The animal kingdom is diverse, and scientists categorize animals into broad groups based on shared characteristics. A fundamental classification method involves examining embryonic development, specifically how earliest structures form. This blueprint reveals deep evolutionary relationships, classifying many complex animals, including humans, as deuterostomes. Understanding why chordates, including vertebrates, fit this classification provides insight into their biological heritage.
What Defines a Chordate?
Chordates are a diverse group of animals distinguished by shared anatomical features present at some point in their life cycle. These features include:
- A notochord: a flexible, rod-shaped structure providing skeletal support. In vertebrates, this is largely replaced by the vertebral column in adulthood.
- A dorsal hollow nerve cord: located along the back, above the notochord, developing into the brain and spinal cord in vertebrates.
- Pharyngeal slits: openings in the throat region. In aquatic chordates, these may develop into gills; in terrestrial forms, they appear only during embryonic stages.
- A post-anal tail: a muscular tail extending beyond the anus, though it may be reduced or absent in some adult chordates.
- An endostyle: or its derivative the thyroid gland, a groove in the pharynx involved in filter-feeding or iodine metabolism.
The Hallmarks of Deuterostome Development
Deuterostome classification hinges on specific embryonic development patterns. One key aspect is radial cleavage, where early cell divisions occur either parallel or perpendicular to the embryo’s main axis, resulting in tiers of cells that align neatly above each other. This contrasts with other developmental patterns where cells are offset.
Deuterostomes exhibit indeterminate cleavage. This means the developmental fate of individual cells in the early embryo is not fixed. If an early embryonic cell were separated, it could potentially develop into a complete organism, which is why identical twins are possible in some deuterostomes.
Perhaps the most defining characteristic of deuterostome development concerns the fate of the blastopore, the first opening that forms during gastrulation. In deuterostomes, this blastopore develops into the anus. The mouth then forms later, as a second opening at the opposite end of the digestive tract. The coelom, or body cavity, in deuterostomes typically forms through a process called enterocoely, where pouches of the primitive gut pinch off to create the mesoderm-lined cavity.
Deuterostomes Compared to Protostomes
To appreciate deuterostome characteristics, it helps to compare their embryonic development with protostomes, the other major group of bilaterally symmetrical animals. The fundamental difference lies in the fate of the blastopore. In protostomes, the blastopore develops into the mouth, with the anus forming later as a second opening. This gives protostomes their name, meaning “mouth first.”
Regarding early cell division, protostomes typically exhibit spiral cleavage, where cells are rotated relative to each other, creating an offset arrangement. Their cleavage is often determinate, meaning the developmental path of each early embryonic cell is already set. If a cell is removed, the developing embryo may have missing parts.
Additionally, the coelom in many protostomes forms through a process called schizocoely. This involves a solid mass of mesoderm splitting to create the body cavity. These contrasting developmental pathways illustrate the deep evolutionary divergence between these two large animal groups, influencing their fundamental body plans.
Why Chordates Fit the Deuterostome Profile
Chordates consistently exhibit deuterostome developmental traits. During their early embryonic stages, chordates undergo radial cleavage. The cellular divisions align perpendicularly or parallel to the animal-vegetal axis, resulting in stacked, aligned cells.
This radial cleavage is coupled with indeterminate cleavage, meaning that the early embryonic cells retain developmental flexibility. For example, if cells from an early chordate embryo are separated, each cell can still develop into a complete, albeit smaller, organism.
Crucially, the blastopore of a developing chordate embryo forms the anus. The mouth subsequently develops from a different opening later in the developmental process. This “anus first, mouth second” pattern is the defining feature of deuterostomes and directly aligns chordates with this group. Furthermore, the coelom in chordates typically develops via enterocoely, where the mesoderm arises from outpocketings of the embryonic gut, completing their deuterostome developmental profile.
Other Major Deuterostome Groups
Chordates are a prominent deuterostome group, part of a larger evolutionary lineage including other significant animal phyla. Echinoderms, a group of marine invertebrates, are also classified as deuterostomes. This phylum includes familiar creatures like starfish, sea urchins, and sea cucumbers.
Despite their radial symmetry as adults, echinoderms share the same fundamental embryonic developmental patterns with chordates, including radial and indeterminate cleavage, and the anus-first formation from the blastopore. Another related group is the Hemichordates, which include acorn worms. These marine animals share some chordate features like pharyngeal slits, and their developmental patterns also place them within the deuterostome superphylum. These groups collectively demonstrate the shared evolutionary history characterized by a distinct mode of embryonic development.