Genomics. Sea anemone provides a new view of animal evolution.

C R E D IT : A D A M R E IT Z E L Genome sequencers have just jumped down to a lower branch on the tree of life, and the view has given them a new perspective on animal evolution. The newly decoded DNA of a few-centimeter-tall sea anemone looks surprisingly similar to our own, a team led by Nicholas Putnam and Daniel Rokhsar from the U.S. Department of Energy Joint Genome Institute in Walnut Creek, California, reports on page 86. This implies that even very ancient genomes were quite complex and contained most of the genes necessary to build today’s most sophisticated multicellular creatures. “The work is truly stunning for its deep evolutionary implications,” says Billie Swalla, an evolutionary developmental biologist at the University of Washington, Seattle. Until now, researchers have relied heavily on the sequenced genomes of the fruit fly, nematode, and that of a few other invertebrates to understand genome evolution leading up to the vertebrates. But the new work drives home how streamlined these invertebrate genomes have become. In contrast, the sea anemone’s genome “has not changed much and retains many of the features present in our last common ancestor,” says Jacek Majewski, a geneticist at McGill University in Montreal, Canada. It “seems to fill the niche essential to answer many evolutionary questions.” Animals divide into two groups, sponges and eumetazoans. The eumetazoans consist of comb jellies, cnidarians such as anemones, and bilaterians, which include everything else: limpets, lions, lobsters, and us. Comb jellies and cnidarians branched off before bilaterians diversified into the variety of animal groups known today, and they are considered relatively “simple” organisms. Cnidarians, for example, have a mouth but no anus; two tissue layers, not three; a nerve net, but no central nervous system per se. Biologists have had plenty of bilaterian genomes to work with. But to look back in time, they needed a nonbilaterian genome for comparison—genes and genome features common to both bilaterians and nonbilaterians likely existed in their common ancestor 750 million years ago. In late 2004, Putnam, Rokhsar, and their colleagues began deciphering the 450-million-base genome of the cnidarian of choice, the starlet sea anemone, Nematostella vectensis. The draft genome is already producing many surprises. Among the anemone’s 18,000 or so protein-coding genes, the researchers have identified 7766 that are also