian period.

Defining Characteristics of Animals

  • Animals are heterotrophic organisms.
    • Distinction: Unlike fungi, animals consume food in a more complex manner and engage in movement.
  • Animals undergo a unique developmental process:
    • Stages in Development:
    • Haploid (n)
    • Diploid (2n)
    • Egg formation via meiosis followed by fertilization, forming a zygote.
    • Zygote undergoes cell division to reach the eight-cell stage, followed by the blastula and gastrula formation.
    • Formation of three distinct tissue layers:
      • Ectoderm (outer layer)
      • Endoderm (inner layer)
      • Mesoderm (middle layer, develops into various organs)

Symmetry in Animals

  • Animals exhibit different types of symmetry:
    • Radial Symmetry:
    • Example: Sea anemones
    • Bilateral Symmetry:
    • Example: Lobsters
  • Early developmental stages lead to different embryonic patterns:
    • Protostomes and Deuterostomes exhibit different embryonic development outcomes.

Tissue Types and Evolutionary Context

  • The differences in body structure and tissue layers (triploblastic vs diploblastic) reflect evolutionary history.
  • Triploblastic animals have three tissue layers:
    • Animals like Flatworms, Molluscs, Annelids display this.
  • Diploblastic animals like Cnidarians have only two tissue layers.

Major Groupings of Animals

  • Phylum Porifera (Sponges):
    • Lack true tissues, exhibit asymmetry, primarily marine, act as suspension feeders.
  • Cnidarians:
    • Characterized by stinging cells (cnidocytes), exist in two body forms (polyp and medusa).
    • Exhibit radial symmetry.
    • Two tissue layers: Ectoderm and Endoderm.

Cnidarian Anatomy and Life Cycle

  • Cnidarians go through a life cycle that involves:
    • Asexual (budding
    • Sexual (external fertilization in medusa form)
  • Major classes of cnidarians:
    • Hydrozoa (solitary or colonial)
    • Scyphozoa (typical jellyfish, medusa prominent)
    • Anthozoa (polyp form is dominant)

Evolutionary Insights from Ctenophores

  • Ctenophores illustrate the complexity of animal evolution with a through gut structure. They possess:
    • Soft body
    • Nervous and muscular systems
    • Historical classification challenged by new findings highlighting their potential primitive traits.

Conclusion and Thought Questions

  • Reflect on common characteristics of all animals.
  • Consider the distinction of sponges and evolutionary implications of body symmetry, tissue layers, and the formation of nervous systems across different clades.
  • Assess how finding filamentous organisms on the beach can inform whether they are algae, slime molds, or hydrozoan colonies.