Advanced Features of Vertebrates Compared to Protochordates

 

Advanced Features of Vertebrates Compared to Protochordates

 

1. Notochord and Vertebral Column: In vertebrates, the notochord is either completely replaced by the vertebral column or coexists with it.

2. Heart Structure: Vertebrates have a divided heart with chambers and contain red blood cells (RBCs) in their blood.

3. Sexual Dimorphism: Each vertebra in vertebrates typically exhibits distinct sexual characteristics, except in certain fish species.

4. Fertilization Site: In vertebrates, fertilization commonly occurs in the animal pole pathway.

5. Endostyle Absence: Unlike protochordates, adult vertebrates lack an endostyle, although it is present in the larval stage of certain species like Petromyzon.

6. Neural Crest Cells: Vertebrate nervous system development involves the presence of neural crest cells.

Primitive Features of Protochordates:

1. Absence of Cranium and Vertebral Column.

2. Presence of Atrium: Protochordates possess an atrium between the pharynx and body wall.

3. Endostyle Presence: Except for hemichordates, protochordates have an endostyle, which serves as the precursor to the thyroid gland in higher vertebrates.

4. Neural Crest Cell Absence: Protochordates lack neural crest cells during nervous system development.

5. Persistent Pharyngeal Slits: Protochordates maintain pharyngeal slits or clefts throughout their life.

6. Simple Heart Structure: Protochordates typically have fewer heart chambers and lack blood corpuscles in their blood.

Origin of Chordates:

Chordates emerged during the Cambrian period, around 500 million years ago, coinciding with the early evolution of invertebrates. They likely originated from freshwater forms, although early chordate fossils are predominantly found in marine sediments. Chordates share similarities in embryonic development, coelom type, and larval stages with deuterostome ancestors such as echinoderms, hemichordates, and pogonophorans. Fossil evidence of the earliest vertebrates dates back to the Silurian-Devonian period, approximately 400 million years ago.

 

Theories of Chordate Origin:

1. Dipleurula Concept: Proposed by Semon and elaborated by Bather, this theory suggests a hypothetical larval form, Dipleurula, as the common ancestor of primitive deuterostomes, including echinoderms.

2. Echinoderm Theory: Originating from Johannes Muller and supported by Garstang and DeBeers, this theory posits that chordates, hemichordates, and echinoderms share a common ancestor. It is supported by embryological and serological evidence, emphasizing similarities in coelom formation and larval stages between echinoderms and chordates.

The Deuterostome Line of Chordate Evolution:

Deuterostomes, including echinoderms, hemichordates, and chordates, exhibit several shared characteristics suggesting a close evolutionary relationship. These features include early indeterminate cleavage, development of the blastopore into the anus, enterocoelous coelom formation, and similarities in larval stages. Such evidence indicates a common evolutionary lineage among these diverse organisms.