Deuterostome phylogeny showing the two major deuterostome branches - Ambulacaria which includes the echinoderm and hemichordate phyla; and the Chordates, which include the tunicate, cephalachordate, and vertebrate subphyla.Spindle diagram
The ancestral origins of the chordates and vertebrates lies within the bilaterian deuterostomes. The Deuterostomes are an eclectic collection of phyla with diverse bodyplans that includes our chordate ancestors. Although the origins of the deuterostomes is murky, it appears that deuterostomes evolved during the late Edicarian/early Cambrian period from a segmented bilaterian ancestor that had pharyngeal gill slits, a hollow nerve cord, along with circular and longitudinal muscles.
The early stem deutetostomes were undergoing a great deal of developmental reorganization as can be seen by the diversity of its major phylogenetic body plans. The branch leading to chordates diverges early and may have been subject to much of the same genetic and developmental instability that accompanied the remodelling of the earlier protosome-like bilaterian bodyplan. By the early Cambrian about 525 million years ago, Myllokunmingia ,which appears to be a very primitive early jawless vertebrate, can be found in the fossil record of the Lower Cambrian Maotianshan shales of China.
Vertebrates undergo several important innovative bursts that organize their anatomy and nervous systems. These seem to be intimately linked to genome duplication events, a somato-visceral fusion of life-stages, and some major adaptive reorganizations. From the beginning, the neural crest has been the great innovation of vertebrates; a fourth germ layer from which, the tissues that weld the somatic and visceral divisions are derived.[1] Following the history of these tissues tells the story of vertebrate evolution and neural organization.
The early genetic and morphological instability of Deuterostome evolution
Early on, Bilaterians bifurcated into two major superphyla. Each superphyla is characterized by differences in their modes of development - Protostomes which develop the mouth first, from the blastopore during gastrulation, and then the anus secondarily; and the Deuterostomes which develop the anus first via the blastopore during gastrulation, and then the mouth secondarily. This distinction between protostomes and deuterostomes appears to be, either the consequence of an inversion of the dorsal-ventral (DV) axis of the body in a branching population of early protostome bilaterians;[2][3][4] or, the parallel branch of bilaterians that resulted from the breaking of radial symmetry, and establishment of bilateral mirror symmetry, at the origin of the bilaterians as the anterior-posterior axis (AP-axis) and dorsal-ventral axis (DV-axis) was established.[5]
Protostomes can be grouped into two types as well: ecdysozoa and lophotrochozoa. Protostomes as a superphyla are a highly diverse group that comprise the vast majority of animals in terms of phyla and body plans, but deuterostomes are a rather more eclectic group that comprised of three phyla: echinoderms, hemichordates, and chordates. Hemichordates seem to share features of both superphyla, as well as unique features of it's own. Much of what is understood about the deuterostomes comes from studies and observations of chordates, and vertebrates in particular - the other two phyla being less understood.
Within the deuterostomes there appears to have been a great deal of genomic instability, replication, and reorganization within the genetic toolkit that constrains the ontololgy of developmental epigenesis prior, and well into, the early evolutionary establishment of these three phyla and their respective body plans.
While instability might seem to be a highly undesirable situation, for somatic selective systems that succeed in adapting, it can be a wellspring of creativity leading to novel results.
Vertebrate origins - Genome duplications and a botched metamorphosis
The British zoologist Walter Garstang (9 February 9, 1868 – 23 February 23, 1949) was one of the first to suggest that the chordate ancestor of vertebrates was a tunicate-like organism that expressed two fundamentally different bodyplans over the course of its life as it metamorphosized from its juvenile form to its adult form. Garstang's Hypothesis proposed that progenesis or neoteny was responsible for the emergence of vertebrates - a situation where the somatic larva sexually matures but does not metamorphosize.[6]
In 1972, the American vertebrate paleontologist and anatomist Alfred Sherwood Romer (December 28, 1894 – November 5, 1973) revived this idea in an attempt to get a clearer understanding of the evolutionary roots of the vertebrate body plan and nervous system structure, but with a twist. Romer adopts Garstang's hypothesis and goes on to describe a somatic mobile juvenile larval bodyplan with a notochord, centralized nervous system, segmented striated musculature, and head-senses that is responsible for dispersal in the ecology such that it can find a place to settle down and metamorphosize into an adult; and, a second body plan, a sessile filter-feeding adult anchored to the seafloor with pharyngeal gill slits, smooth muscle, and an enteric nerve net that runs along the GI tract from the pharynx to the sacral end. Through an accident of evolution, most likely the whole-genome duplication-events that lie at the origin of vertebrates, the two body plans were temporally coexpressed and become fused together.
