Although they are gelatinous like jellyfish, comb jellies form their own phylum, known as ctenophores. Trees of life typically root the comb jellies' lineage between the group containing jellyfish and sea anemones and the one containing animals with heads and rears — which include slugs, flies and humans. Comb jellies paddle through the sea with iridescent cilia and snare prey with sticky tentacles. They are much more complex than sponges — they have nerves, muscles, tissue layers and light sensors, all of which the sponges lack. 

“It’s just wild to imagine” that comb jellies evolved before sponges, says Billie Swalla, a developmental biologist at the University of Washington in Seattle and a leading member of the team sequencing the genome of the comb jelly Pleurobrachia bachei. But the team is suggesting just that, in results they presented at the annual meeting of the Society for Integrative and Comparative Biology, held on 3–7 January in San Francisco, California.

Despite comb jellies' complexity, DNA sequences in the Pleurobrachia genome place them at the base of the animal tree of life, announced Swalla's colleague Leonid Moroz, a neurobiologist at the University of Florida in Gainesville. Another team presented results from genome sequencing for the comb jelly Mnemiopsis leidyi, and found that the phylum lands either below, or as close to the base as, sponges on the tree.