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.Metazoa | Ctenophora | Porifera | phylogenomics | evolution R esolving the phylogenetic relationships close to the root of the animal tree of life, which encompass the phyla Porifera (sponges), Cnidaria (jellyfish, corals, and their allies), Ctenophora (comb jellies), Placozoa (the "plate animals" of the genus Trichoplax), and Bilateria (the group containing all remaining phyla), is fundamental to understanding early animal evolution and the emergence of complex traits [reviewed by Dohrmann and Wörheide (1)]. Traditionally, sponges have been recognized as the sister group to the remaining animals (the "Porifera-sister" hypothesis). Under this scenario, true epithelia (with belt desmosomes connecting neighboring cells) and extracellular digestion are conventionally thought to have been primitively absent in sponges, having evolved in the common ancestor of Placozoa, Ctenophora, Cnidaria, and Bilateria. Within this group, gap junctions between neighboring cells, ectodermal and endodermal germ layers, sensory cells, nerve cells, and muscle cells evolved only once in the common ancestor of Ctenophora, Cnidaria, and Bilateria. Thus, Porifera-sister is consistent with the view that the last common ancestor of the animals was relatively simple and more complex body plans evolved after sponges had separated from the other animal lineages. However, a s...

The relationships at the root of the animal tree have proven difficult to resolve, with the current debate focusing on whether sponges (phylum Porifera) or comb jellies (phylum Ctenophora) are the sister group of all other animals [1-5]. The choice of evolutionary models seems to be at the core of the problem because Porifera tends to emerge as the sister group of all other animals ("Porifera-sister") when site-specific amino acid differences are modeled (e.g., [6, 7]), whereas Ctenophora emerges as the sister group of all other animals ("Ctenophora-sister") when they are ignored (e.g., [8-11]). We show that two key phylogenomic datasets that previously supported Ctenophora-sister [10, 12] display strong heterogeneity in amino acid composition across sites and taxa and that no routinely used evolutionary model can adequately describe both forms of heterogeneity. We show that data-recoding methods [13-15] reduce compositional heterogeneity in these datasets and that models accommodating site-specific amino acid preferences can better describe the recoded datasets. Increased model adequacy is associated with significant topological changes in support of Porifera-sister. Because adequate modeling of the evolutionary process that generated the data is fundamental to recovering an accurate phylogeny [16-20], our results strongly support sponges as the sister group of all other animals and provide further evidence that Ctenophora-sister represents a tree reconstruction artifact. VIDEO ABSTRACT.

The authors wish to note the following: "The contrast of our final projection map was inverted, so that we interpreted the background density rather than the actual protein density in terms of structural features of the potassium channel-Fv complex. In addition, we indexed the 2D crystals with unit cell parameters of a = b = 175 Å, while the correct indexing would be a = b = 124 Å. Given these analysis errors, the resulting density map and our interpretation of the structural features are not correct. Accordingly, we would like to retract this paper. We acknowledge Yoshinori Fujiyoshi, Rod MacKinnon, Kazutoshi Tani, and Tom Walz for identifying the errors and pointing them out to us."

Sponges and evolutionary origins Sponges represent our distant animal relatives. They do not have a nervous system but do have a simple body for filter feeding. Surveying the cell types in the freshwater sponge Spongilla lacustris , Musser et al . found that many genes important in synaptic communication are expressed in cells of the small digestive chambers. They found secretory machinery characteristic of the presynapse in small multipolar cells contacting all other cells and also the receptive apparatus of the postsynapse in the choanocytes that generate water flow and digest microbial food. These results suggest that the first directed communication in animals may have evolved to regulate feeding, serving as a starting point on the long path toward nervous system evolution. —BAP

Opsin proteins covalently bind to small molecular chromophores and each protein-chromophore complex is sensitive to particular wavelengths of light. Multiple opsins with different wavelength absorbance peaks are required for color vision. Comparing opsin responses is challenging at low light levels, explaining why color vision is often lost in nocturnal species. Here, we investigated opsin evolution in 27 phylogenetically diverse insect species including several transitions between photic niches (nocturnal, diurnal, and crepuscular). We find widespread conservation of five distinct opsin genes, more than commonly considered. These comprise one c-opsin plus four r-opsins (long wavelength sensitive or LWS, blue sensitive, ultra violet [UV] sensitive and the often overlooked Rh7 gene). Several recent opsin gene duplications are also detected. The diversity of opsin genes is consistent with color vision in diurnal, crepuscular, and nocturnal insects. Tests for positive selection in relation to photic niche reveal evidence for adaptive evolution in UV-sensitive opsins in day-flying insects in general, and in LWS opsins of day-flying Lepidoptera specifically.