Man being a vertebrate himself, it is not surprising that the problem of searching into the origin and ancestry of vertebrates has long been of particular interest to zoologists. But, like that of most of the animal phyla, the origin of vertebrates also remains obscure. Over the years, several hypotheses have been proposed to explain the origin of vertebrates, but none could stand the test of close scrutiny.
Time of origin. The earliest known truly vertebrate animals were freshwater forms, abundant during the late Silurian and middle Devonian periods. Their fossils are collectively known as ostracoderms. They are placed with living cyclostomes (lampreys, hagfishes), in the jawless group called Agnatha. As their name refers to, 'their body was covered by a dermal bony armour forming an elaborate rounded solid shield on the head. Like cyclostomes, they had presumably a persistent notochord and no vertebrae. The mouth was anterior, ventral and lacked jaws and teeth.
They had no paired appendages homologous with those of vertebrates. Paired eyes, median nostrial and pineal eye were present. A variable number of pharyngeal gill pouches opened by lateral common or separate gill openings. They were adapted for filter feeding.
The oldest fragmentary fossils belonging to ostracoderms occur in the late Cambrian and middle Qrdovician. Absence of any vertebrate fossils in rocks older than the Cambrian, permits only speculation about the earlier history of the vertebrates. Which group was ancestral to the first tree vertebrates (ostracoderms)? In fact there have been no fossils intermediate between the ostracoderms, which are already vertebrates, and any other earlier group of animals. As a result, there has been a great deal of speculation about the time of origin and the early progenitors of the
Subphylum Vertebrata [ 105
dorsal spinal cord notochord
Fig, 3, Hypothetical body plan of ancestral vertebrate.
vertebrates (chordates). Probably the vertebrate organisation had been evolving for several millions of years before the appearance of the first, late Cambrian fossils.
Place of origin. The American geologist Chamberlain gave the idea of freshwater origin of vertebrates, in 1900. It was also supported by Romer and Homer Smith. They argued that dilute body fluids, compared to sea water, and the glomerular kidney to get rid of excess water evolved as adaptations to freshwater conditions.
However, evidence for a . marine origin of vertebrates is also overwhelming. The protochordates and deuterostome invertebrate phyla are exclusively marine forms. All known Cambrian and Ordovician vertebrates also occur as marine fossils. Further, a glomerular kidney is found in hagfishes (Myxiniformes), which are exclusively marine and have body fluids similar to sea water in salt concentration. As Professor James Robertson argues, the primary function of a glomerular kidney is excretory and not osmoregulatory, and it is valuable to an active and mobile vertebrate irrespective of whether it is adapted to sea, to freshwater, or to life on land.
Hypothetical vertebrate ancestor (Prevertebrate). Whatever this ancestral vertebrate or prevertebrate may have been, there are no fossil records to show. There is reason to believe
that it was soft bodied, without any hard exo-or endoskeleton, which could be fossilized (Fig. 3).
The simplest chordates living today are the invertebrate chordates or protochordates belonging to the subphyla Hemichordata, Urochordata and Cephalochordata. They possess the notochord, dorsal nerve cord, pharyngeal gill-slits and postanal tail, fundamentally associated with the vertebrate body plan. They lack the vertebrae and some other features of the earliest as well as the living vertebrates, but they show closest affinities and certainly a common origin with the vertebrates. Therefore, it seems most reasonable and logical to draw inferences about the imaginary, generalized or ancestral vertebrate among them. The ancestor of vertebrates (chordates) can be reconstructed from our present knowledge of existing protochordates.
The American geologist Chamberlain, who proposed the theory of freshwater origin of vertebrates in 1900, also gave the plan of a hypothetical protovertebrate (Fig. 3). It was an aquatic, motile, actively swimming, fish-like animal having a bilaterally symmetrical body with definite head and tail ends. As in higher invertebrates and chordates, the basic internal organization would be some sort of modified tube-within-a-tube arrangement with the major internal organs present inside a large body cavity
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or coelom. It would also possess all the diagnostic chordate-vertebrate features. It had an internal skeleton in the form of a mid-dorsal longitudinal flexible rod, the notochord, surrounded by the vertebrae. There were internal supports in fins, bony plates in the skin and a rigid cranium or skull housing the tripartite brain and associated sense organs. Muscles were V-shaped and segmentally arranged along the sides of the body, particularly the tail, forming myotomes used in locomotion. Mouth was a simple anteroventral opening without jaws. It probably fed on microorganisms, filtered from the water or from bottom detritus. The water taken in through the mouth passed outwards through paired, lateral pharyngeal gill-slits, bathing the internal gills for aquatic respiration. A liver and primitive kidneys were present. Whether the prevertebrate possessed a circulatory system with a single differentiated heart, is uncertain. The nervous system included a tripartite brain and a hollow nerve cord, dorsal to notochord. Sense organs were well developed including the lateral-line organs. The endocrine system was probably already well developed in the prevertebrate. Finally, the gonads were paired. The female laid eggs and the fertilization was external.