Romer saw the great adaptive evolutionary challenge of vertebrate anatomy as one of somatovisceral integration between these two divisions. Romer hypothesizes that at the origin of vertebrates these two bodyplans, which were sequentially expressed, came to be expressed simultaneously - and fused only at the hindbrain-gill slits and the sacral nerve. Originally, the only points of communication between the two "animals" was via the unmyelinated neurons of the parasympathetic nervous system. The rest of vertebrate evolution revolves around adaptions that allow the integration of these two bodyplans. Romer describes the gradual emergence of the myelinated sympathetic nervous system with the appearance of jawed vertebrates and its increasingly sophisticated development of control over the enteric nervous system and viscera by the somatic division as we move along the evolutionary progression of vertebrate anatomy and physiology.
The most reasonable cause of disruption to the ancestral protovertebrates life cycle is likely to be the first genome duplication event that lies at the origin of vertebrates. Under normal circumstances, once the tunicate-like ancestral somatic juvenile located a place to settle down and anchor, it would metamorphosize into a sessile filter feeding pharyngeal gill basket as it assume the adult body plan. To make this transition, phagocytes are charged with dismantling and clearing the somatic tissues of the juvenile that are no longer needed. Metamorphosis stimulates the maturation of the rudimentarily established viscera and the formation of the protective tunicate basket as an exoskeleton.
A disruption in the process of phagocytic removal of the larval tissues, leaving them intact, while stimulating the maturation of the adult body plan, may have resulted in the simultaneous expression of both body plans at the same time - fused at the points of common structure. In this case, the hind-brain-pharnygeal gill arch region and the sacral end. Such a union would be an awkward merger and would respond poorly to situations that require globally coordinated actions in the ecology.
The neural crest as the fourth germ layer - Welding together of two life-stages
The Swiss anatomist Wilhelm His (July 9, 1831 – May 1, 1904) was the first to identify the neural crest tissue in the embryos of vertebrates in 1886.[7] In studies published in 1898, observation of jaw cartilage and tooth dentine being formed by neural crest cells was demonstrated by the pioneering neuroembryologist Julia Barlow Platt (September 14, 1857 – 1935). Although unappreciated at the time, Platts work demonstrated the ability of the neural crest to form a vast swath of cell types[8][9] - something typical of the early germ layers, not a secondary tissue.
In 1983, Glenn Northcutt and Carl Gans published their famous paper The genesis of neural crest and epidermal placodes - a reinterpretation of vertebrate origins in the Quarterly Review of Biology, thereby cementing the status of the neural crest as one of the defining features of vertebrates.[10] In 1998, the developmental biologist Brian K. Hall has proposed that the neural crest is a fourth germ-layer - making vertebrates the only quadroblastic animals to have evolved as of yet.[1]
The neural crest is the primary tissue that is responsible for welding the somatic and visceral divisions together into a cohesive and unified whole. The number of tissue types derived from the neural crest are extensive and not limited to nervous system tissue, giving rise to mesenchymal cell types:
neurons
cartilage
bone
connective
pigment cells
In the developing vertebrate embryo, the neural crest is organized as four major domains:
Vagal/sacral neural crest - participates in the construction of the enteric nervous system and the parasympathetic ganglia. This unmyelinated system is the ancient integratory point of somato-visceral communication.
Cranial neural crest: participates in the formation of the craniofacial tissues, cranial/pharyngeal arch nerves and components modifying the pharyngeal arch system.
Trunk neural crest: participates in the construction of the sympathetic system.
Cardiac neural crest: participates in the construction of the cardiopulmonary loop.
Neural Crest Questions
"Thus, there are many interesting unanswered questions about neural crest development. For example, how do migrating neural crest cells interact with each other, both molecularly and mechanically? How are lineage decisions coupled with migration, and how do neural crest cells interact with the rapidly developing surrounding tissue? As neural crest cells have stem cell properties, what is their degree of fate restriction versus multipotency to form diverse cell types? How does this vary along the rostrocaudal body axis? In the adult, do neural crest-derived cells participate in tissue repair? "[11] - Weiyi Tang and Marianne E. Bronner (2020)
Developmental biologists today, in trying to unravel the mysteries of the neural crest, are finally homing in on the detailed genetic, molecular and cellular mechanisms that Edelman was grappling with and trying to conceptually organize in his morphoregulatory hypothesis with its epithelia-mesechymal transitions via genetically constrained and epigenetically modulated CAM and SAM cycles. Marianne E. Bronner and her colleagues have elucidated a gene regulatory network underlying the development of neural crest tissue in vertebrates.[12][13]
The relationship between striated muscle tissue and bone formation
The vertebrate central nervous system maps itself to the sensorisheets and the muscle ensembles. Similarly, the bones map themselves to the muscles that connect to them, developing in accordance with the relative orientation and tension they exert at their attachment points on the bone. Muscles, and muscle tension, sculpt the bone that forms from the embryonic cartilages that emerge as the endoskeleton develops.
