Origin of Chordata.

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🧬 Origin of Chordata

🎯 Objective:

We will understand the basic characters of chordates, origin, and ancestry of chordates.

📘 Introduction:

The animal kingdom is basically divided into two sub-kingdoms:

  • Nonchordata – includes animals without notochord
  • Chordata – includes animals with notochord or chorda dorsalis (present at some stage in life)

The Chordata is the animal phylum with which everyone is most intimately familiar since it includes humans and other vertebrates.

All chordates have the following features at some stage in life (in many vertebrates, only embryonic stage):

  • Pharyngeal slits – openings connecting the throat to the outside of the neck; often function as gills.
  • Dorsal tubular nerve cord – runs along the back; connects brain to muscles/organs.
  • Notochord – cartilaginous supporting rod beneath the nerve cord.
  • Post-anal tail – body extension beyond the anus.

🌱 Origin of Chordates:

It is believed that chordates originated from invertebrates. Determining the exact group is difficult, but ancestors were likely soft-bodied.

❌ (a) Coelenterate Theory – Rejected

  • Proposed origin from coelenterates (e.g., jellyfish, corals)
  • Claimed radial symmetry, coelenteron, and cnidoblasts lost over evolution
  • Suggested chordates independently developed advanced features
  • Rejected due to lack of evidence and significant structural differences

❌ (b) Annelid Theory – Rejected

  • Proposed chordates evolved from annelid-like ancestors

  • Similarities:

    • Bilateral symmetry
    • Metamerism
    • Coelom
    • Closed circulatory system
    • Hemoglobin

  • Differences:

    • Coelom: schizocoelic (annelids) vs enterocoelic (chordates)
    • Hemoglobin in plasma (annelids) vs in RBCs (chordates)
    • Nerve cord: double & ventral (annelids) vs single, hollow & dorsal (chordates)
    • Embryology and body orientation differ


✅ (c) Echinoderm–Hemichordate Theory – Most Accepted

Suggests chordates, hemichordates, and echinoderms evolved from a common ancestor based on several evidences:

🧫 (i) Embryological Evidence

  • Both echinoderms and chordates show:
    • Enterocoelic coelom formation
    • Mesoderm from archenteron
    • Deuterostome development (blastopore becomes anus)
  • Larval Similarities:
    • Bipinnaria larva (echinoderms) resembles tornaria larva (hemichordates)
    • Nervous system arises from dorsal ectodermal strip in both groups

🧠 These embryological parallels support deuterostome ancestry for echinoderms, hemichordates, and chordates.

🧪 (ii) Serological Evidence

  • Protein similarities in body fluids of chordates and echinoderms suggest close evolutionary relationships
  • Although adult echinoderms are radially symmetrical, this is secondarily derived
  • Larval echinoderms are bilaterally symmetrical, reinforcing a common ancestry


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