It was from such beginnings that the whole array of vertebrates evolved.
Origin of Chordata. We shall now consider the origin of the earlier chordate ancestors of vertebrates. That the chordates have originated from the invertebrates is not doubted by most zoologists now-a-days. Since the earlier chordate ancestors were all soft bodied forms, they left no
Subphylum Vertebrata
fossil remains to give us clues as to their origin.
Therefore, the only basis for judging the origin of the group comes from the resemblances between the lower chordates (protoehordates and the invertebrates. Some structural features shared by them, su:h as bilateral symmetry, anteroposterior body axis, triploblastic coelomate condition, metameric segmentation, etc., may be because of their common ancestry.
Theories of invertebrate ancestry of chordates. Several theories have been advanced to explain the origin of chordates either directly from some invertebrate group or through the intervention ot some protochordate. Almost every invertebrate phylum—Coclenterata, Ncmertean, Phoronida, Annelida, Arthropoda and Echinodermata—has been suggested. But these theories are far from being satisfactory and convincing and have only a historical value. Only the f chmkh rm theory has received some acceptance and shall be considered and evaluated under deuterostomc line of chordate ancestry.
Division of Bilaleria. The greatest group of metazoan phyla, the Hiluteria, is divided into two major divisions—Protostomia and Deuterostomia.
The basis of division is the basic difference in embryonic and larval developments. The divisions probably represent two main lines of evolution within the Animal Kingdom. Their main differences are summarized in the Table 4.
Denlerostome line of chordate evolution. A mere glance at the Table 4, showing common features of all Deuterostomia, suggests strong Table 4. Basic Difference between Protostomia and Deuterostomia.
C h a r a c t e r Protostomia Deuterostomia
1. Cleavage Spiral and determinate Mostly radial and indeterminate
2. Blastopore Fbrrns mouth Forms anus
3. Mesoderm formation By cell cleavage between ectoderm and endoderm
By outpocketing from dorsolateral, endodermal wall of archenteron.
4. Coelom formation Schizocoelous, by a split of mesoderm Enterocoelous, by fusion of gut pouches (except vertebrates)
5. Type of larva Trochophore Tornaria or Bipinnaria (except vertebrates)
6. Phosphagen Arginine Creatine
7. Major phyla included Annelida, Mollusca and Arthropoda Echinodermata, Pogonophora, Hernichordata, and Chordata
125 ] Subphylum Vertebrata
evidence of embryological and biochemical nature of a closer evolutionary relationship between the three principal deuterostome phyla—
Echinodermata, Hemichordata and Chordata. For example :
(1) Early cleavages of zygote are indeterminate, i.e. in, each early blastomere is capable of developing into a whole adult if separated.
(2) Blastopore of gastrula forms the anus, while mouth is formed as a secondary opening.
(3) Pockets or folds arise from the endoderm of developing archenteron of the embryo. The fusion of spaces in the pockets forms the coelom (enterocoelous, except in vertebrates) and their walls become the mesoderm, (4) The pelagic larvae of echinoderms and
hemichordates bear a close structural resemblance. The vertebrates, however, do not have floating larvae, having been lost in the course of evolution.
(5) Biochemically, all deuterostomes use an identical phosphagen, the creatine, in the energy cycle of their muscular contraction.
The phosphagen of invertebrates is arginine.
However, ceitain hemichordates as well as echinoids use both arginine phosphate as well as creatine phosphate. These facts are interpreted to show that the hemichordates are connecting link between chordates and nonchordates.
(6) Serological tests demonstrate that the proteins of the three deuterostome phyla are more closely related to one another than to those of any other phyla.
The precise relationship of the three deuterostome phyla remains unknown, but there is little doubt that they share a common evolutionary history. Several workers have attempted to explain the deuterostome line of chordate evolution. Some of the proposals are as follows :
1. Echinoderm ancestry. On the basis of anatomical, embryological, palaeontological, biochemical and serological evidences, various workers had tried to establish that the chordates probably had originated directly from some primitive echinoderm or some echinoderm larva.
