Chordates originated from invertebrates some 500 million years ago during Cambrian Explosion.
Chamberlain: Evolved from freshwater forms as all modern chordates have glomerular kidneys that are designed to remove excess water from the body. But fossils of chordates have been recovered from marine sediments and the modern protochordates are all marine forms. Glomerular kidneys are also found in some marine species like sharks. Hence, marine origin of chordates seems more possible.
Hemichordata were the first chordates that evolved on earth. They gave rise to Ascidians. Larvae of Ascidians evolved into Cephalochordates and fish by neoteny.
Enchinoderm Origin (Muller)
-Based on comparative studies of larval stages of echinoderms and hemichordates.
-Tornaria larva of Hemichordates resembles echinoderm larvae (Bipinnaria, Auricularia, Dipleurula) which possess ciliary bands and apical tuft of cilia.
-Echinoderms are also deuterostomes and possess mesodermal skeletal elements.
-Fossil Calcichordates are Echinodermata with Calcium Carbonate skeleton (vertebrates have skeleton with hydrated calcium and phosphate). They had a large pharynx with a series of gill slits and each covered with flaps for filter feeding, small segmented body and post natal tail.
-Perforated Pharynx for filter feeding appears to have evolved in diverse groups of animals during Cambrian-Ordovician periods when planktons were abundant in water.
Hemichordata Origin (Romer)
-Ancestral Deuterostomes were sedentary tentacle feeders whose mucus laden ciliated tentacles trapped plankton as they waved in water, like modern Hemichordates (Cephalodiscus and Rhabdopleura).
-Pharyngeal gill slits evolved in these ancestors, which made the pharynx sieve like to trap planktons.
-Extant Pterobranchs possess both ciliated arms and pharyngeal slits.
-Tornaria larva of hemichordates shows phylogenetic relationship with echinoderm larvae and hemichordates show affinity with chordates.
Urochordate Origin (Garstang and Berrill)
-Tadpole like larva of Urochordates have typical chordate characters.
-Notochord in tail along with segmented myotomes.
-Dorsal hollow nerve cord.
-Sense organs.
-Pharyngeal gill slits.
-Chordates evolved from some sessile filter feeding urochordate, with larval stage evolving into adult by neoteny.
Cephalochordate Origin (Chamberlain)
-Extant cephalochordates possess all chordate characters in typical state.
-Also show primitive features of non chordates: absence of heart, head, sense organs, respiratory pigment, filter feeding mode of food capture and excretion by solenocytes.
-Amphioxus like fossils have streamlined, ribbon shaped, 5 cm long body having notochord in the posterior 2/3 of body.
Barrington’s Chordate Origin Theory
-Common ancestor of echinoderms and chordates was a sessile ciliary arm feeder that lived in plankton rich environment of Cambrian.
-Modern echinodermata evolved from this ancestor and retained the original mode of feeding, because they continued to inhabit the same environment.
-Perforation of Pharynx with gill slits evolved in a few groups, which was a superior method of gathering food.
-Hence, sedentary animals evolved pharyngeal filter feeding and lost ciliary arm feeding.
-When plankton numbers declined, the free swimming larva did not metamorphose into a sedentary adult, since free swimming was a better mode of obtaining food in time of scarcity.
-Cephalochordate like ancestors evolved by perfection and expansion of chordate characters that were
present in Ascidian tadpole larvae.