Defining Characteristics and Evolution of Chordates
Chordates range from sea squirts to humans, and they all share four key features: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. These structures define the phylum and play central roles in support, movement, and development.
Vertebrates are a subgroup of chordates that evolved a skull to protect the brain and a vertebral column to replace the notochord. This shift allowed for more complex nervous systems and sensory organs, opening the door to the enormous diversity of vertebrate life.
Key Features of Chordates
Every chordate has these four structures at some point in its development, whether during the embryonic stage, larval stage, or into adulthood.
- Notochord
- A flexible rod that runs along the dorsal (back) side of the body
- Provides structural support and aids in movement
- Present in all chordates at some developmental stage, though in most vertebrates it's eventually replaced by the vertebral column
- Dorsal hollow nerve cord
- Forms when a plate of ectoderm rolls into a hollow tube dorsal to the notochord
- In vertebrates, this tube develops into the brain and spinal cord
- This is different from invertebrates like insects, which have a ventral solid nerve cord
- Pharyngeal slits
- Openings in the pharynx (throat region) that connect to the outside of the body
- In aquatic chordates like fish and tunicates, these allow water to exit the pharynx and function in filter feeding or gas exchange
- In some chordates, pharyngeal slits develop into gill arches that support respiratory tissue
- In terrestrial chordates like mammals, the slits don't persist as openings. Instead, they're modified during development into structures like the eustachian tubes, tonsils, and parts of the jaw and ear bones
- Post-anal tail
- A muscular extension of the body that extends past the anus
- Aids in locomotion, especially in aquatic species like fish and amphibian larvae
- In humans, this structure is present during embryonic development but regresses before birth (the coccyx, or tailbone, is a remnant)
Craniates vs. Other Chordates
Chordates split into two broad groups based on whether they have a skull.
- Craniates (vertebrates)
- Possess a cranium (skull) that encases and protects the brain
- Most adults have a vertebral column that replaces the notochord
- Have a highly developed brain and complex sensory organs (eyes, ears, lateral line in fish)
- Include jawless fishes, cartilaginous fishes, bony fishes, amphibians, reptiles, birds, and mammals
- Invertebrate chordates
- Lack a cranium and vertebral column
- Retain the notochord throughout their lives as their primary structural support
- Have simpler nervous systems and sensory structures compared to craniates
- Include tunicates (sea squirts) and cephalochordates (lancelets)
Tunicates are worth noting because only the larval form looks like a typical chordate, with a visible notochord and tail. The adult form is sessile (attached to a surface) and loses most chordate features, which is why they weren't always recognized as chordates.
Notochord Evolution in Vertebrates
The notochord's role changes dramatically across developmental stages in vertebrates.
- Embryonic stage
- The notochord forms from mesodermal cells during gastrulation
- It extends along the dorsal side of the embryo, defining the head-to-tail (longitudinal) axis
- It induces the overlying ectoderm to form the neural tube, which becomes the spinal cord and brain. This induction step is critical: without the notochord's signaling, the nervous system won't develop properly
- Larval stage (in some vertebrates)
- The notochord persists as a continuous, flexible rod
- It provides support and aids in swimming, as seen in tadpoles and fish larvae
- Adult stage
- In most vertebrates, the notochord is replaced by the vertebral column. Vertebrae form around the notochord from somites (blocks of paraxial mesoderm). Remnants of notochord tissue persist as the nucleus pulposus, the gel-like center of intervertebral discs that cushions the vertebrae.
- In some basal vertebrates like hagfish and lampreys, the notochord persists throughout life, providing support and flexibility in the absence of a fully developed vertebral column.
Unique Features of Vertebrate Development
Three features distinguish vertebrate development from that of other chordates:
- Chondrocranium: a cartilaginous structure that forms around the developing brain and serves as the precursor to the bony skull. In cartilaginous fish like sharks, this remains cartilage throughout life.
- Neural crest cells: a population of cells unique to vertebrates that migrate away from the neural tube during development. They give rise to a remarkable range of structures:
- Craniofacial bones and cartilage
- Peripheral nervous system neurons and glia
- Pigment cells (melanocytes)
- Parts of the heart and adrenal glands
- Endoskeleton: an internal skeletal system (as opposed to the exoskeleton of arthropods) that provides support, protects internal organs, and grows with the organism. It can be made of cartilage, bone, or both.