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The new head hypothesis revisited.

R Glenn Northcutt1

  • 1Neurobiology Unit, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, 92093, USA. egnorthcutt@ucsd.edu

Journal of Experimental Zoology. Part B, Molecular and Developmental Evolution
|July 9, 2005
PubMed
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The New Head Hypothesis regarding vertebrate evolution is re-examined. Key claims about neural crest and head development are evaluated, leading to a revised understanding of deuterostome evolution and vertebrate origins.

Area of Science:

  • Evolutionary Developmental Biology
  • Comparative Genomics
  • Phylogenetics

Background:

  • The Tunicate Hypothesis proposed a specific model for deuterostome evolution.
  • The New Head Hypothesis (1983) posited unique vertebrate traits and their evolutionary significance.

Purpose of the Study:

  • To re-examine the claims of the New Head Hypothesis using evolutionary developmental biology.
  • To investigate the origins of neural crest and neurogenic placodes.
  • To re-evaluate deuterostome evolution and vertebrate origins.

Main Methods:

  • Analysis of existing literature on evolutionary developmental biology.
  • Comparative examination of vertebrate and protochordate tissues.
  • Taxonomic data review.

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Main Results:

  • The link between vertebrate-specific tissues and the shift to active predation is supported.
  • The concept of the vertebrate rostral head as a neomorphic unit is validated.
  • The origin of neural crest and neurogenic placodes from an epidermal nerve plexus is rejected; germ layer rearrangement is proposed.
  • The Tunicate Hypothesis is rejected; a revised Dipleurula Hypothesis is presented.

Conclusions:

  • Vertebrate origins involve significant developmental innovations, not solely inherited traits.
  • Germ layer rearrangement in early chordate development is a key factor in vertebrate origins.
  • The Dipleurula Hypothesis offers a more robust framework for understanding early deuterostome evolution.