Characteristics key to Vertebrate Evolutionary Advance

Vertebrate of more than 500 million years ago

The Earth is weighted down by insects so, on the basis of mass, one could make the case that these invertebrates are the most successful life forms on the planet.  By this metric plants and microscopic organisms would also have to be considered.  The weight of such numbers disregards the achievement of vertebrates in developing ingenious measures to overcome material constraints on body size, degree of activity, mobility, adaptability and awareness.  There are no organisms to equal vertebrates in complexity and sophistication of organization. 

Internal skeleton grows as animal enlarges

Invertebrate external skeleton constrains animal's size

Fundamental to the vertebrate’s ability to range in size from an insect-like shrew to the sea-going monster that is the blue whale is its internal framework or skeleton.  The cartilage or bone of these animals is a living tissue that grows internally as the animal itself enlarges, enabling size to be an issue governed only by gravity.  Invertebrates, like insects, have a nonliving skeleton external to the body that is worn like a suit of armor.  The constraint of being contained by a non-growing capsule places a severe limit on the animal’s potential size.  Marine arthropods such as crabs and lobsters do achieve greater size than insects but nothing comparable to many fish and their fixed exoskeleton plates leave them slow and clumsy in comparison to the darting sinuous nature of fish swimming about their environment.  The internal vertebral skeleton, endoskeleton, is an even bigger advantage for terrestrial animals, providing great structural strength with an economy of material.

Vertebrate paired limbs were very adaptive to new niches

Related to the internal skeleton are the paired appendages of vertebrates, providing them with an extraordinary means of mobility.  These appendages originated as swim stabilizers among creatures similar to the ostracoderms around five hundred million years ago.  They developed into distinguishable pectoral and pelvic appendages.  Later they would refine into the fins used by modern day fish.  A separate line of evolution would provide the jointed limbs of terrestrial animals. 

Sharks have an external slit for each gill

Paired limbs provided animals with the speed necessary to chase down and feed on other animals.  Active predation of this intensity requires a much higher metabolism than is capable of the filter feeding diet of early ostracoderms.  A muscular pharynx improved water circulation through the pharyngeal slits while a web of capillary beds in the region enabled efficient respiration – the exchange of gases between the water and the animal’s steadily evolving gills.  Muscular aortic arches and a ventral heart all added to the animal’s ability to provide the oxygen levels necessary for a high metabolic rate. 

Vertebrates have evolved an elaborate nervous system

Undoubtedly the most critical adaptation for vertebrate development was the rare evolutionary event of a new cell type that resulted in the formation of an advanced nervous system.  This involves cells lying near the embryonic notochord being transformed from the outer layer ectodermal cells into neural crest cells and epidermal placodes.  Together they vastly improved upon the animal’s sensory ability as well as its motor skills and its capacity to integrate input stimuli with the fine-tuned muscle response necessary to capture its prey. 

Natural selection shapes common bones for separate needs

It’s always important to keep in mind that five hundred million years of vertebrate evolution was the result of two separate forces, neither of which could be characterized as a preconceived design.  First, were the random events played out over the eons that occurred within the constraints of the genetic DNA process.  The product of those events would result from the many environmental factors that contributed to shaping the nature of the animal population contemporary to the time.  The animals we see today display the effects of this ongoing process of environmental selection.

Dorsal Hollow Nerve Cord

Nerve Cord foundation for nervous system

The dorsal hollow nerve cord is fundamental to all vertebrates and it serves as the basis for their elaborate central nervous system.  This nerve cord extends beyond animals with vertebrae to also include other chordates in which the vertebrates represent the major part.  Among these animals are the fish-like lancelets, or amphioxus, and tunicates.  The invertebrate tunicates, or sea squirts, seems to be a highly unlikely organism to have a dorsal hollow nerve cord as its sedentary lifestyle and primitive body plan appears to have no need for such a sophisticated nervous system arrangement.  In fact the adult has no such structure and the nerve cord appears only in its mobile larval form.  The lancelet retains its nerve cord but its anterior end does not elaborate into anything one might consider a brain and its head has only rudimentary, unpaired sense receptors. 

Embryo with early neurulation 

The nervous system is made up of cells that give life awareness of its surroundings and, in its highest implementation, a consciousness of the self and a sense of wonder and perspective of the self within all existence.  Imagine that.  Cells are working together to produce the power of thought.  This alone makes creation of nerve cells the most extraordinary development of advanced life forms. 

Development of the nerve cord involves a complex choreography of various types of cells moving with synchronization in both time and space.  It begins at a very early stage of the embryo, during the blastula, when cells first begin to divide and differentiate into what will become three distinct layers of cells.  The outermost layer, the ectoderm, proceeds to form the skin, anterior and posterior parts of the digestive tract as well as much of the nervous system, including the eyes and ears.  The innermost layer, the endoderm, provides lining for the gut and the glands associated with the digestive tract.  The respiratory surfaces of vertebrates also originate from endoderm.  The last of the three layers to usually differentiate is the mesoderm, or middle layer.  Products of mesoderm include the muscles, skeleton, connective tissue and the circulatory and urogenital systems. 

Nerve cord among key chordate characteristics

Neurulation begins when mesodermal cells, called chordamesoderm, collect to form the notochord which becomes the embryo’s body axis.  Presence of the chordamesoderm induces the ectoderm overlying the notochord to develop two longitudinal folds, creating a mid-dorsal furrow between them.  The crests of the two folds grow towards one another, forcing the furrow deeper into the dorsal mesoderm that lies adjacent to the notochord.  These neural folds fuse together to make a tube of isolated ectoderm beneath the surface of the embryo.  This neural tube becomes the basis for the central nervous system. 

During the formation of the neural tube within the embryo of vertebrates another group of cells differentiate themselves from the ectoderm.  Arising in the area between the developing neural tube and the closing ectoderm overhead is a distinct group called neural crest cells.  These cells have great evolutionary importance because they are responsible for the creation of most every characteristic that sets vertebrates apart from all other organisms.  They disperse laterally and ventrally from their point of origin to settle and differentiate into a variety of forms throughout the body.  These migrating neural crest cells become the basis for most of the peripheral nervous system.  They form the autonomic system and several endocrine glands.  They are responsible for much of the head’s skeleton and connective tissue as well as other elements that make up the nervous system.

Anterior nerve cord elaborates into vertebrate brain

A nervous system has developed among insects and other invertebrates but no organism comes close to the refinement of its abilities to comprehend its surroundings and provide reasoned solutions to confronted problems like the power of the nervous systems exhibited among the higher vertebrates.