Larvae and the origins of major phyla

Garstang's vision, of the adaptations of ancestral larvae providing body-plans for descendants, was a useful model for the origins of new phyla. However, the concept of ‘balanced developmental programme’, in which the future adult structure constrains the earlier stages as well as the converse, cannot easily accommodate the Garstang model. Another modern view of larval adaptation, which sees larval development as controlled in a separate (parallel) fashion, cannot easily ‘move’ the reproductive function from one programme to the other. Taken with Gould's clarification of the contrast between neoteny and progenesis, this leaves little ‘room’ for Garstang's model, later adults capitalizing on early caenogenetic innovation. However, some marine larvae are now known to be products mostly of maternal instruction, the zygote nuclei not yet having been involved in their morphogenesis; they are phylotypic (phyletic) stages. Were progenesis to proceed right ‘back’ to such a larva (e.g. the trochophore, nauplius or dipleurula) part of the zygote genome could be ‘freed’ for prescription of a wholly new kind of morphogenesis of the adult. Part of the genome would always be tied to ovarian prescription, part to biological ‘givens’ like t-RNA and histone sequences. Mammals show that maternal prescription can largely be dispensed with, and nematodes show that zygotic-controlled morphogenesis can be greatly reduced; these, we suggest, are evolutionary routes away from ancestral body-plans.