The origin of chordates, the early ancestors of vertebrates, is a subject that has fascinated zoologists for many years. While the exact details of their emergence remain elusive due to the absence of fossil remains, it is widely accepted that chordates evolved from invertebrates. The early chordate ancestors were soft-bodied organisms, leaving no tangible evidence to provide direct insights into their origin. Consequently, scientists must rely on the examination of similarities between lower chordates (protochordates) and invertebrates to understand the evolutionary path of chordates.

Several structural features shared by both lower chordates and invertebrates suggest a common ancestry. Bilateral symmetry, which refers to the body plan exhibiting a left-right symmetry axis, is observed in both groups. This shared characteristic hints at their evolutionary connection and indicates that they likely originated from a common ancestor.

Another common feature is the anteroposterior body axis, which refers to the arrangement of body parts along a front-to-back orientation. The presence of this axis in both lower chordates and invertebrates suggests a shared evolutionary history. Additionally, both groups possess a triploblastic coelomate condition. Triploblastic refers to the presence of three germ layers during development, while coelomate indicates the presence of a fluid-filled body cavity called a coelom. These shared traits further support the notion of a common origin for chordates and invertebrates.

Metameric segmentation, characterized by the division of the body into repeated segments, is another feature that both lower chordates and invertebrates exhibit. This segmentation can be observed in various aspects of their anatomy, such as the arrangement of muscles, nerves, and other internal structures. The presence of metameric segmentation in both groups suggests an ancestral link and provides valuable clues about the evolutionary history of chordates.

Although the absence of fossil evidence from early chordate ancestors poses challenges to fully understanding their origin, the resemblances shared with invertebrates offer important insights. By examining the structural features common to both groups, such as bilateral symmetry, anteroposterior body axis, triploblastic coelomate condition, and metameric segmentation, scientists can infer a common ancestry between chordates and invertebrates.

As research and discoveries continue to unfold, further investigations into the molecular and genetic aspects of chordate development may shed more light on the precise origin of these fascinating creatures. By combining paleontological and molecular evidence, scientists strive to uncover the missing pieces of the puzzle, ultimately enhancing our understanding of the origin and evolution of chordates.

Theories of invertebrate ancestry of chordates

The origin of chordates from invertebrates has been a subject of much speculation and debate, leading to the formulation of various theories. While none of these theories can be considered fully satisfactory or convincing, they hold historical value in understanding the evolution of chordates. Among the invertebrate phyla proposed as potential ancestors of chordates are Coelenterata, Nemertean, Phoronida, Annelida, Arthropoda, and Echinodermata. However, the echinoderm theory has gained some acceptance and will be explored further as a potential deuterostome line of chordate ancestry.

The classification of the Bilateria, the largest group of animal phyla, is divided into two major divisions: Protostomia and Deuterostomia. This division is based on fundamental differences in embryonic and larval development and is believed to represent two major evolutionary lines within the animal kingdom.

The deuterostome line of chordate evolution shows common features among all Deuterostomia, providing strong evidence of a close evolutionary relationship between the three principal deuterostome phyla: Echinodermata, Hemichordata, and Chordata. Several characteristics support this notion. Firstly, early cleavages of the zygote in deuterostomes are indeterminate, meaning that each early blastomere has the potential to develop into a whole adult if separated. Additionally, the blastopore of the gastrula in deuterostomes forms the anus, while the mouth is formed as a secondary opening, which is a unique feature not seen in protostomes.

The formation of coelom in deuterostomes is also significant. Pockets or folds arise from the endoderm of the developing archenteron of the embryo, and the fusion of spaces within these pockets forms the coelom. This process is known as enterocoelous development, except in vertebrates. Another shared characteristic is the structural resemblance between the pelagic larvae of echinoderms and hemichordates, suggesting a closer connection between these groups and chordates. It is worth noting that vertebrates do not have floating larvae, as this feature has been lost during their evolution.

Biochemical evidence further supports the relationship between deuterostomes. For example, all deuterostomes utilize creatine, a phosphagen, in the energy cycle of muscular contraction. In contrast, invertebrates use arginine as their phosphagen. However, certain hemichordates and echinoids utilize both arginine phosphate and creatine phosphate. This suggests that hemichordates may serve as a connecting link between chordates and non-chordates.

Serological tests examining protein relationships also demonstrate a closer affinity between the three deuterostome phyla than with other phyla. While the precise relationship among the three deuterostome phyla remains unknown, it is widely accepted that they share a common evolutionary history.

Various proposals have been put forth to explain the deuterostome line of chordate evolution, but none have been definitively proven. Further research and investigation are necessary to unravel the intricacies of the evolutionary relationship between these groups and to gain a deeper understanding of the origin of chordates from invertebrates.

