What Do Many Organisms With Deuterostome Development Have In Common

Deuterostomes, a diverse group of animals, share a fundamental characteristic during their embryonic development: the blastopore, the opening that forms during gastrulation, develops into the anus, while the mouth forms later. This developmental pattern, known as deuterostome development, is just one of several commonalities that unite this group, which includes echinoderms (such as starfish and sea urchins), chordates (including vertebrates like humans), and hemichordates (acorn worms). Beyond their unique embryological development, deuterostomes share a suite of other fascinating traits in their anatomy, physiology, and evolutionary history. Let's dive deeper into these shared characteristics and explore what makes deuterostomes so special.

Defining Deuterostomes: More Than Just an Anus First

Deuterostome development is the cornerstone of this group, setting them apart from protostomes, where the blastopore develops into the mouth. But this developmental difference is just the tip of the iceberg. Deuterostomes also typically exhibit:

Radial Cleavage: During early embryonic development, the cells divide parallel or perpendicular to the animal-vegetal axis, resulting in neatly aligned tiers of cells.
Indeterminate Cleavage: The fate of each cell in the early embryo is not determined early on. If cells are separated at the two- or four-cell stage, each cell has the potential to develop into a complete embryo. This characteristic is crucial for the development of identical twins in humans.
Enterocoelous Coelom Formation: The coelom, or body cavity, forms from outpouchings of the archenteron, the primitive gut. These pouches eventually pinch off to form the coelomic cavities.

These developmental features, along with molecular and genetic similarities, provide strong evidence for the shared ancestry of deuterostomes.

Key Phyla Within Deuterostomia: A Closer Look

To fully appreciate the shared characteristics of deuterostomes, it's important to understand the major phyla within this group:

Echinodermata (Starfish, Sea Urchins, Sea Cucumbers, etc.): Exclusively marine animals, echinoderms are characterized by their radial symmetry (often pentaradial) as adults, a water vascular system for locomotion and feeding, and an endoskeleton made of calcareous ossicles.
Chordata (Vertebrates, Tunicates, Lancelets): Perhaps the most familiar deuterostomes, chordates possess a notochord (a flexible rod for support), a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail at some point in their development. Vertebrates, a subgroup of chordates, have a vertebral column that replaces the notochord during development.
Hemichordata (Acorn Worms, Pterobranchs): Marine worms that share characteristics with both echinoderms and chordates. They possess pharyngeal slits and a rudimentary notochord-like structure called a stomochord.
Xenacoelomorpha (Xenoturbella, Acoelomorphs): A relatively newly recognized group of deuterostomes. They are simple, bilaterally symmetrical worms with a sac-like gut.

Understanding the diversity within Deuterostomia allows us to better identify the common threads that run through the group.

Shared Anatomical and Physiological Traits

While deuterostome development is the defining feature, several other anatomical and physiological characteristics are widely shared among these diverse animals:

1. Coelom: The Body Cavity

The presence of a coelom is a significant feature in most deuterostomes. The coelom is a fluid-filled body cavity that lies between the body wall and the digestive tract. It provides several advantages:

Organ Protection: The coelom cushions internal organs, protecting them from injury.
Hydrostatic Skeleton: In some deuterostomes, like echinoderms and hemichordates, the coelom acts as a hydrostatic skeleton, providing support and enabling movement.
Circulation: The coelomic fluid can facilitate the transport of nutrients and waste products.
Space for Organ Development: The coelom provides space for the development and growth of internal organs.

2. Endoskeleton: Internal Support

While not universal, an endoskeleton is a common feature in many deuterostomes:

Echinoderms: Possess an endoskeleton made of calcareous ossicles, providing support and protection.
Chordates: Vertebrates have an endoskeleton made of bone or cartilage, providing strong internal support and allowing for complex movement.

The endoskeleton allows for greater size and complexity compared to animals with an exoskeleton.

3. Pharyngeal Slits: Filter Feeding and Respiration

Pharyngeal slits, openings in the pharynx (the region of the digestive tract just behind the mouth), are another shared characteristic:

Chordates: Pharyngeal slits are present in all chordate embryos and function in filter feeding in invertebrate chordates like tunicates and lancelets. In aquatic vertebrates like fish, the pharyngeal slits develop into gills for respiration.
Hemichordates: Possess pharyngeal slits that are used for filter feeding and respiration.
Echinoderms: While adult echinoderms do not have pharyngeal slits, there is evidence that their ancestors did.

The presence of pharyngeal slits in diverse deuterostomes suggests that this feature was present in their common ancestor.

4. Water Vascular System: Unique to Echinoderms

While not shared with all deuterostomes, the water vascular system is a defining characteristic of echinoderms and deserves mention due to its complexity and importance:

Function: The water vascular system is a network of hydraulic canals that branch into tube feet. These tube feet are used for locomotion, feeding, gas exchange, and excretion.
Mechanism: Water enters the system through a sieve-like structure called the madreporite and circulates through canals, powering the tube feet.

This unique system highlights the evolutionary innovations within the deuterostome lineage.

