ORIGIN OF CHORDATA (PROTOCHORDATA AND EUCHORDATA)
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 fossil remains to give us clues as to origin of chordata. Therefore, the only basis for judging the origin of the group comes from the resemblances between the lower chordates (protochordates and the invertebrates).
Some structural features shared by them, such 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—Coelenterata, Nemertean, 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 echinoderm theory has received some acceptance and shall be considered and evaluated under deuterostome line of chordate ancestry.
Division of Bilateria :- The greatest group of metazoan phyla, the Bilateria, 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.
Deuterostome line of chordate evolution :- Common features of all Deuterostomia, suggests strong evidence of embryological and biochemical nature of a closer evolutionary relationship between the three principal deuterostome phyla— Echinodermata, Hemichordata and Chordata. For example :-
- Early cleavages of zygote are indeterminate, i.e. in, each early blastomere is capable of developing into a whole adult if separated.
- Blastopore of gastrula forms the anus, while mouth is formed as a secondary opening.
- 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.
- 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.
- Biochemically, all deuterostomes use an identical phosphagen, the creatine, in the energy cycle of their muscular contraction. The phosphagen of invertebrates is arginine. However, certain 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.
- 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 :-
- 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 The hemichordata larva (tornaria) is strikingly similar to the larva (bipinnaria or dipleurula) of echinoderms.
- 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 tornaria larva raises doubts about the common ancestry of echinodermates and hemichordates Garstang and de Beer proposed the Neotenous larva theory .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 assume that Carapoid echinoderms might have evolved from tornaria like creatures which have begun to settle down to lead sedentary life. The water vascular system might have developed out of ciliated grooves of these creatures. Besides this, it was also claimed that in the lower silurian period, one carapoid echinoderm had the calyx perforated by a series of 16 small apertures. These apertures can be compared with the gill-slits of Branchiostoma.
There is a strong suggestive evidence that the early evolutionary stage of Deuterostomia group was sessile or sedentary. The pharynx perforated by gill-slits, a characteristic feature of chordates, is also a likely adaptation to sedentary habit. No doubt, hemichordates are sedentary and have pharyngeal gill slits and a hollow dorsal nerve cord. But the presence of a true notochord is doubtful and their adult body plan is quite different from vertebrates. Therefore the prospects of some hemichordate as a likely ancestor of vertebrates seems to be impossible so that they are put under a separate phylum of their own.
The urochordate or ascidian theory of vertebrate origin was advocated by W. Garstang in 1928 and later elaborated by N.J. Berrill (1955) in his book. “Origin of Vertebrates”, Romer (1959) and others. The adult tunicates or ascidians reflect the primitive sessile marine and filter feeding condition of the ancestral chordates. But their body plans are so divergent that it is impossible to imagine a direct evolutionary transformation of an adult ascidian into a vertebrate.
On the other hand, the ascidian larvae are tadpole-like, elongated, bilaterally symmetrical and free-swimming creatures with pharyngeal gill-slits, notochord, dorsal hollow nerve tube, and a muscular postanal tail. They represent only slightly modified living caricature of the ancestral chordate that gave rise to the vertebrate line of evolution. According to this theory. certain of these larvae failed to 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 sessilitv is a specialized condition wherever it occurs in the Animal Kingdom.
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.
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 a 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.
While a great deal is known about modern chordates, including the lower forms, their origin remains obscure. Scientists have not succeeded in determining which lower forms have given rise to them. Their early ancestors most likely were soft-bodied and left no definite fossil remains.
They must have originated prior to Cambrian period as the oldest fossils of known vertebrates have been discovered in late Cambrian strata.
Most scientists consider that the chordates have originated from invertebrates. Several theories attempt to explain the origin of chordates from nonchordate groups, but they have serious drawbacks and are far from being satisfactory. One theory advocates the descent of Chordata from the Echinodermata as such.
The remarkable similarities between the echinoderm (bipinnaria) and hemichordate (tornaria) larvae is taken as good evidence for common ancestry. Garstang suggested that probably free-swimming auricularian larvae of some ancestral echinoderms evolved into chordates through paedogencsis, i.e., prolongation of larval life without undergoing metamorphosis and reproducing sexually. Most zoologists (Romer, Berril, Barrington, etc.) now favour the deuterostome line of chordate evolution, according to which the phyla Echinodermata, Hemichordata and Chordata show common ancestry on embryological and biochemical evidences.
The protochordates provide the connecting link between early chordate ancestors and verebrates. The differentiation probably occurred much earlier than Cambrian period. The earliest traces of vertebrates have been found in the rocks of late Cambrian and Ordovician. A number of fishes followed in Silurian and became abundant in the Devonian. The subsequent periods show the evolution of amphibians, reptiles, birds and mammals