Summary of Animal Origins and Evolution Concepts

Protists Leading to Animals
  • Common Ancestor: Molecular analyses indicate sponges and other animals share a last common ancestor resembling extant choanoflagellates.
  • Divergence: Sponges diverged approximately 670-700 million years ago.
Choanoflagellate Protists
  • Sister Group: Choanoflagellates are the extant sister group of animals, consisting of small, unicellular, heterotrophic protists (about 140 species).
  • Characteristics:
    • Funnel-shaped collar of microvilli.
    • Finger-like projections to capture bacteria.
  • Evolution Hypothesis: Animals are hypothesized to have evolved from a choanoflagellate-like ancestor, supported by:
    • Similar cell morphology.
    • Presence of collar cells in sponges.
    • DNA sequence homology.
Multicellularity
  • Eukaryotic Cells: Evolution of eukaryotic cells led to diverse forms, with multicellularity evolving independently across multiple lineages.
  • Advantages: Enhances complexity and adaptability.
  • Fossil Evidence: Multicellular eukaryotes emerged about 1.2 billion years ago; whole-body animal fossils date back to approximately 560 million years ago.
Geological Setting

  • Neoproterozoic Era (~1 billion – 541 million years ago):

    • Rise of planktonic algae, leading to the Neoproterozoic Oxygenation Event (800-650 million years ago).
    • Earliest multicellular animal fossils from the Ediacaran biota (635-541 million years ago).

  • Palaeozoic Era (~541 – 252 million years ago):

    • Cambrian explosion marked significant diversification, with rapid appearance of most animal phyla.
    • Emergence of hard body parts (shells, exoskeletons).
    • Hypotheses explaining the Cambrian explosion include increased oceanic oxygen and evolution of predation.
Geological Timeline in Animal Evolution
  • Palaeozoic Era: Animal diversity increased despite being negatively impacted by mass extinctions.
  • Mesozoic Era: Emergence of dinosaurs and mammals; development of feathers and powered flight.
  • Cenozoic Era: Mammals occupied diverse ecological niches following the End-Cretaceous mass extinction.
Invertebrates
  • Dominance: Most extant animals are invertebrates, found in diverse habitats, ranging in size from microscopic to giant.
  • Paraphyletic Group: Invertebrates form a paraphyletic group.
Reflective Questions for Exam Preparation
  • What geological background led to the origin of animals?
  • When did the first animals appear in the fossil record?
  • What are some characteristics of basal animals?