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Concept 34.1: Chordates have a notochord and a dorsal, hollow nerve cord

  • Vertebrates are animals with a backbone and include over 60,000 species, which is fewer than the approximately 1 million insect species, but vertebrates show a much greater range in body size and form.

  • Vertebrates display extreme diversity in body mass, from the massive plant-eating dinosaurs (up to 40,000 kg) and the blue whale (over 100,000 kg, the largest animal ever), to tiny fish like Schindleria brevipinguis (8.4 mm long, with a mass about 100 billion times smaller than a blue whale).

  • All vertebrates are part of the phylum Chordata, which also includes two groups of invertebrate deuterostomes: cephalochordates (e.g., lancelets) and urochordates (e.g., tunicates).

  • Chordates are bilaterally symmetrical animals within the clade Deuterostomia, and their closest relatives outside the group are echinoderms (e.g., sea stars).

  • The phylogenetic tree of chordates shows major evolutionary branch points, including:

    • The origin of chordates (notochord)

    • Cephalochordates and urochordates as invertebrate chordates

    • Vertebrates (with vertebrae), including cyclostomes (jawless vertebrates like hagfishes and lampreys)

    • Gnathostomes (jawed vertebrates, e.g., sharks, rays, bony fishes)

    • Osteichthyans (bony fishes with lungs or lung derivatives)

    • Lobe-finned fishes and tetrapods (vertebrates with limbs and digits)

    • Amniotes (vertebrates with amniotic eggs, including reptiles, birds, and mammals)

  • Derived traits distinguish each major clade, such as jaws in gnathostomes and amniotic eggs in amniotes, but some lineages have lost or reduced these traits (e.g., hagfishes and lampreys have reduced vertebrae).

  • Evolutionary relationships are branching, not linear, so it is incorrect to view evolution as a straight line "leading to" humans or mammals; instead, all groups share common ancestors at various branch points.

Derived Characters of Chordates

  • All chordates share four key derived characteristics: a notochord, a dorsal hollow nerve cord, pharyngeal slits or clefts, and a muscular post-anal tail, though some traits may only be present during embryonic development.

  • The notochord is a flexible, rod-like structure located between the digestive tube and nerve cord, providing skeletal support and a structure for muscle attachment. In most vertebrates, it is replaced by a more complex skeleton, with remnants forming parts of the intervertebral disks in humans.

  • The dorsal, hollow nerve cord develops from ectoderm and is unique to chordates, running above the notochord. It forms the central nervous system (brain and spinal cord), unlike the solid, ventrally located nerve cords of other animals.

  • Pharyngeal slits or clefts are grooves in the pharynx region that, in many chordates, develop into slits allowing water to exit the body without passing through the digestive tract. These structures function in suspension feeding in invertebrate chordates, as gills for gas exchange in most vertebrates, and develop into parts of the ear and head/neck structures in adult tetrapods.

  • The muscular, post-anal tail extends beyond the anus and contains skeletal and muscular elements, aiding in locomotion for many aquatic species. In many chordates, this tail is reduced during development, while nonchordates typically have a digestive tract running the length of the body.

Lancelets

  • Lancelets (Cephalochordata) are the sister group to all other living chordates, characterized by their bladelike shape and retention of key chordate traits such as a notochord, dorsal hollow nerve cord, pharyngeal slits, and post-anal tail throughout their life cycle.

  • Larval lancelets feed on plankton by alternating upward swimming and passive sinking, trapping food particles in their pharynx; adults burrow into sand with only their anterior end exposed, using cilia and mucus nets to filter food from seawater.

  • Lancelets exhibit segmentation through muscle blocks (somites) along the notochord, enabling side-to-side undulations for swimming; although globally rare, they can reach densities of over 5,000 individuals per square meter in certain locations.

Tunicates

  • Chordate evolutionary relationships are depicted as a branching sequence: Cephalochordata and Urochordata (tunicates) diverge early, with tunicates being more closely related to other chordates than lancelets (Cephalochordata).

  • Tunicate larvae display all four main chordate characteristics: notochord, dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. These features are most evident during the brief larval stage, which is adapted for swimming and locating a substrate.

  • Larval tunicates use tail muscles and the notochord for movement, guided by light- and gravity-sensitive cells.

  • Adult tunicates undergo a dramatic metamorphosis: the tail and notochord are resorbed, the nervous system degenerates, and internal organs rotate 90∘. Most chordate features are lost except for pharyngeal slits, which function in suspension feeding.

  • Adult tunicate anatomy includes an incurrent siphon (for water intake), pharynx with slits (for filtering food), excurrent siphon (for water exit), and a thick tunic at the base. The anus empties into the excurrent siphon, and some species can expel water forcefully when threatened ("sea squirts").

  • Genetic differences: Tunicates have only 9 Hox genes, while other chordates (including lancelets) have 13. This gene loss suggests tunicate larvae have a distinct genetic basis for their body plan compared to other chordates.

Early Chordate Evolution

  • Lancelets and tunicates provide important insights into vertebrate evolution, as lancelets retain key chordate features as adults and branch from the base of the chordate phylogenetic tree, suggesting the ancestral chordate had a mouth, notochord, dorsal hollow nerve cord, pharyngeal slits, and post-anal tail.

  • Lancelets have a simple nervous system, with only a slightly swollen tip at the anterior end of their dorsal nerve cord, but the same Hox genes (BF1, Otx, Hox3) that organize the vertebrate brain are expressed in similar anterior-to-posterior patterns in both lancelets and vertebrates, indicating a shared genetic mechanism for brain region development.

  • Hox genes such as BF1, Otx, and Hox3 control the development of major brain regions (forebrain, midbrain, hindbrain) in vertebrates, and their expression patterns in lancelets suggest evolutionary continuity in the genetic control of nervous system development.

  • Tunicate genomes reveal genes associated with vertebrate organs (like the heart and thyroid gland) that are present in both tunicates and vertebrates but absent in nonchordate invertebrates, indicating these genes were likely present in early chordates.

  • Tunicates possess embryonic cells with characteristics similar to the vertebrate neural crest, a key vertebrate trait, suggesting that these cells may represent an intermediate evolutionary stage in the development of the neural crest.

Concept Check 34.1

  • All chordates share four derived characters at some point in their life: a notochord, a dorsal hollow nerve cord, pharyngeal slits or clefts, and a muscular post-anal tail.

  • Humans are chordates but lack most of these features as adults because these structures are present only during embryonic development and are modified or lost before birth.

  • Lungs or lung derivatives are predicted to be present in vertebrate groups branching after the common ancestor of osteichthyans (bony fishes and their descendants), including ray-finned fishes, lobe-finned fishes, amphibians, reptiles (including birds), and mammals, as these groups evolved from ancestors with lungs or lung-like structures.