Origin of Chordates – Dipleurula concept, Echinoderm theory

It is considered that the origin of chordates is deeply associated with the deuterostome lineage, where the early ancestors are sharing characters with hemichordates and echinoderms. In this group, the basic features such as a notochord, dorsal hollow nerve cord (DHNC), pharyngeal gill slits, post-anal tail and endostyle are present at some stage of development. These are the defining features and these structures together forms the axial framework in the body.

It is thought that the earliest chordate ancestor was a free-living marine form which was elongated and showing a simple body plan with a pharynx having many openings. This helped in filter feeding and it is the process which is seen in some modern hemichordates and amphioxus type animals.

It is believed that the formation of the notochord and dorsal nerve cord is the important evolutionary step. These structures developed along the dorsal side of the embryo and helped in producing a new larval form which was capable of active swimming. The tail region supported by notochord and segmented muscle blocks (myomeres) gave better locomotion. This is referred to as a major step towards active movement in the ancestral chordates.

Fossils from early Cambrian such as Pikaia and Haikouichthys show these characters and it indicates that the chordate body plan was already established during that time. These fossils reveal the segmented muscles and a rod-like structure which is identified as notochord.

In this step, the shift in the dorsoventral axis is also important because there is evidence that the pattern of Bmp-Chordin signalling got inverted compared to the protostomes. This indicates that the central nervous system was already beginning to be organized in the earlier deuterostome ancestors. In enteropneust hemichordates a collar-type neural tube is present and this provide information that some form of nervous system centralization was present even before true chordates evolved.

Earlier, tunicates were considered to represent the primitive chordate condition and it was suggested that the chordates originated by paedomorphosis of a sessile ancestor. But now it is shown by molecular studies that tunicates are highly modified and derived.

This means that the ancestral chordate was not a sessile form but rather a free-living animal with a simple elongated body. It is now understood that the basic chordate characters evolved in an active filter-feeding organism which had a complete life cycle without attachment to any substrate.

Theories of invertebrate ancestry of chordates

Theories Based on Protostome Derivation (The Inversion Hypothesis)

The inversion hypothesis is the idea that the chordate body plan is an inverted form of the ancestral protostome-like organism. It is the process where the dorsal–ventral orientation in early bilaterians is assumed to have changed, leading to the dorsal nerve cord in chordates while protostomes show the ventral nerve cord. It is referred to as a classical explanation for opposite body orientation seen in these two groups.

Classical Model

The classical form of this theory was first suggested by Étienne Geoffroy Saint-Hilaire (1822) and later expanded by Anton Dohrn (1875). It is assumed that the ancestral organism was annelid-like having a ventral nerve cord and a dorsal heart. These features are retained in most protostomes. It is the process where the chordate ancestor was thought to undergo a complete inversion of the body axis so the nerve cord shifted to the dorsal side (DHNC) and the heart moved to the ventral region. Some of the main features are– axial muscle blocks became dorsolateral and opposite to that of protostomes. This is referred to as the first major attempt to explain the reversal of the body axis.

Molecular Evidence

The modern form of support comes from developmental studies on dorsoventral patterning. It is observed that BMP signalling pathways such as BMP-4 and its antagonists (Chordin) in vertebrates show opposite expression orientation when compared with Drosophila where Dpp (BMP homolog) is dorsal and Sog is ventral. These are conserved pathways but expressed inversely. The reaction is as follows– conserved genes controlling D-V polarity remain the same but their spatial arrangement change in protostomes and chordates. This gives some support that both groups share a common ancestral toolkit.

Refutation Based on Developmental Constraints

Even though conserved genes are present, the hypothesis is mostly rejected as a literal body flip. The processes is different because protostomes exhibit spiral cleavage and schizocoelous coelom formation, whereas deuterostomes have radial cleavage and enterocoelous coelom formation. These are fundamental embryological patterns. It is the reason why a complete inversion during evolution is considered incompatible with the distinct developmental pathways in the two lineages.

Alternative Explanation (Independent Condensation Theory)

Among the important alternate views is the independent condensation hypothesis. It is the process that explains the opposite nervous system orientation without assuming an inversion event. It is suggested that the bilaterian ancestor possibly had a diffuse neural network or multiple nerve cords. Nervous tissue then condensed toward the ventral region in protostomes and toward the dorsal region in chordates. These are described as independent evolutionary pathways. This process occurs when selective pressures act on different regions of the body, resulting in opposite organization of nerve cord and signalling molecules.