Echinoderm ancestry

• Based on various lines of evidence, there have been attempts to establish a potential ancestry of chordates directly from primitive echinoderms or echinoderm larvae. The resemblance between the hemichordate larva, known as tornaria, and the larva of echinoderms, such as bipinnaria or dipleurula, has been particularly striking. In fact, when the tornaria larva was first discovered, it was initially mistaken for an echinoderm.
• Both the tornaria and echinoderm larvae are small, transparent, free-swimming, and bilaterally symmetrical. They share similar ciliated bands arranged in loops, a dorsal pore, sensory cilia at the anterior end, and a complete digestive system with a ventral mouth and posterior anus. These remarkable similarities in larval form and features led researchers like Johannes Muller and Bateson to propose a common ancestry for echinoderms and hemichordates.
• However, the presence of an apical plate with eye spots in the tornaria larva has raised doubts about the direct common ancestry of echinoderms and hemichordates. In response to this, Garstang and de Beer put forth the Neotenous larva theory, suggesting that the auricularia larva of echinoderms might have undergone sexual maturity, giving rise to chordates.
• Garstang (1894) envisioned a scenario where the ciliated bands and underlying nervous tissue of the auricularia larva of echinoderms concentrate to form ridges, leaving a groove between them. Subsequently, the lips of the groove would fuse, resulting in the formation of a tube. This tube would resemble the nervous system of chordates.
• The fossil records of Carpoid echinoderms from the Cambrian and Ordovician periods also provide insights into a potential echinoderm ancestry. Torsten and Gislen suggested that Carpoid echinoderms might have evolved from tornaria-like creatures that started to settle down and adopt a sedentary lifestyle. The water vascular system of echinoderms could have developed from the ciliated grooves of these creatures. Additionally, it has been claimed that in the Lower Silurian period, one Carpoid echinoderm had a calyx (body) perforated by a series of 16 small apertures, which can be compared to the gill slits of Branchiostoma (a lancelet or amphioxus, a type of chordate).
• The similarity observed in the larval forms of echinoderms and hemichordates further supports the idea that both groups might have descended from a common ancestor. Although the exact details of the echinoderm ancestry of chordates remain uncertain, these lines of evidence provide valuable insights into the potential evolutionary connection between these groups and shed light on the origin of chordates. Further research and investigation are necessary to continue unraveling the intricate evolutionary history of chordates and their relationship to echinoderms.

Hemichordate ancestry

• While there is suggestive evidence that the early evolutionary stage of the Deuterostomia group, to which chordates belong, was likely sessile or sedentary, the exact ancestry of chordates from hemichordates remains uncertain. Hemichordates, which are sedentary organisms, do possess some similarities to chordates, such as pharyngeal gill slits and a hollow dorsal nerve cord. However, the presence of a true notochord, a defining characteristic of chordates, is doubtful in hemichordates. Furthermore, their adult body plan significantly differs from that of vertebrates.
• Due to these differences and uncertainties, it is considered highly unlikely that hemichordates serve as direct ancestors of vertebrates. Instead, they are classified under a separate phylum of their own. While hemichordates share certain features with chordates, such as the presence of gill slits and a dorsal nerve cord, their overall body plan and the absence of a true notochord indicate that their relationship to chordates is more distant.
• As research continues and more discoveries are made, our understanding of the evolutionary relationships within the Deuterostomia group may evolve. However, at present, the idea of hemichordates as likely ancestors of vertebrates seems improbable, and they are recognized as a distinct phylum with their own unique characteristics and evolutionary trajectory.

Urochordate ancestry

• The urochordate, or ascidian theory of vertebrate origin, proposed by W. Garstang in 1928 and further developed by N.J. Berrill and others, suggests a connection between urochordates (ascidians) and the evolutionary origins of vertebrates. Ascidians, in their adult form, exhibit the primitive sessile marine and filter-feeding characteristics of ancestral chordates. However, the stark differences in body plans between ascidians and vertebrates make it difficult to envision a direct evolutionary transformation from adult ascidians to vertebrates.
• On the other hand, the larvae of ascidians display tadpole-like characteristics. They are elongated, bilaterally symmetrical, and free-swimming creatures with pharyngeal gill slits, a notochord, a dorsal hollow nerve tube, and a muscular post-anal tail. These larvae represent slightly modified versions of the ancestral chordate, which gave rise to the evolutionary lineage leading to vertebrates. According to this theory, certain larvae failed to undergo metamorphosis into adults and instead became sexually mature precociously, retaining their larval features. These neotenous larvae then evolved into cephalochordates (such as lancelets) and eventually gave rise to vertebrates. The theory also suggests that the sessile nature observed in primitive chordates, pterobranch hemichordates, and certain echinoderms is a result of their common ancestry with ascidians.
• However, the ascidian theory of chordate origin has some drawbacks. One significant limitation is that it assumes sessile urochordates to be ancestral to chordates, while in reality, their sessile lifestyle represents a specialized condition that occurs in various lineages throughout the Animal Kingdom. This specialization raises questions about the direct ancestral relationship between ascidians and vertebrates.
• While the ascidian theory provides valuable insights into the potential evolutionary connections between ascidians and chordates, it is not considered a perfect explanation for the origins of chordates. Further research and investigation are necessary to refine our understanding of the evolutionary relationships between urochordates and vertebrates and to uncover the precise origins of chordates within the diverse array of marine invertebrate lineages.