HYPOTHETICAL LARVAE
ECHINODERM / \ CHORDATE
starfish acorn worm
Fig. 4. Similarity of laival fauns of echinoderms and hemichordates has lent support to the idea that both came from a common ancestor.
The hemichordata larva (tornaria) is strikingly similar to the larva (bipinnaria or dipleurula) of echinoderms (Fig. 4). It was, in fact, mistaken for an echinoderm when first discovered. Both are small, transparent, free swimming and bilaterally symmetrical. Both have similar ciliated bands in loops, a dorsal pore, sensory cilia at the anterior end and a complete digestive system of ventral mouth and posterior anus. This striking larval resemblance led Johannes Muller and Bateson to suggest a common ancestry for the echinoderms and the hemichordates. But presence of apical plate with eye,spots in tornai ia larva raises doubts about the tomnion ancestry of echinodermates and hemicfi!.-idaies Garstang and de Beer proposed the
Neoterons mi va thcoiy .suggesting that probably the auricularia larva of echinoderms became sexually mature and later this neotenic larva gave rise to the chordates.
Garstrong (1894) imagined that if ciliated bands together with underlying nervous tissue of auricularia larva of echinodermates, concentrates to form ridges leaving a groove between them and if lips of the groove fuses subsequently, it will give rise tube. It will resamble with the nervous system of chordates.
Cambrian and ordovician fossil records of Carpoid echinoderms lead Torsten and Gislen to
Subphylum Vertebrata [ 109
metamorphose into adults, but became neotenous*, that is, sexually mature by developing gonads precociously, . and later evolved into the cephalochordates and vertebrates. The sessile nature of life of the primitive chordate ancestory, pterobranch hemichordates and primitive echinoderms by the workers is considered resulting from common ancestry.
However, the ascidian theory of chordate origin does not seem to be perfect. The principal drawback is that the theory considers sessile urochordates to be ancestral to chordates. Whereas, they are highly specialized because scssilitv is a specialized condition wherever it occurs in the Animal Kingdom.
' 4. Cephalochordate ancestry. The cephalochordates, particularly the lancelets (Branchiostoma lanceolatum) are an interesting group of animals. They possess the three basic , chordate features in diagrammatic form. According
to Colbert, the living Amphioxus (^Branchiostoma) answers the logical structure of a model prevertebrate. Homer Smith's reconstruction of the hypothetical protovertebrate in his book, From Fish to Philosopher (1953), also greatly resembles Amphioxus. But, the excretory system of cephalochordates consisting of flame cells called solenocytes, is altogether different from that of vertebrates. The- solenocytes are ectodermal and therefore not homologous with 'the mesodermal vertebrate kidneys. Further, lack of strong cephalization and sense organs, and the unique forward extention of notochord indicate that the cephalochordates may hint about the likely ancestral body plan of vertebrates, but they are not themselves ancestral. Both may represent divergent paths of evolution from a common remote ancestor.
5. Harrington's hypothesis. The most plausible hypothesis by E.J.W. Harrington (1965) is based on the deuterostome line of chordate evolution. The common echinoderm—chordate ancestor was in all probability a small, sessile or semisessile, lophophorate or arm feeding creature.
It fed by ciliary method by trapping food particles in va set of waving tentacles. From this ancestral stalk were derived early stalked echinoderms and pogonophores. The next logical step was the derivation of a sessile fiber feeder or stem chordate. The cumbersome external tentacles were replaced by an internal filtering apparatus in which food -.is entrapped inside pharynx which develops external gill-slits and a mucus-secreting endostyle.
Cephalodiscus, a living pterobranch hemichordate, shows the transitional stage between the two modes of feeding because it has a single pair of gill-slits besides the crown of tentacles.
Pharyngotremy, that is, perforated pharynx with internal food-trapping mechanism, resulted in the evolution of free-living hemichordates on one hand and the sessile ancestral urochordates (tunicates) on the other. Some ancestral tunicates, instead of producing ciliated larvae common to the earliest groups, formed tadpole larvae with all the typical somatic features of the chordates. According to Garstang, 'the larva became elongated and increased in' size, the longitudinal ciliary bands shifted mid-dorsally and changed to the hollow nerve cord, the adoral cilia developed into the endostyle, and muscle fibres evolved in the tail.
This typical chordate larva by paedogenesis suppressed the sessile adult stage, developed gonads precociously and became the ancestor of cephalochordates (Branchiostoma), vertebrates and the larvaceans probably representing three cases of parallel evolution.
