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DNA Study Shows Comb Jellies were First Animals

Dec 13, 2013 07:25 AM EST
The iridescent comb rows and internal structures of Mnemiopsis leidyi.
The iridescent comb rows and internal structures of Mnemiopsis leidyi.
(Photo : Stefan Siebert, Brown University)

Comb jellies have now replaced sponges as our first ancestors, latest study shows.

For long, sponges (belonging to the phylum Porifera) held the title of being the first animal. Earlier, University of St. Andrews in the U.K. researchers had reported that Otavia antique- a sponge- could have been our earliest ancestor.

But, a new study by researchers at National Human Genome Research Institute (NHGRI) and colleagues has challenged this long-held idea and has shown that comb jelly (of the phylum Ctenophora) came before sponges.

What's interesting about the study is that gelatinous marine creatures such as the comb jelly are actually more complex than the simple porous organisms like the sponges. The new study could change scientists' idea of evolution of life by showing that early organisms had genes for complex processes (such as nervous system and muscles), but some creatures just lost these genes over time, The National Geographic reported.

For the study, researchers analyzed the genome of the sea walnut or Mnemiopsis leidyi, a comb jelly found in the Atlantic Ocean.

"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, at the University of Bergen, Norway and lead author of the study. "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 research team found that these jellies branched out of the evolutionary tree much before than the simple sponges. This is the first time that researchers sequenced Ctenophora 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, Ph.D., senior author of the study and senior scientist in NHGRI's Division of Intramural Research. "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 latest genetic analysis of comb jellies showed that although comb jellies have muscle cells, the organism lacks many genes that specify the muscle types. Researchers say that the absence of the muscle genes suggests that these creatures evolved muscle cells independently after diverging from the evolutionary tree, according to a news release.

Also, comb jellies have a primitive nervous system called the nerve net along with several genes required for a nervous system. Sponges' genomes too have these genes, showing that early forms of the organism might have had the ability to develop a nervous system, but lost it over time.

The study was published in the journal Science.

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