The vertebrate bodyplan exhibits many lines of evidence indicating a deep structural duality in it´s anatomical, physiological, developmental, and behavioral organization.
The anatomical, physiological, and behavioral features characteristic of the bodyplan duality correlate with two distinct modes of existence:
The evolutionary roots of this duality of structure and modes of existence reach back to the Paleozoic era to a sessile invertebrate pharyngeal filter-feeding organism possessing a motile somatic larval life-stage for facilitating the dispersal of offspring to suitable environments for maturation.
Sessile bodyplans are usually associated with animals that occupy a fixed location within the environment and live their enitre lives in that locality.
Visceral bodyplans exhibit internally-oriented sensorymotor systems much more conducive to adaptively optimizing internal hedonic states and functions - they tend to exhibit diffuse decentralized nervous systems or nerve-nets whose behavior is often characterized by localized action or peristalsis across neighboring regions.
The adult tunicate bodyplan is representative of the visceral bodyplan organization and a sessile lifestyle.
Somatic bodyplans exhibit externally-oriented sensorymotor systems capable of cooridinating motile locomotor activity over a broadly distributed range of environments.
Motile somatic bodyplans contain multiple sensory modalities to facilitate purposeful navigation within the environment - they tend to exhibit centralized heirarchally organized nervous systems that cooridinate the global binding of diverse and highly-distributed cell populations within the bodyplan during moments of action on the environment.
The juvenille, or larval, tunicate bodyplan is representative of the somatic bodyplan organization and a motile lifestyle.
A key evolutionary event surrounding the origin of the Craniates and Vertebrates appears to be the permanent association of the motile somatic body of the juvenille larva with the pharyngeal filter-feeding visceral body of the mature adult throughout the entire lifetime of the organism.
I will tenatively postulate that this somato-visceral fusion is a consequence of the persistence of a abberent pattern of life-cycle metamorphosis, possibly due to a HOX cluster duplication event, in the chordate lineage leading to vertebrates.
There appear to be at least two potential contributing factors leading to this event:
The fusion event may have been preceeded by a period of progressive developmental neoteny in the anscestral chordate population.
The versatility of the motile bodyplan favored the emergence of a sexually mature organism that foregoes metamorphosis and retains major features of the juvenille bodyplan into adulthood.
Indeed, cephalachordates may represent a branch of this ancestral neotenous chordate population that diverged prior to the advent of the genome duplication event leading to vertebrates.
Neoteny would have resulted in the retention of the juvenille bodyplan by inhibition of metamorphosis, thereby aborting it´s consequent bodyplan reorganization.
A key event on the way to vertebrates is the duplication of the HOX Cluster and other parts of the genome that control gene expression patterns, cellular differentiation and adaptive specialization within the bodyplan.
HOX Cluster transcription factor duplications facilitate to the emergence of new differentiation pathways, while extra copies of structural genes allow for the emergence of the vertebrate gene superfamilies such as cell and substrate adhesion molecules (CAMs and SAMs), Immunoglobins, receptor families, etc...
Below is a speculative scenario for the evolutionary radiation of the chordates:
|
Of course, a wide range of hypotheses exist concerning the origin and radiation of the Chordates:
The GBR Hypothesis of Chordate Origins
The Veriform Ancestor Alternative to the GBR Hypothesis of Chordate Origins
What is clear is that at some point in the evolutionary history leading to vertebrates, the somatic and visceral bodyplans became fused at the pharyngeal-hindbrain complex and the sacral end of the organism and established the basic relations that define the conserved vertebrate bodyplan.
The general organizing principles of the both bodyplans that comprise the life-cycle of the anscestral chordate organism, i.e. the juvenile larval stage and the mature adult stage, will be a primary point of reference for understanding the foundations and evolutionary potential of the vertebrate bodyplan.
Special attention will be paid to the role of the anatomical fusion points and integrating tissues in establishing a functional relationship between the somatic and visceral divisions within a single unified organism.
Of particular interest will be the physiological and behavioral consequences of simultaneous co-existence of both the juvenille and adult bodyplans within the same organism at the same time.
As we shall see, the addition of a somatic locomotor unit to the hedonic pharyngeal unit allows early chordate filter-feeders to abandon the sessile visceral life-style and adopt the migratory mode of existence that characterizes the vertebrates.
Crucially, by examining the origin and evolution of the pharyngeal-hindbrain complex, and the nature of the interface between the somatic and the visceral division, we can understand how the integration of the vertebrate as a unitary organism is cooridinated and maintained throughout vertebrate evolution.
The evolving integrative capacities of pharyngeal-hindbrain complex are manifested as innovations in the structure of the autonomic nervous system and it´s functionality that increase the level of cooridination between somatic body and visceral body within the organism and the environment.
Changes in the developmental trajectories of the pharyngeal arch components during evolution result in concommitant changes in the structure of the brainstem and autonomic nervous system that can be correlated with shifts in behavioral repertoires and potentials.
