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To corroborate such position and avoid the criticisms of saturation and long-branch effects thrown on the SSU molecule, we have searched for independent molecular data bearing good phylogenetic information at deep evolutionary nodes. Here we report a phylogenetic analysis of DNA sequences from the myosin heavy chain type II (myosin II) gene from a large set of metazoans, including acoels and nemertodermatids. Our study demonstrates, both for the myosin II data set alone and for a combined SSU + myosin II data set, that Platyhelminthes are polyphyletic and that acoels and nemertodermatids are the extant earliest bilaterians. Hence, the common bilaterian ancestor was not, as currently held, large and complex but small, simple, and likely with direct development. This scenario has far-reaching implications for understanding the evolution of major body plans and for perceptions of the Cambrian evolutionary explosion.<\/jats:p>","DOI":"10.1073\/pnas.172390199","type":"journal-article","created":{"date-parts":[[2002,9,30]],"date-time":"2002-09-30T12:42:53Z","timestamp":1033389773000},"page":"11246-11251","update-policy":"https:\/\/doi.org\/10.1073\/pnas.cm10313","source":"Crossref","is-referenced-by-count":161,"title":["A phylogenetic analysis of myosin heavy chain type II sequences corroborates that Acoela and Nemertodermatida are basal bilaterians"],"prefix":"10.1073","volume":"99","author":[{"given":"I.","family":"Ruiz-Trillo","sequence":"first","affiliation":[{"name":"Departament de Gen\u00e8tica and Departament de Biologia Animal, Universitat Barcelona, Av. 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