Humans belong to the group of animals known as deuterostomes. This classification stems from fundamental differences in early embryonic development, which broadly divides the animal kingdom into two major groups: protostomes and deuterostomes. The differing developmental pathways, particularly in the formation of the digestive tract and body cavity, define these two superphyla.
Protostomes
Protostomes are characterized by specific patterns during their early embryonic development. In these organisms, the blastopore, the initial indentation formed during gastrulation, develops into the mouth. The anus then forms later, at a different location on the embryo. This developmental sequence is a defining characteristic, giving protostomes their name, which translates to “first mouth”.
Another distinguishing feature of protostomes is their cleavage pattern, the series of cell divisions immediately following fertilization. Protostomes typically exhibit spiral cleavage, where cells divide at an oblique angle to the polar axis of the embryo. This results in daughter cells that lie in the furrows between the parent cells. Furthermore, protostome cleavage is often determinate, meaning the developmental fate of each embryonic cell is fixed very early in development; if a cell is removed, the embryo will not develop completely.
The formation of the coelom, or body cavity, also differs in protostomes. Their coelom typically forms through a process called schizocoely. In this method, a solid mass of mesoderm, one of the primary germ layers, splits to create the coelomic space. Examples include mollusks, annelids (segmented worms), and arthropods (insects, spiders, crustaceans).
Deuterostomes
Deuterostomes follow a distinct developmental pathway. In deuterostomes, the blastopore, the initial opening that forms during gastrulation, develops into the anus. The mouth then forms later, at a secondary opening, which is why deuterostome means “second mouth”.
The cleavage pattern in deuterostomes is radial, where cells divide either parallel or perpendicular to the polar axis of the embryo. This results in tiers of cells that are directly aligned over one another. Additionally, deuterostome cleavage is generally indeterminate, meaning that the developmental fate of early embryonic cells is not fixed. If a cell is separated from the early embryo, it retains the potential to develop into a complete organism, which is the biological basis for identical twin formation.
Coelom formation in deuterostomes occurs through a process called enterocoely. Here, the coelom develops from outpocketings of the archenteron, the primitive gut. These pouches pinch off, eventually forming the mesodermal layer and the coelomic cavity. Important examples of deuterostome animal groups include echinoderms (like starfish and sea urchins) and chordates, a phylum that includes all vertebrates.
Humans: A Deuterostome Example
Human embryonic development aligns with the characteristics of deuterostomes. Our early development clearly demonstrates the defining features of this group, placing us firmly within the Deuterostomia superphylum.
During human gastrulation, the blastopore develops into the anus. This means that the first major opening formed in the human embryo, the blastopore, will differentiate into the posterior opening of the digestive tract, with the mouth forming later. This fundamental process, where the anus precedes the mouth in development, is a hallmark of deuterostomes.
The cleavage pattern in early human embryos is also consistent with deuterostome development. Human embryos undergo radial cleavage, where initial cell divisions occur in planes that are either parallel or perpendicular to the main axis of the embryo. Furthermore, human embryonic cells exhibit indeterminate cleavage, meaning that at early stages, the cells are totipotent or pluripotent; they can each develop into a complete organism or contribute to any tissue type. This regulative development allows for remarkable flexibility, such as the ability to form identical twins if early embryonic cells separate.
The formation of the coelom in humans follows the enterocoelous pattern, characteristic of deuterostomes. The mesoderm, which forms the lining of the body cavity, arises from outpocketings of the primitive gut, or archenteron. These pouches expand and fuse to create the coelomic space, which eventually gives rise to major body cavities like the thoracic and abdominal cavities. These detailed developmental patterns confirm that humans are indeed deuterostomes, highlighting a deep evolutionary connection to other animals within this classification.