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Evolution of Chordates and Mammals

Apr 14, 2025

Lecture Notes: Understanding Chordates and Evolutionary Biology

Introduction

  • Definition of human classification: mammalian amniotic tetrapodal sarcopterygiian osteichthyan gnathostomal vertebrate cranial chordate.
  • Exploration of the phylum Chordata, which includes 52,000 species of vertebrates and thousands of invertebrates.
  • Focus on the evolutionary milestones that lead to mammals, the most complex class within Chordata.

Key Concepts in Evolutionary Biology

  • Synapomorphic Traits: Traits that set groups of animals apart from their ancestors.
    • Chordates have four synapomorphies present at some stage in their life cycle.

Four Synapomorphic Traits of Chordates

  1. Notochord

    • A cartilage structure located between the digestive tube and nerve cord.
    • In vertebrates, a skeleton develops around it.
    • In humans, reduced to vertebral discs.

  2. Dorsal Hollow Nerve Cord

    • Develops into the central nervous system.
    • Different from solid, ventral nerve cords in other phyla.

  3. Pharyngeal Slits

    • Functions vary: filter feeding in invertebrates, gill slits in fish, and develop into head/neck structures in land vertebrates.

  4. Post-Anal Tail

    • Aids movement in aquatic animals, becomes coccyx in humans.

Evolutionary History of Chordates

  • Origin traced to Cambrian explosion over 500 million years ago.
  • Three subphyla: Cephalochordata, Urochordata, Vertebrata.
    • Cephalochordata: Oldest, retains all chordate traits.
    • Urochordata: Tadpole-like larvae with chordate characteristics.
    • Vertebrata: Most species-rich, characterized by a backbone.

Vertebrate Evolution & Key Adaptations

  • Craniates: Vertebrates with a head, sensory organs, and brain.
  • Agnathans: Jawless vertebrates like lampreys.
  • Gnathostomes: Jawed vertebrates; includes Chondrichthyes (cartilage fish).
  • Osteichthyes: Bony fish, vital for the evolution towards mammals.

Evolution of Tetrapods

  • Origin of lobe-finned fishes (Sarcopterygii): Lead to tetrapods (land-adapted).
  • Transition from aquatic to terrestrial life.
  • Coelacanths: Living fossils, provide insights into early vertebrate adaptations.

Amphibians and Amniotes

  • Amphibians: First tetrapods with 3-chambered hearts; require water for reproduction.
  • Amniotic Egg: Allowed life on land, leading to reptiles, birds, and mammals.

Reptilia and Evolution of Birds

  • Reptiles: Early amniotes, ectothermic.
  • Birds (Aves): Evolved from archosaurs, endothermic with 4-chambered hearts.

Class Mammalia

  • Unique traits: Hair, mammary glands, three ear bones.
  • Internal development of embryos, avoiding environmental predation.

Conclusion

  • Evolutionary journey from the origin of chordates to the emergence of mammals.
  • Review and further exploration options available for reinforcement of concepts.

Additional Resources

  • Access review materials and further explanations on Facebook and Twitter, or via comments section.

Note: This summary is based on a lecture from Crash Course Biology. It aims to provide a concise overview of the evolutionary history and classification within the phylum Chordata, leading up to mammals.

Full transcript