Animals such as amphioxus are commonly considered candidates for the title of vertebrate progenitor. Under the proposed model, such animals may be in the vertebrate lineage, but they are not its beginning. They are in a sense its end, in that evolution proceeds by reduction and they are the most reduced chordates. Their evolutionary potential is minimal, since they have lost the parts which could have been adapted to new functions.
Fig.6 Amphioxus.
This does not mean that such animals were not among the earliest vertebrates; their reductive evolution from the vertebrate prototype could have been immediate and extreme. Some early successful chordates may have been morphologically simple, drastic reductions from the protovertebrate form, while other branches which were more conservative,retaining more parts, only later became dominant, after their additional parts were usefully adapted.
The fossil evidence does not show a preponderance of these 'primitive' types among early chordates. Among the oldest known vertebrates are fishes at least as complex as modern fishes. They had complex skeletons, fully developed organs of perception, electric organs, and armor plates; even their brains had the same components as the brains of modern vertebrates.
The progenitorship of the simple chordates is based on their simplicity and their resemblance to the vertebrate embryo. This is valid only if one assumes that evolution always proceeds from the simple to the complex, and that embryonic development mirrors evolution. It is more probable that among segmented organisms reduction is the essential evolutionary mechanism, as the evidence implies, and that simple chordates are like vertebrate embryos because they are vertebrate embryos; the rest of their ontogeny has been lost.
It is understandable that early Darwinists seized upon the idea of the progenitorship of the simple chordates. It fit with pre-Darwinian classificatory schemes. The pattern of reduction and distortion had not yet been recognized, and the biogenetic law was a promising new concept. But this view has not been substantiated, and it has proven to be a sterile idea.
forward to Conclusion