THEORIES ABOUT ORIGIN OF CHORDATES
It is believed that Chordates have originated from invertebrates. It is difficult to determine from which invertebrate group the chordates were developed. Chordate ancestors were soft bodied animals. Hence they were not preserved as Fossils.
Many biology theories were put forward to explain the Origin of Chordates.
1. COELENTERATE THEORY - ORIGIN OF CHORDATES: According to this theory chordates were developed from coelenterates. Radial symmetry coelenteron,cnidoblasts etc, were 1st and advanced characters were developed to give rise to chordates. This theory infers that chordates might have acquired higher characters independently. It is not correct and hence this theory is not acceptable.
2. ANNELID THEORY - ORIGIN OF CHORDATES: This theory suggests that the chordates have evolved from an annelid stock. The annelids show bilateral symmetry, mesmerism, head, lateral Coelome complete digestive tract, closed circulatory system, hemoglobin, etc., like chordates. The resemblance is enhanced if, an annelid is turned upside down. But the mouth would be dorsal which is unlike that of chordates. Metamerism and appendages of annelids differ in nature from those of the chordates. Bilateral symmetry, head and complete digestive tract occur in other non-chordate phyla also. Coelome is schizocoelic in annelids and enterocoelic in lower chordates. Haemoglobin is dissolved in the plasma in annelids but it is present in the red blood corpuscles in chordates. Annelid nerve cord is double, and, ventral in contrast to single, hollow, dorsal nerve cord of chordates. Some striking differences exist between the annelids and the chordates in their embryology. Hence it is difficult to accept this theory.
3. ECHINODERM - HEMICHORDATE THEORY - ORIGIN OF CHORDATES: This theory infers origin of chordates, hemichordates and echinoderms from a common ancestor. This theory is based on the following evidences.
a) EMBRYOLOGICAL EVIDENCE - ORIGIN OF CHORDATES: Both echinoderms and chordates have enterocoelic coelome, mesoderm and deuterostomous mouth. There is resemblance between the bipinnaria larva of certain echinoderms and the tornaria larva of hemichordates. In echinoderms chordates the central nervous system develops from a dorsal strip of ectoderm.
b) SEROGICAL EVIDENCE - ORIGIN OF CHORDATES: A close similarity exists between the proteins of the body-fluid of chordates and echinoderms. Hence the chordates are more related to echinoderms.
The radial symmetry of adult echinoderms will disprove their relationship with the bilaterally symmetrical chordates. In echinoderms radial symmetry is secondarily developed from a basically bilateral symmetry. Both the primitive and the early echinoderm larvae show bilateral symmetry.
The generalised chordate ancestry is as follows: This was suggested by N.J.Berrill, (1953) in his book “The origin of Vertebrates”.
ECHINODERM - AURICULARIA - HEMICHORDATA - TORNARIA - PROTO CHORDATE - ASCIDIAN TADPOLE - FREE SWIMMING CHORDATE
Hyman (1959) and others concluded that all the three groups have a common ancestor, Echinoderms and hemichordates branched off from a common evolutionary line which ended in the chordate group.