Cephalochordate ancestry

• Cephalochordates, particularly lancelets (Branchiostoma lanceolatum), are a fascinating group of animals that possess the three fundamental chordate features in a simplified diagrammatic form. They have been regarded as important models for understanding prevertebrate structures. The logical structure of a model prevertebrate, as proposed by Colbert and Homer Smith’s hypothetical protovertebrate reconstruction in his book “From Fish to Philosopher” (1953), both bear resemblance to the living Amphioxus (Branchiostoma).
• However, there are significant differences between cephalochordates and vertebrates that suggest cephalochordates are not direct ancestors but rather provide insights into the ancestral body plan of vertebrates. One notable distinction is the excretory system of cephalochordates, which consists of flame cells called solenocytes. This system is entirely different from the mesodermal vertebrate kidneys and is ectodermal in nature. Therefore, the solenocytes of cephalochordates are not homologous to the kidneys of vertebrates.
• Additionally, cephalochordates lack strong cephalization (the development of a distinct head region) and specialized sense organs. These features set them apart from vertebrates, which typically exhibit well-developed sensory structures. Furthermore, the unique forward extension of the notochord in cephalochordates is another distinguishing characteristic. These differences suggest that cephalochordates and vertebrates may have evolved along divergent paths from a common remote ancestor.
• Although cephalochordates provide valuable insights into the likely ancestral body plan of vertebrates, they are not themselves direct ancestors. They represent an independent lineage that has retained certain primitive chordate characteristics. To uncover the precise ancestry of vertebrates and the evolutionary transitions leading to their complex body plans, further research and investigation into the diverse array of chordate groups are necessary.

Barrington’s hypothesis

• Barrington’s hypothesis, proposed by E.J.W. Barrington in 1965, offers a plausible explanation for the evolution of chordates within the deuterostome lineage. According to this hypothesis, the common ancestor of echinoderms and chordates was likely a small, sessile or semisessile lophophorate or arm-feeding creature. This ancestral organism utilized ciliary feeding by trapping food particles in its tentacles. From this ancestral stalk, early stalked echinoderms and pogonophores were derived.
• The next stage in this evolutionary process was the emergence of a sessile fiber feeder or stem chordate. The external tentacles were replaced by an internal filtering apparatus, where food was trapped inside the pharynx that developed external gill-slits and a mucus-secreting endostyle. Cephalodiscus, a living pterobranch hemichordate, represents a transitional stage between the two modes of feeding, exhibiting both a single pair of gill-slits and a crown of tentacles.
• The development of a perforated pharynx with an internal food-trapping mechanism, known as pharyngotremy, led to the evolution of free-living hemichordates and the sessile ancestral urochordates, also known as tunicates. Some ancestral tunicates, instead of producing ciliated larvae like the early groups, gave rise to tadpole larvae with all the typical somatic features of chordates.
• According to Garstang’s interpretation, these larvae underwent elongation and size increase, with the longitudinal ciliary bands shifting to a mid-dorsal position and transforming into the hollow nerve cord. The adoral cilia developed into the endostyle, and muscle fibers evolved in the tail. Through a process called paedogenesis, in which the larval stage becomes reproductively mature and suppresses the sessile adult stage, these chordate larvae became the ancestors of cephalochordates (such as Branchiostoma), vertebrates, and larvaceans. It is suggested that these three cases of parallel evolution emerged from this common ancestor.
• Barrington’s hypothesis provides an intriguing framework for understanding the evolutionary transitions within the deuterostome lineage, shedding light on the possible origins of chordates from a lophophorate-like ancestor. Further research and evidence are necessary to validate and refine this hypothesis and to gain a comprehensive understanding of the precise pathways that led to the diverse array of chordate groups we see today.

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