Man is but a Worm: Chordate Origins - Essay Example

It is possible that the shared morphological characteristics among hemichordates and chordates have been found in their deuterostome ancestor as well. Anatomical structures that are controlled by the genes are analyzed. The main physical characteristics of chordates include notochord, dorsal neural tube, and so on. The homologies that hemichordates share with chordates include pharyngeal gill slits and post-anal tail while other homologies are being studied (Swalla, 2007). Gene sequencing for hemichordates has been done and the axes for hemichordates and lancelets are examined in order to evaluate their homologies.

The popular theory of Garstang about the apparent similarities during the larval development established that the rolling of ambulacraria larva into the posterior neural tube resulted in the evolution of chordate tadpole larvae. Although there exist similarities in the gene expression of both hemichordates and echinoderms, the gene expression data implies that it is hard to find similar homologies. Another theory that has been proven false is that of Romer’s regarding the lophophorates being basal deuterostomes.

The molecular data show that the lophophorates actually belonged to the group lophotrochozoa. Therefore, both the theories of Garstang and Romer have been disproven. Hemichordates and chordates have been assumed to be similar for years and echinoderms have been assumed to be basal deuterostomes. So, the chordate ancestors have been evolved from deuterostomes as they acquired a notochord and neural tubes. Belonging to the same group, ambulacraria, echinoderms and hemichordates possess similar characteristics and homologies including dipleurula larva (Hart, 1994).

Despite these homological similarities the two obvious eyespots in tornaria larvae in ptychoderid hemichordates vanish as the process of metamorphosis continues. The echinoderm ancestors do not have the eyespots found in hemichordates (Arendt and Wittbrodt, 2001). The photoreceptor cells present in deuterostome ancestors were lost in ambulacraria during metamorphosis. The gene sequencing of a lancelet has suggested the presence of 17 chromosomes in ancestral chordates. The expression of gene Hox 1 suggests that the anterior-posterior axes have been arranged in a like manner in hemichordates, lancelets, and vertebrates (Swalla, 2007).

The large cells present in the central nervous system of hemichordates were found to be of two different types. It is suggested that they are involved in various responses including escaping and contraction (Bullock, 1944, 1965). The presence of a dorsal-ventral axis in chordates and the expression of the gene nodal on the right side of chordates and on the left side of sea urchins suggest an axis inversion in chordates (Swalla and Smith, 2008). This has led to the hypothesis that the inversion of the mouth to the other side of the ancestor’s body resulted in the evolution of the vertebrate (Christiaen et al., 2007). The only assumption in this hypothesis is the independent evolution of the central nervous system in hemichordates and chordates.

In an experiment the hypothesis that the dorsal neural tube in hemichordates is homologous to that in chordates is tested. The evolution of notochords as a support