Subsequent to the somatovisceral fusion event the pharyngeal-hindbrain complex will undergo two major transitional shifts, one with the emergence of gnathostomes (jawed vertebrates) and the other with the emergence of amniotes (land vertebrates).
Indeed this system will be dramatically altered as tetrapods move onto the land and switch from cardiopharyngeal respiration to the cardiopulmonary respiration.
In amniotes, the remaining pharyngeal arch components will be freed from their prior responsibilities and co-opted in new ways that will drive the evolution of the mammalian, primate, and human bodyplan and behavioral potential.
|
|
Areas in red and blue roughly correspond to the somatic division. Areas in yellow approximate the visceral division.
Note the relative points of fusion at the Pharyngeal-Hindbrain Complex and the Sacral Plexus. Also of interest is the absence of connections between the two divisions along the majority of the visceral tract. |
The "Somato-Visceral" fusion event appears to be concurrent with a whole genome duplication event that characterizes vertebrates and corresponds to the first HOX-Cluster duplication event.
The gene duplication event gives birth to the vertebrate gene super-families and will be followed by at least one more whole genome duplication at the transition to gnathostomes.
Of critical importance for vertebrates and their evolution is the duplication of the HOX-Cluster, which is responsible for patterning the anterior-posterior axis (AP-axis) in all bilaterians as well as participating in the formation of paired appendages in gnathostomes and tetrapods.
Other important vertebrate innovations that are concurrent with the gene duplication event are:
It is of further interest to speculate on the fate of two populations of cells involved in metamorphosis of the juvenille larva into the adult bodyplan:
It is of notable interest that the antibody-based immune system is a unique feature of vertebrates - while all bilaterian animals, including vertebrates, share a common active host defense system rooted in phagocytosis and induced apotosis.
Immunoglobin antibodies are known to be related to the cell adhesion molecules (CAMs) and may have arisen from the CAM genes as a result of the genome duplication event and subsequent evolutionary divergence of function.
It would also be of interest to trace the fate of the phagocytic cells that are responsible for remodelling the larval body into the adult body-plan in the ancestral urochordate-like organism.
One might speculate that these phagocytic cells were originally responsible for identifying larval cells slated for consumption during metamorphosis (adult-self/juvenille-self recognition) but subsequent to the genome duplication event and the consequent developmental turmoil, these cells found themselves sandwiched between the somatic and viscerally fused bodyplans that came to be the vertebrate bodyplan.
The new physiological ecology providing positive selection pressure for these cells within the bodyplan would eventually lead to a change from self-self recognition to self-nonself recognition as these cells adopted a role consistent with their ancient function, namely defense of self from non-self.
Divergence of a subset of the duplicated CAM genes - along with their regulatory genes - would eventually result in a a new role for these molecules at the cellular and physiological level as cell-surface and humoral antibodies.
Craniates make their first appearance in the early Cambrian fossil record around 530 million years ago.
Myllokunmingia is one of the earliest confirmed fossil craniates discovered within the Lower Cambrian of Chengjiang, China ( ).
Myllokunmingia is thought to have possessed a skull comprised of cartilage but to have lacked the capacity for biomineralization, and therefore these craniates lacked bones.
Haikouichthys is a another small Early Cambrian vertebrate from Chengjiang, China with clear signs of a brain, eyes, pharyngeal arches, a notochord, and rudimentary vertebrae but it is uncertain whether this organism had the capacity for bone formation (, ).
The Myxinoidea, or Hagfish, are the modern representatives of the pre-Vertebrate craniate condition, lacking vertebrae and possessing more than seven pharyngeal arches.
The general Hagfish bodyplan may be considered to most closely resemble the archaic craniate condition, although extant hagfish are considered to undergone considerable bodyplan modification over the course of evolutionary time.
Myxinoidea are also classified as agnathans and/or cyclostomes - and are the only known extant non-vertebrate craniates; as well as being the only non-vertebrate agnathans
|
Craniates possess all the features of chordates plus:
The agnathan vertebrate Priscomyzon riniensis is one of the earliest lampreys (Petromyzontoidea) to appear in the fossil record, dating to the Late Devonian ( ).
Lampreys are believed to have originated during the late Cambrian, but do not show up in the fossil record until about 360 million years ago.
The fossils found in South Africa clearly demonstrate the "living fossil" status of lampreys - their bodyplans having changed very little over the last 360 million years.
|
Adult lampreys are parasitic animals that attach themselves to other fish to feed.
Lampreys possess several specializations including a large oral disc with circumoral teeth.
|
Petromyzontoidea are also classified as agnathans and/or cyclostomes - and are the only known extant vertebrate agnathans.
Lampreys are considered to most closely resemble the archaic vertebrate ancestor.
|
|
Agnathan vertebrates possess all the features of craniates plus the following:
