The Comb Jelly Was Our First Ancestor, Surprising Evolution Study Says

By Josh Lieberman on December 12, 2013 5:07 PM EST

comb jelly
Say hello to the comb jelly, your first ancestor (according to a new study). (Photo: Flickr: jkohen)

What were the first animals--and thus the first human ancestors? Scientists have long thought sponges are all the way at the bottom of the animal tree. (Sponges may not look much like animals, but they are: they do things like produce sperm and eggs, and sponge larvae can swim.) Now, a new study published in Science claims that before sponges hit the evolution scene, Ctenophora, or comb jellies, had evolved.

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In the study, a team of researchers from the National Human Genome Research Institute created the first complete genome sequence of a comb jelly. When the researchers took this new genome sequence and entered it into software that finds evolutionary relationships among organisms, what they found surprised them: the comb jelly didn't sit in the middle of an animal group, but most likely was at the base. It was the first animal.

It's a controversial finding, but it isn't the first time it's been suggested. In 2008, a study published in Nature found comb jellies as the base of all animals by using the same computer method used in the 2013 study. The lead author of the 2008 study called the findings "a complete shocker....So shocking that we initially thought something had gone very wrong." The 2008 study didn't have the benefit of a complete genome sequence, leading some to question the findings. But this new study, which did rely on a complete genome sequence, came to the same conclusion. 

If comb jellies did in fact evolve before sponges, then it could call into question some basic evolutionary concepts. Comb jellies are more evolved than sponges. Like all animals, comb jellies have a nervous system and muscles; sponges do not. But how could an animal like the sponge be less evolved than an animal that preceded it? It isn't clear.

"If the split between [comb jellies] and all other animals was the earliest split in animal evolution, it suggests some unintuitive facts about evolution," said Finnerty. "For example, that sponges, which are very simple animals that lack a nervous system and lack muscle cells, actually came from an ancestor that had those features." Finnerty said that comb jellies "would seem to be representative of a much more complex condition, anatomically and behaviorally."

That doesn't mean that the comb jellies study is wrong, of course, but it does mean that much of what we know about evolution could be turned on its head. Antonis Rokas, a professor of biological sciences at Vanderbilt University in Nashville, Tenn., told The Scientist that the study raises more questions than answers. If the placing of comb jellies at the bottom of the animal tree is right, "it really changes quite a bit of the text book interpretation that we have about the evolution of morphological complexity."

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