The animal kingdom is broadly divided into two major groups based on fundamental differences in their earliest embryonic development. This classification system separates organisms into Protostomes and Deuterostomes, representing two ancient and highly successful evolutionary paths. The names, derived from Greek, hint at the central difference in how the digestive tract forms during the first stages of life. Examining the developmental processes that occur immediately after fertilization reveals which of these two superphyla humans belong to.
Key Developmental Differences
The primary features distinguishing Protostomes (“mouth first”) from Deuterostomes (“mouth second”) are defined by the developmental fate of three embryonic structures. The first difference is the destiny of the blastopore, which is the initial opening that forms during gastrulation. In protostomes, this opening becomes the organism’s mouth, while the anus forms later at the opposite end of the embryo. Conversely, in deuterostomes, the blastopore develops into the anus, and the mouth forms secondarily.
Another defining characteristic is the pattern of cell division, known as cleavage, in the early embryo. Protostomes exhibit spiral cleavage, where new cells are offset and nested in the furrows of the older cells, giving a staggered appearance. This is determinate cleavage, meaning that the developmental fate of each embryonic cell is fixed very early on. If an early cell is isolated, the remaining embryo will develop missing parts because that cell’s contribution was already determined.
Deuterostomes, however, display radial cleavage, where the cell divisions occur parallel or perpendicular to the main axis of the embryo, stacking the cells directly on top of one another. This pattern is associated with indeterminate cleavage, where the fate of the early cells is not set. Because each cell retains the potential to develop into a complete organism, this indeterminate cleavage is the mechanism that allows for the formation of identical twins in humans and other deuterostomes.
The third difference lies in how the coelom, or body cavity, forms within the mesoderm layer. Protostomes utilize schizocoely, where a solid mass of mesodermal tissue splits open to create the internal cavity. Deuterostomes primarily use enterocoely, where the coelom originates from outpockets of the archenteron, the primitive gut that forms during gastrulation. These pouches pinch off and expand to form the body cavity.
Why Humans Are Classified as Deuterostomes
Humans are classified as deuterostomes because our embryonic development follows the ancestral pattern of the Deuterostomia superphylum. As members of the phylum Chordata, all vertebrates, including humans, share these fundamental developmental features. During gastrulation in the early human embryo, the initial opening, the blastopore, becomes the site of the future anus.
The earliest cell divisions in the human embryo follow the radial and indeterminate cleavage pattern characteristic of deuterostomes. This regulative development means that the removal or manipulation of an early embryonic cell does not automatically result in a defective organism. The cells are capable of adjusting their developmental paths, which allows for the formation of identical multiples.
The formation of the coelom in the human embryo aligns with the deuterostome pattern, though it is highly modified in mammals. While the specific process is complex, the underlying ancestral mechanism is derived from the enterocoelous development seen in simpler deuterostomes. These three shared characteristics—blastopore fate, cleavage pattern, and coelom origin—solidify the human position within the Deuterostome lineage.
Where These Classifications Fit in the Animal Kingdom
The division into Protostomes and Deuterostomes provides a high-level framework for classifying the vast diversity of life. Protostomes represent the largest and most species-rich branch of the animal kingdom. This group includes well-known phyla such as Arthropoda, which contains all insects, spiders, and crustaceans, and Mollusca, which encompasses snails, clams, and octopuses.
Other major protostome groups are Annelida, the segmented worms like earthworms, and Nematoda, the roundworms. Together, these phyla constitute the vast majority of all animal species on Earth. Deuterostomes contain significantly fewer species but include the largest and most familiar animals.
The deuterostome phyla include Echinodermata, which are marine animals like sea stars and sea urchins, and the phylum Chordata, which contains all vertebrates. The inclusion of humans, fish, birds, and all other backboned animals within Chordata demonstrates the close evolutionary link shared by this developmental pattern.