At first glance, ctenophores look like little more than striped jelly blobs flitting through the sea, propelled by rows of cilia. But the genome of one of these comb jellies, as they are commonly called, suggests they have a pivotal place in animal history. The DNA sequence of the sea walnut, Mnemiopsis leidyi, puts these creatures at the base of the animal tree of life, researchers report online today in Science. That base is also populated by sponges, jellyfish, and their other cnidarian relatives, and organisms called placozoans. By comparing the genomes of these four groups of organisms and other animals, researchers conclude that ctenophores branched off first, not sponges as many have thought. Yet ctenophores have muscles and nerve cells, cell types that sponges lack, suggesting that animal evolution did not proceed smoothly from the simple to the more complex. Instead, the ancestor to all these animals may have had a more complex nervous system that sponges and cnidarians lost, and the mesodermal tissue that gives rise to muscles may have arisen independently in ctenophores.
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