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In order to study the origin of metazoans, we have compared sequences from the 5′ end of the large subunit ribosomal RNA of a number of protists, fungi, plants and metazoans, including all diploblastic phyla (sequences of 10 new species have been determined, including that of the placozoan, Trichoplax adhaerens). These sequences were analyzed using distance matrix, maximum parsimony and maximum likelihood methods, and the validity of the results was ascertained with bootstrapping and species removal or addition. Triploblasts and diploblasts formed two clearly separated monophyletic units; this divergence, which apparently preceded the diversification of diploblastic animals (i.e. the successive sponge, ctenophore, cnidarian radiations), showed a much more ancient origin of triploblasts with respect to diploblasts than classically assumed. These results do not exclude the possibility that triploblasts and diploblasts arose independently from different protists.

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