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.