5. Complex Nervous System: From Nerve Nets to Brains

Deuterostomes exhibit a wide range of nervous system complexity:

Echinoderms: Have a relatively simple nervous system with a nerve net and radial nerves. They lack a centralized brain.
Hemichordates: Possess a dorsal and ventral nerve cord, but lack a well-defined brain.
Chordates: Have a dorsal hollow nerve cord that develops into the brain and spinal cord in vertebrates. This centralized nervous system allows for complex behavior and intelligence.

The evolution of the nervous system within deuterostomes reflects the increasing complexity of their lifestyles and environments.

6. Similar Molecular and Genetic Mechanisms

Beyond anatomical features, deuterostomes share similar molecular and genetic mechanisms that control development and physiology:

Hox Genes: These genes play a crucial role in determining body plan and segment identity in animals. Deuterostomes have similar Hox gene clusters, suggesting a shared ancestry.
Signaling Pathways: Signaling pathways like the Wnt and BMP pathways are involved in various developmental processes in deuterostomes. The conservation of these pathways highlights their importance in deuterostome evolution.
Transcription Factors: Similar transcription factors regulate gene expression in deuterostomes, influencing development and cell differentiation.

These molecular similarities provide strong evidence for the close evolutionary relationship between deuterostomes.

Evolutionary History: Tracing the Deuterostome Lineage

Understanding the evolutionary history of deuterostomes helps to explain the shared characteristics and the diversity within the group. The fossil record and molecular data suggest that:

Common Ancestry: Deuterostomes share a common ancestor with protostomes, but diverged early in animal evolution.
Early Deuterostomes: The earliest deuterostomes were likely simple, bilaterally symmetrical animals with pharyngeal slits and a coelom.
Echinoderm-Hemichordate Ancestry: Echinoderms and hemichordates are thought to be more closely related to each other than to chordates. This is supported by molecular data and some shared developmental features.
Chordate Evolution: Chordates evolved from a deuterostome ancestor with a notochord, dorsal hollow nerve cord, and pharyngeal slits. The evolution of vertebrates was a major event in chordate evolution, leading to the diversification of fishes, amphibians, reptiles, birds, and mammals.

The evolutionary history of deuterostomes is a complex and ongoing area of research, but it provides valuable insights into the origins of their shared characteristics.

Specific Examples of Shared Traits Across Different Deuterostome Groups

Let's explore how these shared traits manifest in different deuterostome groups:

Echinoderms and Chordates: While seemingly disparate, both groups share the fundamental deuterostome developmental pattern. Furthermore, some larval forms of echinoderms exhibit bilateral symmetry, hinting at their evolutionary connection to bilaterally symmetrical chordates. Genetic studies also reveal surprising similarities in gene expression during development.
Hemichordates and Chordates: The presence of pharyngeal slits is a striking similarity between hemichordates and chordates. In both groups, these slits are used for filter feeding or respiration. Hemichordates also have a stomochord, a structure similar to the notochord of chordates, although its homology is still debated.
All Three Groups (Echinoderms, Hemichordates, Chordates): The coelom is a shared feature that plays important roles in organ development and support. While the specific function of the coelom may vary, its presence in all three groups underscores its importance in deuterostome evolution.

Challenges to the Deuterostome Hypothesis

While the deuterostome clade is widely accepted, there have been some challenges to this hypothesis:

Xenacoelomorpha: The placement of Xenacoelomorpha within Deuterostomia was initially controversial. These simple worms lack many of the typical deuterostome characteristics, such as a well-developed coelom and pharyngeal slits. However, molecular data strongly supports their inclusion in Deuterostomia. Some researchers suggest that Xenacoelomorpha may represent a basal deuterostome lineage, having lost some of the more complex features over time.
Developmental Variability: While deuterostome development is a defining feature, there is some variability in how it occurs in different groups. For example, the mode of coelom formation can vary. These variations highlight the plasticity of developmental processes and the challenges of reconstructing evolutionary history based solely on developmental data.

Despite these challenges, the overwhelming evidence supports the monophyly of Deuterostomia, meaning that all deuterostomes share a single common ancestor.

The Significance of Understanding Deuterostome Development

Understanding deuterostome development and shared characteristics is crucial for several reasons:

Evolutionary Biology: It provides insights into the evolutionary relationships between major animal groups and the origins of vertebrate body plan.
Developmental Biology: It helps us understand the genetic and molecular mechanisms that control development and how these mechanisms have evolved over time.
Medicine: Studying deuterostome development can provide insights into human development and disease. For example, understanding the genes involved in notochord formation in chordates can help us understand the development of the vertebral column in humans.
Conservation Biology: Understanding the evolutionary history and relationships of deuterostomes can inform conservation efforts. By recognizing the shared ancestry and unique adaptations of different deuterostome groups, we can better protect their biodiversity.

Conclusion: A Legacy of Shared Ancestry

Deuterostomes, united by their unique developmental pattern and a suite of shared anatomical, physiological, and molecular characteristics, represent a major branch in the animal kingdom. From the familiar vertebrates to the enigmatic echinoderms and hemichordates, these animals showcase the power of evolution to shape diverse forms while retaining the legacy of shared ancestry. By continuing to explore the intricacies of deuterostome development and evolution, we can gain a deeper understanding of the history of life on Earth and our own place within it. The study of deuterostomes is not just about understanding animals; it's about understanding the fundamental principles of life itself.