Researchers have long-believed complex cell types, such as muscles, evolved only once; new research on a comb jellyfish challenges that idea.

The research team sequenced the genome of the Mnemiopsis leidyi, a comb jellyfish (ctenophore) that can be found in the western Atlantic Ocean, a National Institute of Health news release reported.

Scientists thought the jellyfish and other simple creatures that lacked these complex cells branched off from the evolutionary tree before the advanced features had a chance to develop. This new research suggests the jellyfish's neurons and muscle cells "were either lost multiple times during evolution or evolved independently in the ctenophores," the news release reported.

The team found the box jellies do contain muscle cells, but don't possess any of the other genes that normally specify other animals' muscle types.

"Having genomic data from the ctenophores is crucial from a comparative genomics perspective, since it allows us to determine what physical and structural features were present in animals early on," Andy Baxevanis, Ph.D., senior author of the study and senior scientist in NHGRI's Division of Intramural Research, said. "These data also provide us an invaluable window for determining the order of events that led to the incredible diversity that we see in the animal kingdom."

The absence of these genes suggests the jellyfish developed its muscle cells independently from the rest of the animal kingdom.

The jellyfish has a special type of nervous system called a "nerve net," and have specific related genes. Sponges also possess these "nerve net" genes suggesting they once had an immune system but lost it.

"Expanding our understanding of genomes across the animal kingdom is important for gaining an understanding of evolutionary adaptation at the molecular, cellular and organismal levels," Daniel Kastner, M.D., Ph.D., NHGRI scientific director, said. "The whole-genome sequence of the comb jelly provides a nontraditional model through which new insights about genes and their functions, including those in our own genome, may become better understood."

The team hopes their discovery will lead to a future understanding of our developmental history.

"Our study demonstrates the power of comparative genomics research having an evolutionary point of view, probing the interface of genomics and developmental biology," co-author Jim Mullikin, Ph.D., NISC director, said. "The data generated in the course of this study also provide a strong foundation for future work that will undoubtedly lead to novel findings related to the nature of animal biology."