Researchers have said that comb jellies branched from the rest of the animals before the sponge, a simple animal without complex cell types.
The cornerstone of the study is the researchers' sequencing, assembly, annotation and analysis of the genome of Mnemiopsis leidyi, a comb jelly native to the coastal waters of the western Atlantic Ocean.
The results also show that critical cell types, like neurons and muscle cells, were either lost multiple times during evolution or evolved independently in the ctenophores.
For the past 30 years, researchers have used whole-genome sequencing of organisms to advance their understanding of evolution. They do this by comparing the order of the chemical bases of DNA-150 million base pairs for comb jellies versus 3 billion in humans-that comprise the organism's genome.
While whole-genome sequencing data have been available for four of the five major animal lineages - sponges (Porifera), flat invertebrates (Placozoa), jellyfish (Cnidaria), and animals with left-right symmetry, including humans (Bilateria) -Ctenophora remained the last major animal lineage for which there were no sequenced genomes.
Comb jellies possess muscle cells, but the analysis of the Mnemiopsis genome showed that comb jellies lack the vast majority of genes that specify muscle types in most other animals.
According to the researchers, the absence of such a large number of muscle genes suggests that muscle cells evolved independently in comb jellies, after they diverged from the rest of the animals.
Comb jellies also possess a simple form of nervous system, called a nerve net, and their genome contains many of the genes involved in the nervous system. Sponges also have these genes, suggesting that they may have had the capacity to support a nervous system and perhaps lost it.
The study has been published online in the journal Science.