Theories Based on Deuterostome Derivation (Ambulacraria Linkage)

It is the process where chordates are explained as originating within the Deuterostomia, along with the Ambulacraria group which includes Echinodermata and Hemichordata. Molecular phylogeny places these clades together, forming a major line of bilaterians with shared developmental characters such as radial cleavage and enterocoelous coelom formation.

A. Paedomorphosis Hypothesis (Sessile Ancestor)

This hypothesis describes that the early chordates evolved from a sessile, filter-feeding ancestral organism. It is referred to as a classical theory given by Walter Garstang.

Mechanism– It is suggested that larval stages of echinoderms (auricularia) or hemichordates (tornaria) were selected for motile behaviour. In this step, the free-swimming larva of a sessile ancestor such as tunicate tadpole larva became sexually mature by paedomorphosis (neoteny). The larva already has the notochord and dorsal hollow nerve cord (DHNC). When this larval stage retained its motile characters and reproduced, it is proposed that it formed the early cephalochordate-like animals.

Current Status– This theory is now less accepted. It is the process where genomic analysis showed that tunicates are highly derived forms rather than primitive ancestors. This means the sessile adult form is not considered ancestral.

Vetulicolian Evidence– Vetulicolia from the Cambrian period is suggested as a sister group to tunicates. These fossils were free-swimming filter feeders. These are described as evidence that the primitive lifestyle for tunicates was free-living, not sessile. This undermines the idea that chordates originated from a sessile ancestor through paedomorphosis.

B. Auricularia Hypothesis

This hypothesis is closely linked to larval transformation ideas and focuses on the formation of the DHNC.

Mechanism– It is assumed that the circumoral and aboral ciliated bands of the ancestral dipleurula (auricularia-like) larva migrated towards the dorsal side. These bands fused at the midline and got internalized to form the dorsal nerve cord. The aboral ciliated band is explained to form structures like the endostyle and pharyngeal tracts.

Current Status– It is rejected because lancelet (cephalochordate) neurulation does not involve ciliated bands. The processes is therefore inconsistent with the developmental mechanism seen in present chordates.

C. Progressive or Worm-like Ancestor Hypothesis (Current Leading View)

This is considered the most supported theory based on molecular and developmental data.

Proposed Ancestor– The ancestral chordate was free-living, worm-like, and active. It is similar to enteropneust hemichordates which show a vermiform body and burrowing lifestyle.

Phylogenetic Support– Molecular studies place Hemichordata + Echinodermata as Ambulacraria, the sister group of Chordata. Within chordates, Cephalochordates diverge first, then the group uniting Tunicates and Vertebrates (Olfactores). Amphioxus is considered the closest model to the early chordate body plan, including a single Hox cluster.

Ancestral Features

Some of the main features are–

• Pharyngeal openings for filter feeding, also present in hemichordates.
• A central nervous system precursor is present in adult enteropneusts where a dorsal cord internalizes in the collar region.
• The chordate DHNC is considered to evolve by elaboration of this dorsal precursor system along with the formation of notochord.

This suggests that the ancestor was already showing early features of a centralized nerve cord.

Ecological Niche– It is assumed that early chordates were suspension feeders. Examples include yunnanozoans, early vertebrates like Haikouichthys, modern Amphioxus, tunicates, and larval lampreys. These are described as following a filter-feeding mode consistent with pharyngeal slits.

D. Aboral–Dorsalization Hypothesis

This theory provides a developmental explanation for dorsal axial structure.

Mechanis– It is proposed by Nori Satoh. It is the process where DHNC and notochord formation occurred on the aboral side due to spatial constraints on the oral side. This developmental dorsalization produced a fish-like or tadpole-like (FT) larval form. The FT form improved locomotion and became important during early chordate evolution.

Echinoderm ancestry

I. Phylogenetic Position and Deuterostome Status

• Echinoderms and hemichordates form the Ambulacraria, which is considered the sister group to chordates.
• These organisms share deuterostome characters such as radial cleavage, anus-from-blastopore formation, and enterocoelous coelom development.
• Larvae like bipinnaria or auricularia show bilateral symmetry and resemble tornaria larvae of hemichordates.
• Adult echinoderms develop pentamerous symmetry, water vascular system, and calcareous endoskeleton which is considered highly specialized.

