Man is but a worm: Chordate origins - Essay Example

The history and origins of this organisms dates far back as the postulation of Darwin’s Origin of species. The study of the evolution of these species was pioneered in the year 1866 by a scientist, Kowalevsky who made a discovery that tunicates possess tadpole larvae and this hence led to many hypotheses that tunicates are “ancestral chordates”. A thinking which has remained prevalent to date as tunicates have been studied widely in the world over and are still described as the “ancestral chordates”.

However, numerous theories that regard to chordates have come up over the years of study, it can be argued that there are as many arguments and hypotheses as there are scholars of these organisms. The evidence of echinoderms and hemichordates as being sister groups has been appreciated as they have shown major similarities in their genetic characteristics and body types whereas there have been conflicts and rejection as pertains to the mitochondrial, ribosomal and genomic data among members of the phylum chordata (Brown, Prendergast and Swalla, 2008).

As denoted earlier, chordates are said to have four distinctive features that form their body plan: a notochord, an endostyle, pharyngeal gills and a post-anal tail. Hemichordates on the other hand are the only ones with a tripartite body plan in the adult state with a proboscis, a neck and a posterior abdominal region with gill slits. Gene expression data and the analysis of the morphological data have suggested that hemichordates and chordates share the gill slits and post-anal but other characteristics are still being investigated.

Further, when gene expressions are compared in the larvae of developing hemichordates and echinoderms, numerous similarities have been observed in the expression of transcription factors which is inclusive of the t-brain observed in the apical tuft, brachyury which is found in the blastopore and BMP 2/4 which has been distinctively observed in the hydropore (Brown, Prendergast and Swalla, 2008). However, it is still difficult to find similarities in the characteristics between chordates and ambulacraria larval patterns.

Romer’s theory of chordate evolution which had postulated that the lophophorates were basal deuterosomes was a hypothesis that was disproven in the late 1990’s by molecular phylogenies. Later analysis have made suggestions that the brachiopods and phoronoids both of which are lophophorates have a close relationship, but bryozoans which are still members of the same group have a distant relationship hence insinuating that feeding tentacles have evolved distinguishably in many marine invertebrates.

Hemichordates and Echinoderms are joined in one group, the ambulacraria which exemplifies both morphological and molecular shared characteristics particularly the morphologies exhibited in the larvae that feed with their cilia i.e. the dipleurula larva (Brown, Prendergast and Swalla, 2008). A notable shared anterior feature is the existence of a serotonergic nerve net in the anterior apical tuft of some echinoderm and hemichordate tornaria larvae. Although the larvae of echinoderms and hemichordates have many shared morphological characteristics, the larvae of ptychoderid hemichordates have distinctive prominent eyespots in the tornaria larvae which are seen to systematically become vestigial as metamorphosis continues.

This eyespots, according to