Monday, April 7, 2014

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.

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