The consequence of muscle mutations on vertebrate morphology
Because of the relationship between bone formation and muscle attachment, mutations that occur in vertebrate striated muscles can result in significant alteration of regional, and sometimes the global, morphology. The phylogenetically constrained skeletomuscular system of vertebrates is extremely plastic, offering the possibilities of many forms, while maintaining a central organizing principle over evolutionary time. The occasional inheritable genetic mutation in the muscle ensembles provides a pathway to quick evolutionary transformation within the vertebrate body plan.
The innovations and challenges of the jawless vertebrates
the development of a new adaptive immunity division of the immune system enhances innate immunity via the emergence of variable lymphocyte receptors.[14]
the somatic and visceral divisions of the body are only connected at the pharyngeal-hindbrain region and the sacral end.
for the autonomic nervous systemn, there is no sympathetic division - only, an un-myelinated division of the para-sympathetic radiating out from the pharyngeal-hindbrain complex and sacral end of the organism.
there are no jaws or fins, no myelin.
due to lack of a sympathetic fight or flight system, the primary mode of response in the face of uncertainty is catatonic freezing or "feigning death".
It appears that a second genome duplication event may have facilitated the emergence of jawed vertebrates by providing another duplicate set of genes that could be redirected to new purpose without disruption of earlier established genetic and epigenetic developmental somatic programs.
Key innovations, all of which are key components in the fight or flight system:
adaptive antibody-based immunity based on B and T-cells.[14]
jaws, fins, proprioceptors, and the kinesthetic system.
myelin sheath and fast nerve conduction.
the myelinated sympathetic nervous system and adrenal system gradually exerting more and more control over the visceral division over evolutionary time.
the trigeminal system and head-sensory integration with the jaws and fins.
the emergence of fight or flight behavior as the sympathetic enables global coordination of somatic and visceral divisions.
Many of the most important modifications of the vertebrate body plan in mammals concerns the adaption to land. There are several key features of significant importance:
transformation of the swim bladder, which is off the main circulatory tract of the pharyngeal arch system that oxygenates blood flow to the brain, into the lung.
remodeling the two chamber heart into the four chamber heart so as to establish the cardiopulmonary loop and reestablish fully oxygenated blood flow to the brain, marking the onset of endothermy and a relatively continuous real-time foraging and exploration strategy.
the myelinated division of the parasympathetic nervous system
remodeling the pharyngeal arch nerve components into the cranial nerves via the myelinated parasympathetic.
the emergence of the neocortex and the thalamocortical system from the olfactory cortex.
bringing the limb girdles from their ancestral position as fins, to a position that brings them under the body carriage in early mammals.
Sprawling and erect hip joints - horiz
adapting chemosensory and olfactory senses to air from their ancestral aquatic adaptions.
remodeling the lateral line for sensing water pressure into the auditory system for sensing air pressure.
remodeling vestibular, olfactory, visual, and newly emergent auditory into the head-neck system.
remodeling the pharyngeal arch cartilages, and muscle groups, to become part of the social engagement system linking body posture, head-neck orienting postures, and facial expressions together with vocalizations that are linked to species specific auditory enhancement and entrainment mechanisms.
Primates - Brachiation, bipedalism, grooming, and dexteral intellegence
In 2004, Hansell H. Stedman and his colleagues at the University of Pennsylvania were searching for mutations related to muscular dystrophies when they discovered a species-specific frameshift mutation of a highly conserved primate gene, MYH16.[15] The MYH16 gene encodes a heavy myosin chain that is present in the jaw muscles of primates and is responsible for powerful contractile muscle movement. In primates, the MYH16 gene encoded myosin chain is highly conserved and expressed in the temporalis and masseter muscles that connect the jaw to anchor points on the sagital crest in primates. The frame-shift mutation that Stedman's group discovered is universal to humans, but apparently is disruptive to most other primates due to the masticatory demands of their diet and therefore has been highly conserved within the family.
Stedman and his group propose that the MYH16 frameshift mutation diminshed the pressure that the temporalis and masseter muscles placed on the developing sagital crest, thereby releasing the somatic selective pressure required to induce its formation in the first place. This in turn, allowed for an expansion of the braincase starting approximately 2.4 mya, as the structural constraint on braincase expansion, the sagital crest, became vestigial and ultimately disappeared from ontogeny.
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