Chordate origin and evolution

Chordates are the most diverse and complex phylum in the animal kingdom. To date it

comprises of 48000 recorded species although the actual number is thought to far exceed

this number. Members of this phylum have true coelom which allows for the formation of

internal organs. The phylum may be identified through five distinct characteristics. These

are the notochord, dorsal tubular nerve curve cord, pharyngeal gill slits, endostyle and

post-anal tail. Each of these features serves to distinguish chordates from other phyla.

However not all these features are present in both adult and embryonic stages of the

organism.

The notochord is a stiffened flexible rod which allows for movement. This may

eventually be replaced by the vertebral column. It is the first part of the skeleton to

develop in the embryo. Present in all chordates the dorsal tubular nerve cord has its front

end enlarged to form the brain. Pharyngeal gill slit functions as both a filter feeding

mechanism and in gaseous exchange. While the endostyle originated as part of a feeding

apparatus is confined to chordates. It functions as an endocrine gland. The post anal tail

provides chordates with stability, is ideal for propulsion and plays a role in maintaining

balance. Other characteristics of the phylum chordate are bilateral symmetry, a tube

within a tube body plan, segmented muscles, cephalization, a ventral heart and

endoskeleton.

Chordates are a large group however they are closely related by their unique distinctive

features. Over the years zoologists as well as biologists have wondered about the origin

and evolution of chordate. It is this curiosity that later led some to postulating theories

about chordate ancestry and evolution based on varying assumptions and notable

similarities. These theories include: the annelid theory, the coelenterate theory, the

echinoderm-hemichordate theory, as well as the cephalochordate and urochordate theory.

In the annelid theory of chordate origin chordates are alleged to have evolved from an

annelid ancestor. This theory was said to have come about due to several similarities

between annelids and chordates. These similarities include bilateral symmetry,

mesmerism, lateral coelom, complete digestive tract, closed circulatory system,

hemoglobin, etc. as in the case of the chordates. Annelids also have a double ventral

nervous cord while chordates have single hollow dorsal ones. This theory has brought up

a lot of skeptism as there are none of the five distinctive chordate characters in the

annelids.

The coelenterate theory suggests that chordates arose from coelenterates. However no

evidence has been found that would link the chordates to the coelenterates .like in the

annelid theory the distinct features of the chordates are absent in the coelenterate and

there are no fossil records of any common ancestors between the two.

The cephalochordate theory of chordate origin was proposed by chamberlain who studied

primitive and advanced characteristics of chordates. He proposed that while extant

cephalochordates possess all the distinct of a chordate as well as some non chordate

features such as lack of a heart and sensory organs ,respiratory pigment etc .Fossils have

been discovered in Burgess Shale in the British Columbia, Canada that lend some support

to his claim.

In the urochordate theory of origin W. Garstang and N.J Berrill compared the tadpole like

larva of the urochordate which portrayed similar characteristics of a typical chordate.

These features included the notochord along with the segmented muscles, dorsal hollow

nerve cord sensory organs and pharyngeal gill slits.Garstang believed that chordates arose

from sessile filter feeding urochordate by the larval stage evolving to adult. The

researcher supports this theory as these features can be found in the modern lamprey

larva.

The theory of chordate origin from echinoderm –hemichordates suggests that chordates,

echinoderms and hemichordates arose from a common ancestor. This is based on

embryological and serogical evidences. The embryonic development and genetic make up

of these three organisms seem to have a common link. However there are concerns since

the body symmetry differs.