The Base of the Animal Tree? The identity of the most basal lineages of the animal kingdom evolutionary tree has long been contested. Ryan et al. (p. 10.1126/science.1242592 ; see the Perspective by Rokas ) sequenced the genome of the ctenophore the warty comb jelly or sea walnut, Mnemiopsis leidyi , and conclude that ctenophores alone, not sponges or the clade consisting of both ctenophores and cnidarians, are the most basal extant animals. The results suggest a specific evolutionary process that likely occurred—including repeated gains and loss of mesoderm, expansion of genes associated with the cell cycle, growth signaling, apoptosis, and epithelial and neural cell types. Furthermore, previous hypotheses regarding the evolution of animals may require re-evaluation.

The Genome of the Ctenophore Mnemiopsis leidyi and Its Implications for Cell Type Evolution