Abstract
I challenge the widely held assumption, reflected in current classifications, that larvae and corresponding adults evolved from common ancestors and that animals with lophophores evolved from an ancestral lophophorate. Types of animal development are defined, and their origins are discussed in relation to the common ancestor hypothesis and hybridogenesis, the proposal that new life forms have been generated by hybridization. Echinoderms are commonly classified by their larval characteristics as bilateral enterocoelous deuterostomes. However, some echinoderms without larvae develop as radial schizocoelous protostomes, which would place them in a different superphylum from those with larvae. Current classifications also place some hemichordates in a different superphylum from other hemichordates. A genetic study concludes that lophophorates are not descended from a common ancestor, thus invalidating the clade Lophotrochozoa. I propose that lophophorates and barnacles are chimeras, with components from different phyla. The evidence consistently supports the view that both larvae and lophophores were later additions to life histories. F M Balfour (1851-1882) recognized that larvae were later additions to life histories, and there were no echinoderm larvae until after the establishment of the classes of that phylum. He has been ignored, and, as a result, animal taxonomy has been on the wrong road since the late 19th century.
Highlights
Conventional assumptions on the evolutionary origins of larvae and lophophores have led to fundamental flaws in inferred relationships between animal phyla
These are reflected in current classifications, which place a minority of echinoderms in a different superphylum from the majority, and one class of hemichordates in a different superphylum from other hemichordates
In this paper I review hypotheses on the origins of larvae, lophophores and barnacles, and I call for a radical reassessment of higher-group animal taxonomy
Summary
Conventional assumptions on the evolutionary origins of larvae and lophophores have led to fundamental flaws in inferred relationships between animal phyla. The larval transfer hypothesis states that larvae were later additions to life histories They originated as adults in other taxa, and their genomes were transferred by hybridization. More than a century after Balfour, and after 40 years of accumulating examples of animal development that defy explanation in terms of the common ancestor hypothesis, I, like Balfour, deduced that the basic forms of most larvae were later additions to life histories, and, contrary to Haeckel, ancestral echinoderms were radial, and there were no echinoderm larvae until after the establishment of echinoderm classes. Hybridogenesis is the generation of new life forms and new life histories by hybridization, which is the interbreeding of different organisms at all levels of relationship It includes larval transfer (defined above) and component transfer, the transfer of genetic prescriptions for parts of animals (e.g., lophophores) by hybridization. If there is more than one larval phase in the life history of an animal, metamorphosis is the change-over from the expression of one larval genome to the expression of either the larval genome or the adult genome
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