While an argument over ancient ancestry may seem academic, it's an important question to answer because it influences how researchers think about the nature of animal evolution, said study co-author Andy Baxevanis, a geneticist at the National Human Genome Research Institute in Bethesda, Maryland.

Sponges are simple creatures, lacking muscles or a nervous system, he said, while comb jellies have both. (See a video on how comb jellies hunt.)

"There's been this long-standing tenet in evolutionary biology," Baxevanis explained, that once evolution led to "some kind of complex cell type—like musculature or an eye—you wouldn't lose it."

That's because keeping muscles or a nervous system would be a survival advantage.

But if comb jellies did in fact come before the simpler