Summary of Lecture 19 on Animal Diversification

What are Animals?

Animals are defined by five key characteristics:

  1. Multicellular
  2. Heterotrophic (require organic compounds as food)
  3. Eukaryotic (cells with nucleus)
  4. Possess structural proteins like collagen, nerves, and muscles
  5. Unique developmental sequence regulated by Hox genes

The Cambrian Explosion

  • Occurred around 574 million years ago.
  • Represents a rapid increase in the diversity of animal life, with all major body plans appearing in the fossil record.
  • Characterized by hard body parts and the presence of 35 living phyla.

Molecular Clock Estimates

  • Genetic mutations build up over time and can be used to estimate when species diverged.
  • Best estimates suggest animals originated in the Neoproterozoic era, around 800 million years ago.
  • The contradiction between molecular clocks and the fossil record indicates either estimates may need revision or previous conditions precluded fossil preservation.

Life in the Cambrian

The Burgess Shale is famous for its well-preserved fossils of Cambrian organisms, revealing a variety of life forms such as:

  • Anomalocaris
  • Trilobites
  • Marella
  • Hallucigenia

Sudden Appearance of Animal Life

  • Possible explanations include a rise in oxygen levels and an evolutionary arms race sparked by the emergence of predators.

Current Animal Diversity

  • Taxonomic distribution shows:
    • Insects (55%)
    • Other arthropods (9%)
    • Mollusks (4%)
    • Chordates (3%)

Innovations and Evolution

The evolution of bilaterally symmetrical animals allowed for:

  • Development of central nervous systems
  • Cephalization
  • Enhanced sensory organs primarily located in the head region.

Tetrapods

Tetrapods are chordates that have adapted to life on land:

  • Include amphibians (e.g. frogs), reptiles (e.g. lizards, snakes, turtles), and mammals (e.g. humans).

Mammals Derived Traits

Mammals are characterized by:

  • Hair and lactation
  • High metabolic rates (endothermy)
  • Diverse reproductive methods, including oviparous (egg-laying) and viviparous (live-bearing) types.

Key Messages

  • Vertebrate evolution is variable and non-linear, resulting in diverse adaptations and some losses (e.g., snakes losing legs).
  • Extinction events (e.g., dinosaurs) have cleared ecological niches, enabling the rise of new lineages (e.g., mammals).