Comb Jelly Reveals New Secrets of Animal Evolution

For more than a century, scientists believed that complex cell types, such as neurons and muscles, evolved only once. The simple animals that lack these cell types branched off from the rest of the animals on the evolutionary tree. Now, though, a comb jelly has challenged this belief and has shown that these complex cells were gained and lost several times over the course of evolution.

The comb jelly in question is called Mnemiopsis leidyi, which is native to the coastal waters of the western Atlantic Ocean. In order to find a little bit more about this species, though, the researchers used whole-genome sequencing. The scientists compared the order of the chemical bases of DNA that comprise the organism's genome.

"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," said Andy Baxevanis, one of the researchers, in a news release. "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."

Comb jellies possess muscle cells, but the research from this study showed that this particular comb jelly lacks the vast majority of genes that specify muscle types in most other animals. 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. These jellies also possess a simple form of nervous system, called a nerve net, and their genome contains many of the gnees involved in the nervous system.

"Our analysis of the Mnemiopsis genome thoroughly corroborates previous studies suggesting that ctenophores might be the sister group to the rest of the animals," said Joseph Ryan, one of the researchers, in a news release. "With our whole-genome sequencing data in hand, it is now clear that the cell types that make up muscles and nervous systems were either lost in some animal lineages or that, despite the complexity of these cells, they very well may have evolved multiple times."

The findings reveal a bit more about the process of evolution in animals. More specifically, it shows that there wasn't just "one" path in evolution. Instead, cells evolved multiple times, showing that there are different ways to get to similar results.

The findings are published in the journal Science.