The larvae of the invertebrate chordates (tunicates and amphioxus) are secondary larvae whose relation to the primary dipleurula-type larvae of echinoderms and hemichordates is somewhat obscure. Past authorities have argued that chordates actually derive from motile larvae that have evolved to become sexually mature (paedomorphosis). One such proposal, more specific in its predictions than most, is due to Walter Garstang: he noted the topological similarity between dipleurula ciliary bands (which have their own innervation) and chordate neural plate, and suggested that the latter actually derive from the former. While there is as yet no convincing evidence that chordates are directly derived from the body of a dipleurula-type larva, the core of the idea, that there is continuity in the neurogenic tissues, has some appeal. It is a surprisingly parsimonious way of deriving a characteristic chordate structure, the neural tube, whose origin is otherwise unexplained.
There are two problems with Garstang's hypothesis, however, both a consequence of comparatively recent data on the expression patterns of the genes required to pattern the body during embryonic development. First, dipleurula-type larvae fail to express a full set of antero-posterior patterning genes. Instead, they mainly express genes associated with the head region, which makes them less than ideal models for an ancestral form for chordates, which do clearly have a full set of patterning genes. More specifically, it seems unlikely that the whole length of the nerve cord in chordates could have arisen from a ciliary band of an ancestral larva that has no equivalent of the chordate trunk and tail.
The second problem is that chordates appear to be dorsoventrally inverted relative to other metazoans, including those (i.e. echinoderms and hemichordates) with dipleurula larvae. Note that with Garstang's idea, the dorsal body surface always remains dorsal through the transition to chordates. A variant of Garstang's hypothesis that avoids this problem has been proposed the Nielsen (1999). He accommodates the inversion problem by having the brain originate from a post-oral site on the original ventral side of the body, which, after inversion, becomes dorsal. The mouth is then also dorsal, but this is the case also in some basal chordates, namely tunicates. Secondarily the mouth would have to move to a ventral position in more advanced chordates, i.e. amphioxus and vertebrates. At first sight, a post-oral origin for the brain is very counterintuitive, but the idea is increasingly well supported by the emerging molecular data."
Testing the validity of this and related ideas, in order to better understand the link between lower deuterostomes and protochordates, is an active area of current research in evolutionary biology.
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