Not the comb jellies?
Who came first – sponges or comb jellies? A new study reaffirms that sponges are the oldest animal phylum – and restores the classical view of early animal evolution, which recent molecular analyses had challenged.
Sponges (Porifera), comb jellies (Ctenophora), the true jellyfish and corals (Cnidaria) and plate animals (Placozoa) together make up the so-called non-bilaterian animals. All four phyla are evolutionarily ancient, and were already in existence more than 600 million years ago. However, unraveling the interrelationships between them — and how they relate to the Bilateria, to which all other animals, including humans, belong — has turned out to be one of the most challenging problems in evolutionary biology. “If we are to understand the evolution of certain key features of animals, such as their nervous systems, tissue organization and organs, we first have to clarify the early phylogenetic relationships” Wörheide explains.
So all these phyla of complex animals were already in existence 600 million yeas ago?
The evolution of complexity The traditional view of the issue postulates that the sponges were the first group to diverge from the lineage that gave rise to all other animals — in other words, Porifera are the sister group of all other animal species. This hypothesis is supported by studies of comparative morphological and functional anatomy, which are based on the recognition of structural similarities between the basic body plans of the various animal groups. However, the results of more recent phylogenetic analyses, derived from comparisons between sequences of specific genes and of whole genomes, seemed to point to Ctenophora as the first group that parted company with the lineage from which the rest of the animal kingdom (including sponges) evolved.
So this is a conflict between two different methods of analysis which give two different results, but the authors’ view is ore traditional?
Confirmation of this latter hypothesis would have far-reaching implications for our understanding of evolutionary history because comb jellies and their relatives are relatively complex animals — unlike sponges and placozoans, ctenophores possess muscles and a nervous system. “If ctenophores diverged first, these organ systems likely have been present in the common ancestor of all animals — and sponges and placozoans must subsequently have lost them — or complex traits like nerve cells and muscles must have emerged independently several times in different lineages,” Wörheide explains. “The latter scenario would require a wholesale reassessment of the early evolution of animals.” More.
Well, maybe a wholesale reasssessment can be put off, but not for long?
See also: Stasis: Life goes on but evolution does not happen
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Here’s the abstract:
Understanding how complex traits, such as epithelia, nervous systems, muscles, or guts, originated depends on a well-supported hypothesis about the phylogenetic relationships among major animal lineages. Traditionally, sponges (Porifera) have been interpreted as the sister group to the remaining animals, a hypothesis consistent with the conventional view that the last common animal ancestor was relatively simple and more complex body plans arose later in evolution. However, this premise has recently been challenged by analyses of the genomes of comb jellies (Ctenophora), which, instead, found ctenophores as the sister group to the remaining animals (the “Ctenophora-sister” hypothesis). Because ctenophores are morphologically complex predators with true epithelia, nervous systems, muscles, and guts, this scenario implies these traits were either present in the last common ancestor of all animals and were lost secondarily in sponges and placozoans (Trichoplax) or, alternatively, evolved convergently in comb jellies. Here, we analyze representative datasets from recent studies supporting Ctenophora-sister, including genome-scale alignments of concatenated protein sequences, as well as a genomic gene content dataset. We found no support for Ctenophora-sister and conclude it is an artifact resulting from inadequate methodology, especially the use of simplistic evolutionary models and inappropriate choice of species to root the metazoan tree. Our results reinforce a traditional scenario for the evolution of complexity in animals, and indicate that inferences about the evolution of Metazoa based on the Ctenophora-sister hypothesis are not supported by the currently available data. (paywall) – Davide Pisani, Walker Pett, Martin Dohrmann, Roberto Feuda, Omar Rota-Stabelli, Hervé Philippe, Nicolas Lartillot, Gert Wörheide. Genomic data do not support comb jellies as the sister group to all other animals. Proceedings of the National Academy of Sciences, 2015; 201518127 DOI: 10.1073/pnas.1518127112