Chordate Evolution - Where Did We Get Our Brains?

  • Discussion focuses on various ancestral lineages leading to Chordata and classification hierarchies.

    • Common ancestor discussed:

    • Characterized by unique cell junctions, collagen, and proteoglycans in the extracellular matrix.

  • Key groups in the evolutionary tree:

    • Cnidarians, issues of symmetry, organ systems, and developmental biology discussed.

COMMON CHARACTERISTICS OF CHORDATES

  • Four main features of Chordates:

    1. Hollow dorsal nerve cord:

      • Differentiates into brain and spinal cord in vertebrates.

    2. Pharyngeal slits:

      • Connects the pharynx with the external environment.

    3. Notochord:

      • A flexible, rod-like structure that extends the length of the body.

    4. Post-anal tail:

      • A tail that extends beyond the anus.

DORSAL HOLLOW NERVE CORD

  • Runs along the length of the body.

  • Innervates the muscles attached to the notochord.

  • Forms from the infolding of ectoderm during embryonic development.

PHARYNGEAL SLITS

  • Initially functioned as a filter feeding apparatus in early chordates.

  • In fishes, utilized as gills.

  • In modern chordates, pharyngeal pouches are formed during embryonic development.

DERIVATIVES OF PHARYNGEAL SLITS & BARS IN VERTEBRATES

  • Contributions include:

    • Jaws and jaw support.

    • Gills and gill supports.

    • Aortic arches and other significant vessels.

    • Some muscle types.

  • These features provide evidence of an aquatic ancestry.

NOTOCHORD

  • Definition:

    • Core of large cells with turgid fluid-filled vacuoles surrounded by an outer sheath layer.

    • Exists as a long tube in cephalochordates and tunicate larvae.

    • In nearly all vertebrates, it is replaced by the vertebral column during embryonic development (exception: jawless fish).

CHORDATE CLASSIFICATION

  • Classification of Chordata includes:

    • Non-Vertebrate Chordates

    • Examples:

      • Urochordata (e.g. Ascidiacea, Thaliacea, Appendicularia)

      • Cephalochordata (e.g. Lancelets)

    • Vertebrate Chordates

    • Examples:

      • Class Myxini (hagfish)

      • Class Cephalaspidomorphi (lampreys)

      • Class Chondrichthyes (cartilaginous fish)

      • Class Osteichthyes (bony fish)

      • Class Amphibia

      • Class Reptilia (paraphyletic)

      • Class Aves (birds)

      • Class Mammalia

SUBPHYLUMS

  • Cephalochordata (Lancelets)

    • Characteristics: no brain, Hox genes used for cranial bulb formation in the lancelet's nerve cord. Vertebrates use the same Hox genes to form the brain.

  • Urochordata (Sea Squirts)

    • Characteristics:

    • Lifecycle includes both sexual and asexual reproduction.

    • Larval form: Tadpole-like, free-swimming, poor feeding abilities. Exhibits fundamental chordate features.

    • Adult forms: Generally filter feeders; some are immobile, while others are free-living. Body covered by a 'tunic' made of secreted protein and cellulose-like material.

ASCIDIACEA AND THALIACEA

  • Thaliacea (Salps):

    • Pyrosomes form colonies with multiple tiny ascidian-like zooids.

    • Asexual reproduction can yield large swarms quickly under favorable conditions (e.g., 5000 individuals per m³).

    • Ecological significance: Important for carbon sequestration and found within marine food webs, serving as food for marine turtles.

  • Ascidia:

    • Sessile filter feeders in their adult stage.

    • Unique anatomy includes siphons, nerve ganglion, and digestive components.

MODEL FOR NEURODEGENERATION

  • Research on Botryllus schlosseri, a colonial sea squirt, indicates that its distinctive lifecycle can be utilized for comparative studies on neurodegenerative processes across different phylogenies.

VERTEBRATE CHORDATES

  • Vertebrate Characteristics:

    • Presence of a vertebral column.

    • Endoskeleton.

    • Developed head and neural crest.

    • Significantly differentiated brain with complex internal organs, including:

    • Complete digestive system

    • Muscular gut

    • Osmoregulatory and excretory systems

    • Closed circulatory system

CENTRAL NERVOUS SYSTEM OF VERTEBRATES

  • Vertebral Column:

    • Replaces the notochord during embryonic development, enclosing the nerve cord.

    • Jawless fishes lack vertebrae; their notochord persists.

    • Serves as an anchor for the endoskeleton.

HEAD AND NEURAL CREST

  • Provides paired sense organs for foraging.

  • These are typically housed anteriorly in proximity to the mouth.

HIGHLY-DIFFERENTIATED BRAIN

  • Brain regions evolved to manage sensory processing:

    • Hindbrain: Coordinates motor reflexes, centered in the cerebellum.

    • Midbrain: Processes visual information.

    • Forebrain: Handles olfactory information and has increased cerebellum and cerebrum sizes, notably in mammals, enhancing functions such as coordination of movement, processing visual and auditory signals, social behavior, communication, and self-awareness.

CONCLUSION

  • Summary of learning outcomes reviewed, reiterating the understanding of Chordata's phylogeny, features of non-vertebrate and vertebrate forms, and the evolution of vertebrate brains.

READING

  • Additional Information:

    • Satoh et al. (2014)"Chordate evolution and the three-phylum system." Proc. R. Soc. B, 281: 20141729.