II. Echinoderms and Ancestral Deuterostome Morphology

• The ancestral deuterostome is reconstructed as a bilaterally symmetrical worm with pharyngeal openings.
• Modern echinoderms lack gill slits, but fossil evidence suggests these were present earlier in the lineage.
• Stylophorans like Jaekelocarpus oklahomensis and Lagynocystis pyramidalis show internal bars interpreted as homologous to gill bars of hemichordates.
• It is the process where filter-feeding bars may have been ancestral and were later lost in most echinoderms.
• Calcichordates were earlier proposed as connecting forms due to skeletal resemblance and possible pharyngeal pouch openings, but this view is debated.

III. Hypotheses Based on Echinoderm Development

• Auricularia hypothesis describes a chordate ancestor evolving from an auricularia-like larva through migration of ciliated bands to form DHNC.
• Paedomorphosis hypothesis uses echinoderm and hemichordate larval forms to explain retention of motile larval characters.

IV. Molecular Insights

• Sea urchin Hox cluster shows rearranged gene order and is used more in adult patterning than larval development.
• Nodal signaling acts on the right side in sea urchins, opposite to vertebrates, supporting axial changes after divergence.
• Pharyngeal gene cluster such as Pax1/9, Eya, FoxI, FoxC, and FoxL1 is retained, even though modern echinoderms lack gill slits.
• This genetic conservation supports that the early deuterostome ancestor had a pharyngeal filtering system.

Hemichordate ancestry

I. Phylogenetic Position and Internal Taxonomy

• Hemichordates belong to Deuterostomia along with echinoderms and chordates.
• Hemichordates and echinoderms together form Ambulacraria, which is supported as the sister group to chordates.
• The phylum has two main groups– Enteropneusta (acorn worms) and Pterobranchia.
• Enteropneusts are free-living, worm-like animals having proboscis, collar, and trunk.
• Pterobranchs are sessile and colonial, and some studies suggest they are derived from enteropneusts.
• Tornaria larva of many enteropneusts resembles auricularia larva of echinoderms, supporting Ambulacraria monophyly.

II. Hemichordates as a Model of the Ancestral Deuterostome

A. Ancestral Lifestyle and Morphology

• The ancestral deuterostome is considered a free-moving worm with pharyngeal openings like modern enteropneusts.
• Hemichordates have gill slits used in filter feeding or respiration, which is taken as an ancestral deuterostome trait.
• The pharyngeal gene cluster (Pax1/9, Eya, FoxI, FoxC, FoxL1) is conserved in both hemichordates and chordates.
• Stylophoran fossils show internal bars similar to gill bars in enteropneusts, suggesting an early filter-feeding system.

B. Centralization of the Nervous System

• Hemichordates have a diffuse nerve plexus with dorsal and ventral nerve cords.
• The collar region has an internalized dorsal cord formed by a process similar to neurulation.
• This collar cord is suggested to be homologous to early forms of the chordate DHNC.
• Studies on Ptychodera flava show centralized neuronal masses indicating that centralization predates chordates.

C. Stomochord Controversy

• Early workers thought the stomochord was homologous to the notochord, which led to the name Hemichordata.
• It is now described as a pharyngeal outgrowth without notochord function.
• The stomochord shows molecular similarity to the chordate endostyle, especially due to FoxE expression.
• This supports a link to filter-feeding structures rather than axial support.

D. Developmental Genetic Insights

• Anteroposterior patterning genes are conserved and show similar expression patterns to vertebrate neural regions.
• Hemichordates use Bmp on the dorsal side and Chordin on the ventral side, forming an inverted D-V axis compared to chordates.
• The Bmp gradient in hemichordates does not segregate neural and epidermal tissues, unlike in chordates.
• Chordates later used this axis to centralize nervous tissue, while hemichordates retained a diffuse system.

Urochordate ancestry

I. Phylogenetic Position of Urochordata

• Urochordates include ascidians, appendicularians, and thaliaceans.
• Molecular data supports Tunicates + Vertebrates forming Olfactores, with cephalochordates diverging first.
• Tunicates are considered highly derived, not primitive chordate forms.
• Internal relationships remain debated because different groups show different lifestyles.
• Appendicularians are sometimes placed basally, while other studies include them inside Pleurogona.

II. Urochordate Ancestral Morphology and Lifestyle

A. Paedomorphosis (Sessile Ancestry)

• Earlier theories considered adult ascidians as primitive and sessile.
• Tadpole larva of ascidians carries notochord, DHNC, pharyngeal slits, and post-anal tail.
• The idea was that this larva became sexually mature by paedomorphosis.
• It is now less accepted because tunicates are shown to be highly specialized.