* N'.B. T h e development of gonads in the ascidian larva is wrongly called neoteny. It m u s t - b e correctly termed paedogenesis or paedomorphosis. Neoteny properly refers to the retention 'of an embryonic or larval trait in the adult body. E x a m p l e s of neoteny a r e retention of embryonic cartilaginous endoskeleton in adult elasmobranch fishes and the external gills in adult aquatic salamanders. In contrast, paedogenesis emphasizes the premature development of gonads in the larval body, which never undergoes metamorphosis.
Subphylum Vertebrata
IMPORTANT QUESTIONS
» Long Answer Typt Questions
1. What are vertebrates ? Describe their general characters.
2. Differentiate between—(i) Chordates and vertebrates, (ii) Lower vertebrates and higher vertebrates, (iii) Protostomia and Deuterostomia.
3. Write on essay on the origin and ancestry of vertebrates.
4. Give the outline of evolutionary history of vertebrates,
» Short Answer Typt Question
3. Enlist the 'big five" diagnostic vertebrate characteristics,
» Multiple Choke Qutittons
1. The term vertebrata is synonymous to ; (a) Craniata (b) Chordata (c) Urochordata (d) Protochordata
2. In vertebrates, the pharyngeal gill slits are not more than :
(a) 6 pairs (b) 7 pairs (c) 8 pairs (d) 9 pairs 3. Heart in vertebrates is :
(a) Dorsal, muscular (b) Ventral, skeletal (c) Ventral, muscular (d) Dorsal, skeletal 4. Respiratory pigment in vertebrates is :
(a) Chlorophyll (b) Haemocyanin (c) Heparin (d) Haemoglobin 5. Excretion in vertebrates takes place by :
(a) Unpaired kidney (b) Unpaired pro nephron (c) Paired mesonephric kidneys
(d) Paired mesonephric and irietanephric kidneys 6. Brain in vertebrates, is the enlarged :
(a) Anterior end of dorsal nerve cord (b) Posterior end of dorsal nerve cord (c) Anterior end of ventral nerve cord (d) Posterior end of dorsal nerve cord
?. Spinal nerves in vertebrates are formed by the union of : (a) A pair of dorsal roots (b) A pair of ventral roots (c) A dorsal and a ventral nerve root
(d) Two dorsal and one ventral nerve root 8. Which period is known as the age of fishes :
(a) Cambrian (b) Ordovician (c) Silurian (d) Devonian
9. The first fossil records of vertebrates were found in the rocks of :
(a) Ordovician (b) Cambrian (c) Jurassic (d) Devonian 10. The first jawed vertebrates (Placoderms), arose in ;
(a) Cambrian (b) Ordovician (c) Devonian (d) Silurian 11. The first tetra pods arose in I
(a) Ordovician period (b) Devonian period (c) Silurian (d) Carboniferous 12. The 'age of amphibians' is used for :
(a) Ordovician period (b) Devonian period (c) Silurian (d) Carboniferous 13. Which era is known as 'the age of reptiles' :
(a) Archaeozoie (b) Proterozoic (c) Paleozoic (d) Mesozoic 14. Mammals arose during the :
(a) Cretaceous period (b) Triassie period (c) Jurassic period <d) Permian period 15. The connecting link between reptiles and birds :
(a) Apteryx (b) Strulhio {c) Archaeopteryx (4) Crvcodilus
16. Which era is known as the age of mammals ; (a) Archaeozoie (b) Paleozoic (c) Mesozoic (d) Cenozoic 17. In deuterostomes, blastopore forms :
(a) The mouth (b) The anus (c) Brain (d) Kidney 18. Which phosphagen is utilized by vertebrates in
cycles of muscle contraction ;
(a) Creatine (b) Arginine (e) Guanine (d) Cysteine
ANSWERS
1. (a) 2.
(b)
3. (c) 4 . (d) 5. (d) 6. (a) 7 . (c) 8. (d) 9 . (a) 10. (c) 11. (d) 12. (d) 13. (d) 14. (b) 15. (c) 16. (d) 17. ( b ) 18. (a).The earliest known vertebrates to appear in fossil record were jawless primitive fishlike animals collectively known as the ostracoderms, and placed under the class Ostracodermi. They resembled the present day cyclostomes (lampreys and hagfishes) in many respects and together with them, constitute a special group of jawless vertebrates, the Agnatha.
Occurrence
Ostracoderms were encountered first as fragmented fossils occurring in the rocks of late Cambrian and middle Ordovician periods, dating back approximately 5(X) million years. They were quite abundant during the upper Silurian and Devonian periods. Most of their fossils were preserved in the bottom sediments of freshwater streams. However, opinion is sharply divided as to whether their habitat was freshwater or marine.