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Of the four ancient animal groups in contention for the title of first animal ancestor-sponges, comb jellies, jellyfish, and a group called the placozoa-only the comb jellies lacked a complete genome sequence from one of its members. That's because keeping muscles or a nervous system would be a survival advantage.īut if comb jellies did in fact come before the simpler sponges, that view would be turned upside down, suggesting instead that animals started off with genes for complex biology and some groups then lost them over time.īaxevanis and colleagues came to this conclusion after producing the first complete genome sequence of a comb jelly using a species called the sea walnut (Mnemiopsis leidyi). "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." Sponges are simple creatures, lacking muscles or a nervous system, he said, while comb jellies have both. 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. A traditional view pegs sponges-marine creatures that look more like rocks or corals-as our ancient ancestors.īut a new genetic study is stirring the waters, suggesting comb jellies, gelatinous marine animals that look similar to jellyfish, are actually the first animals to have evolved over 600 million years ago. In a prehistoric version of "the chicken or the egg" question, researchers have long debated which animal group came first.