B. Evidence for a Primitive Free-Living Ancestor

• Vetulicolia fossils are considered sister group to Tunicates inside crown Chordata.
• Nesonektris aldridgei shows a rod-like posterior structure resembling a notochord.
• Vetulicolians were free-swimming, so early tunicates are inferred to be pelagic.
• Sessile adult forms of ascidians are considered a derived condition.
• The common ancestor likely had a thick cuticle, large pharynx, posterior notochord, and terminal anus.

III. Cambrian Evidence for Early Tunicates

• Shankouclava shankouense is the earliest confirmed tunicate from the Lower Cambrian.
• Its tail shows dorsal segmentation suggesting active movement.
• It had a tunic similar to modern ascidians.
• Megasiphon thylakos indicates that the basic tunicate body plan formed around 500 Ma.

IV. Distinctive Urochordate Features

• Tunicates synthesize cellulose (tunicin) forming the tunic, a unique feature among animals.
• The cellulose synthase gene (CesA) likely came from horizontal gene transfer.
• Tunicate genomes show reduced and dispersed Hox clusters, supporting their derived status.
• The endostyle is present and is homologous to the vertebrate thyroid gland.
• Genes such as Nkx2-1 and FoxE help in endostyle formation in Ciona intestinalis.

Cephalochordate ancestry

• Cephalochordates is considered the most basal living chordates and it is the group that retain all fundamental chordate characters throughout life.
• It is the model for ancestral chordate body plan because these animals have notochord, dorsal hollow nerve cord (DHNC), pharyngeal gill slits, endostyle and post-anal tail in adult stage.
• The body is adapted for fish-like locomotion and it is the notochord which is muscularized and act as a mechanical organ for undulatory swimming.
• These animals show a single intact Hox gene cluster and this genomic stasis is used to indicate the primitive chordate condition before whole-genome duplications in vertebrates.
• The cephalochordate ancestor is derived from a motile Ambulacrarian ancestor which was worm-like with pharyngeal gill slits.
• Evidence from hemichordates show that nervous system centralization is older than true chordates and the dorsal collar cord in enteropneusts resemble early neurulation.
• The major innovation in chordate lineage is the formation of notochord and its integration with dorsal nerve tissue to form the axial complex.
• The aboral-dorsalization hypothesis states that the notochord and the DHNC was formed on the aboral side of the embryo leading to fish-like larval form.
• Cephalochordates diverged earlier than tunicates and vertebrates, supporting that tunicates are highly derived even though adult forms are simple.
• The notochord type in cephalochordates resemble “stack of coins” structure which is considered the ancestral condition for chordates.
• Some features like anteriorly projecting notochord and asymmetric muscle segments in Amphioxus may be derived within cephalochordate lineage.
• Fossils from Cambrian such as Pikaia gracilens show primitive lancelet-like body with myotomes indicating undulatory swimming.
• Vetulicolian fossils like Nesonektris aldridgei show rod-like posterior structure that resemble notochord and support early chordate affinities.
• Cephalochordates thus represent the key link showing how ancestral deuterostome characters got integrated with notochord and DHNC to produce streamlined motile chordate body plan.

Barrington’s hypothesis

• This hypothesis was proposed by E.J.W. Barrington (1965) to explain chordate evolution within deuterostomes.
• It is suggested that the common ancestor of echinoderms and chordates was a small sessile or semisessile lophophorate animal using ciliary feeding.
• This ancestor trapped food by ciliated tentacles and gave rise to early stalked echinoderms and pogonophores.
• The next stage is described as a sessile fibre feeder in which external tentacles were replaced by an internal filtering pharynx.
• The pharynx developed gill-slits and an endostyle which secreted mucus for trapping food.
• Cephalodiscus (pterobranch hemichordate) represent an intermediate stage because it has one pair of gill-slits along with a crown of tentacles.
• Development of a perforated pharynx is referred to as pharyngotremy and this step produced free-living hemichordates and sessile urochordates.
• Some ancestral tunicates produced tadpole larvae instead of only ciliated larvae.
• The tadpole larva showed chordate features like notochord and dorsal hollow nerve cord.
• According to Garstang’s idea, elongation of this larva shifted ciliary bands to the mid-dorsal region and formed the hollow nerve cord.
• The adoral cilia formed the endostyle and muscle fibres developed in the tail region.
• Through paedogenesis, these larvae became sexually mature and the sessile adult stage was suppressed.
• It is this paedogenetic larva which gave rise to cephalochordates, vertebrates, and larvaceans by parallel evolution.
• The hypothesis explains the transition from lophophorate-like ancestor to early chordates using feeding modifications and larval evolution.