When scientists classify animals, one major distinction comes down to how they develop as embryos. This splits the animal kingdom into two big groups: Deuterostomia and Protostomia.
The differences between protostomes and deuterostomes are fundamental to understanding how animal life evolved and diversified into the huge range of creatures we see today. These differences arise during the earliest stages of development.
We’ll dive into the key distinctions, focusing on what happens to the blastopore (the first opening that forms), differences in cell division (cleavage), and how the body cavity (coelom) develops.
Blastopore Fate: Mouth or Anus?
One of the key differences between protostomes and deuterostomes lies in what happens to the blastopore during embryonic development. The blastopore is the first opening that forms in the developing embryo, and its fate determines whether an animal is classified as a protostome or a deuterostome.
Protostomes: “Mouth First”
In protostomes, the blastopore becomes the mouth. If an anus develops later, it forms at the opposite end of the embryo. The word “protostome” literally means “mouth first.”
Examples of protostome phyla include mollusks (like snails and clams), annelids (segmented worms), and arthropods (insects, crustaceans, and spiders).
Deuterostomes: “Mouth Second”
In deuterostomes, the blastopore becomes the anus. The mouth forms later at the opposite end of the embryo. The word “deuterostome” means “mouth second.”
Examples of deuterostome phyla include echinoderms (starfish and sea urchins) and chordates (animals with a backbone, including humans).
Cleavage Patterns: Spiral vs. Radial
One key difference between protostomes and deuterostomes lies in the way their cells divide during the early stages of embryonic development. This is referred to as cleavage, and the pattern can be either spiral or radial.
Protostomes: Spiral and Determinate Cleavage
Protostomes exhibit what’s called spiral cleavage. This means that as the cells divide, they do so at an angle, creating a spiral arrangement in relation to each other. It’s like stacking oranges in a pyramid – each layer is slightly offset from the one below.
They also undergo determinate cleavage. This is a fancy way of saying that the developmental path of each cell is decided very early on. If you were to remove a single cell from an early protostome embryo, development would stop. The remaining cells wouldn’t be able to compensate for the missing one. This also makes it impossible for protostomes to form identical twins.
Deuterostomes: Radial and Indeterminate Cleavage
In contrast, deuterostomes display radial cleavage. Here, the cells divide in a way that’s parallel or perpendicular to the main axis of the embryo (the animal-vegetal axis). This results in a more symmetrical, radial arrangement.
Deuterostomes also have indeterminate cleavage. This means that the fate of each cell isn’t set in stone from the beginning. If you were to remove a cell from an early deuterostome embryo, the remaining cells could still develop into a complete, viable organism. This is how identical twins can form. Each cell has the potential to become a whole new individual.
How the coelom forms: schizocoely vs. enterocoely
The coelom is a fluid-filled cavity that develops in the mesoderm, or middle layer, of most animals. It acts as a protective cushion for the organs, and in some animals, it functions as a hydrostatic skeleton.
Protostomes and deuterostomes differ in how the coelom is formed.
Protostomes: schizocoelous coelom formation
In protostomes, the coelom forms through a process called schizocoely. During this process, the mesoderm splits apart to create the coelomic cavity.
Think of it like this: the mesoderm starts as a solid block of tissue, and then a space opens up within that block, creating the coelom.
Deuterostomes: enterocoelous coelom formation
In deuterostomes, the coelom forms through enterocoely. In enterocoely, the coelom develops from pouches that bud off from the archenteron. The archenteron is the primitive gut that forms during gastrulation, the stage when the cells of the blastula, an early form of the embryo, begin to differentiate.
Imagine the archenteron as a balloon. Small bubbles form on the side of the balloon, and these bubbles eventually pinch off and become the coelomic cavities.
Other key differences and what they tell us about evolution
The nervous systems of protostomes and deuterostomes also develop differently. Protostomes usually develop a ventral nerve cord, which runs along their belly. In contrast, chordate deuterostomes (like us!) develop a dorsal nerve cord, which runs along their back and becomes the spinal cord.
Molecular evidence, like the arrangement of Hox genes, also supports the idea of a fundamental split between these two groups early in animal evolution. Hox genes are a family of genes that control body plan development. The different patterns of these genes in protostomes and deuterostomes suggest they branched off from each other a very long time ago.
Scientists believe that both protostomes and deuterostomes evolved from a common ancestor, a simple organism that lived hundreds of millions of years ago. By studying the differences and similarities between these groups, we can learn more about the early evolution of animals and how the incredible diversity of life on Earth came to be.
Frequently Asked Questions
Are humans a protostome or deuterostome?
Humans are deuterostomes. This means that during our embryonic development, the blastopore (the first opening that forms) becomes our anus, and the mouth forms later. This is a key characteristic that distinguishes us from protostomes, which have the opposite development.
Do protostomes form a mouth first?
Yes, protostomes are characterized by the blastopore developing into the mouth first during embryonic development. The name “protostome” literally means “mouth first.” This is a defining feature that separates them from deuterostomes, where the blastopore becomes the anus.
What is a deuterostome in biology?
In biology, a deuterostome is a major group of animals characterized by a specific pattern of embryonic development. During gastrulation, the blastopore forms the anus, and the mouth develops later. Deuterostomes also exhibit radial cleavage and indeterminate cell fate during early development. This group includes echinoderms (like starfish) and chordates (which include vertebrates like us!).
Final Thoughts
To recap, protostomes and deuterostomes differ in key developmental processes. The blastopore becomes the mouth in protostomes, while it forms the anus in deuterostomes. Protostomes exhibit spiral cleavage, while deuterostomes have radial cleavage. And the coelom forms differently in each group.
Understanding these differences is crucial for tracing the evolutionary relationships between animals. As research continues, our understanding of protostomes and deuterostomes may evolve, further refining our knowledge of animal phylogeny.