Chordates, from sea squirts to humans, share key features that set them apart. The notochord, dorsal hollow nerve cord, pharyngeal slits, and post-anal tail define this diverse group. These structures play crucial roles in support, movement, and development.

Vertebrates, a subgroup of chordates, took evolution a step further. They developed a skull to protect their brain and a backbone to replace the notochord. This allowed for more complex nervous systems and sensory organs, paving the way for diverse adaptations.

Defining Characteristics and Evolution of Chordates

Key features of chordates

• Notochord
  • Flexible rod runs along dorsal side of body
  • Provides support and aids in movement
  • Present at some stage of development in all chordates (embryonic, larval, or adult)
• Dorsal hollow nerve cord
  • Develops from ectoderm plate that rolls into a tube dorsal to notochord
  • Forms brain and spinal cord in vertebrates (humans, fish, birds)
• Pharyngeal slits
  • Openings in pharynx connect to outside of body
  • Allow water to exit pharynx in aquatic chordates (fish, tunicates)
  • Develop into structures like eustachian tube and tonsils in terrestrial chordates (humans, birds)
  • In some chordates, pharyngeal slits develop into gill arches for respiratory function
• Post-anal tail
  • Muscular extension of body posterior to anus
  • Aids in locomotion, especially in aquatic species (fish, amphibian larvae)

Craniates vs other chordates

• Craniates (vertebrates)
  • Have cranium (skull) that encases brain
  • Possess vertebral column that replaces notochord in most adults
  • Have highly developed brain and complex sensory organs (eyes, ears)
  • Include jawless fishes, cartilaginous fishes, bony fishes, amphibians, reptiles, birds, and mammals
• Other chordates (invertebrate chordates)
  • Lack cranium and vertebral column
  • Retain notochord throughout their lives
  • Have simpler nervous system and sensory organs compared to craniates
  • Include tunicates (sea squirts) and cephalochordates (lancelets)

Notochord evolution in vertebrates

• Embryonic stage
  • Notochord forms from mesodermal cells during gastrulation
  • Extends along dorsal side of embryo, defining longitudinal axis
  • Induces formation of neural tube (future spinal cord and brain) from overlying ectoderm
• Larval stage (in some vertebrates)
  • Notochord persists as continuous, flexible rod
  • Provides support and aids in swimming motion (tadpoles, fish larvae)
• Adult stage
  1. In most vertebrates, notochord is replaced by vertebral column
     • Vertebrae form around notochord from somites (segments of paraxial mesoderm)
     • Notochord tissue contributes to intervertebral discs (cushioning between vertebrae)
  2. In some primitive vertebrates, notochord persists throughout life
     • Provides support and flexibility in absence of complete vertebral column (hagfish, lampreys)

Unique features of vertebrate development

• Chondrocranium: cartilaginous precursor to the skull that forms around the developing brain
• Neural crest cells: specialized cells that migrate from the neural tube and give rise to various structures, including:
  • Craniofacial bones and cartilage
  • Peripheral nervous system
  • Pigment cells
• Endoskeleton: internal skeletal system that provides support and protection for internal organs