The phylum Chordata includes creatures as different as the blue whale, the sea squirt, and humans. An animal is defined as a chordate because it possesses a specific set of anatomical features at some point during its life. These characteristics unite a group that has colonized nearly every environment on Earth.
Defining Characteristics of Chordates
Every animal in the phylum Chordata is identified by four specific anatomical structures present during some phase of its life cycle. The persistence of these features into adulthood varies, but their appearance during embryonic development is a shared, defining trait.
The notochord is a flexible, rod-shaped structure that provides skeletal support. It runs along the length of the body, located between the digestive tube and the nerve cord. For some chordates, the notochord serves as the primary structural support throughout their lives. In many others, it functions as a scaffold during embryonic development, eventually being replaced by a vertebral column.
All chordates possess a dorsal hollow nerve cord, distinct from the solid, ventral nerve cords of many other animal phyla. This tube of nervous tissue is positioned above the notochord. In the majority of chordates, this embryonic structure develops into the central nervous system, which consists of the brain and spinal cord.
Pharyngeal slits are openings in the pharynx, the part of the throat just behind the mouth. In aquatic chordates, these slits often function as part of a filter-feeding system or develop into gills for respiration, allowing water that enters the mouth to exit the body. In many land-dwelling chordates, these slits are present only during embryonic stages and are repurposed into structures of the head, ear, and neck.
A post-anal tail is a posterior extension of the body that goes beyond the anus. This feature contains skeletal elements and muscles and is a primary means of locomotion in many aquatic species. For some terrestrial vertebrates, the tail aids in balance, communication, or courtship displays. In other groups, including humans, this tail is present during the embryonic phase but becomes vestigial in the adult form.
Major Groups of Chordates
The phylum Chordata is categorized into three major subphyla, distinguished by how they retain the chordate characteristics into adulthood. Two of these groups are invertebrates, lacking a backbone, while the third and most familiar group consists of the vertebrates.
The subphylum Cephalochordata consists of small, blade-shaped marine animals known as lancelets, which inhabit shallow ocean floors. Lancelets are often presented as a textbook example of the phylum because they retain all four key chordate features throughout their entire lives. The notochord provides the primary structural support in the adult animal.
Urochordata, or tunicates, includes sea squirts, which are sessile as adults and live in shallow marine waters. The larval stage of a tunicate is free-swimming and possesses all four chordate characteristics. Upon settling and undergoing metamorphosis, it loses its notochord, post-anal tail, and most of its nerve cord, retaining only the pharyngeal slits for filter-feeding.
The subphylum Vertebrata is the largest and most diverse group of chordates, including fish, amphibians, reptiles, birds, and mammals. The defining feature of this group is the development of a vertebral column, or backbone. This skeletal innovation laid the groundwork for greater size and mobility. All vertebrates are chordates, but not all chordates are vertebrates.
The Vertebrate Lineage
The history of vertebrates began over 500 million years ago. The earliest known vertebrates were jawless fish that appeared during the Cambrian period. These animals possessed a cranium but lacked jaws and paired fins, likely feeding by filtering debris from the seafloor.
The evolution of jaws, which arose from the gill arches supporting the pharyngeal slits, was a significant development. This transformed vertebrates from passive filter-feeders into active predators. Jawed fishes, including cartilaginous and bony fish, diversified rapidly. Bony fish developed a strong, lightweight skeleton that supported larger body sizes.
The transition from water to land was first taken by amphibians that evolved from lobe-finned fish ancestors around 365 million years ago. While these early tetrapods could live on land, they remained tied to water for reproduction. The development of the amniotic egg, with its protective membrane and shell, was the next innovation. This allowed reptiles to lay their eggs on land, freeing them from aquatic environments and enabling them to colonize diverse terrestrial habitats.
From these early land-colonizing reptiles, several lineages diverged. One path led to the evolution of modern reptiles and birds, while another lineage gave rise to mammals. These groups further adapted with features like feathers, hair, and changes in metabolism, leading to today’s diversity of vertebrate life.
The Human Connection
Humans are members of the phylum Chordata, exhibiting all four defining characteristics during embryonic development. These features, though modified or absent in our adult form, provide clear evidence of our evolutionary heritage. Tracing these structures reveals our deep connection to the rest of the phylum.
The notochord appears in the human embryo, providing support and signaling for the developing nervous system. As development proceeds, the vertebral column forms around the notochord, which largely disappears. Remnants persist in adults as the pulpy nucleus of the intervertebral discs that cushion our spine.
Our dorsal hollow nerve cord develops into the brain and spinal cord. These structures form the central nervous system, the control center for the entire body.
Pharyngeal slits also appear in the human embryo as grooves in the neck region. Instead of developing into gills, these structures are repurposed to form parts of the middle ear, Eustachian tubes, and various glands in the neck.
Finally, a post-anal tail is visible during human embryonic development, extending beyond the anal region. This tail is a clear link to our tailed ancestors. By the end of the embryonic period, this structure regresses, and its remaining vertebrae fuse to form the coccyx, or tailbone.