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Because of their simple organization the Acoela have been considered to be either primitive bilaterians or descendants of coelomates through secondary loss of derived features. Sequence data of 18S ribosomal DNA genes from non-fast evolving species of acoels and other metazoans reveal that this group does not belong to the Platyhelminthes but represents the extant members of the earliest divergent Bilateria, an interpretation that is supported by recent studies on the embryonic cleavage pattern and nervous system of acoels. This study has implications for understanding the evolution of major body plans, and for perceptions of the Cambrian evolutionary explosion.
"Since the first Metazoa were almost certainly radial animals, the Bilateria must have sprung from a radial ancestor, and there must have been an alteration from radial to bilateral symmetry. This change constitutes a most difficult gap for phylogeneticists to bridge, and various highly speculative conjectures have been made" (1, p. 5). So began Libbie Hyman's discussion on the origin of bilaterian Metazoa, and despite a century of morphological studies and a decade of intensive molecular work the nature of the simplest bilaterian animal remains elusive (1, 2). Paleontological and molecular data indicate that most bilaterian phyla appeared and diversified during the Cambrian explosion (3, 4). Three main clades emergedthe Deuterostomia, the Ecdysozoa, and the Lophotrochozoa (5), although their branching order is unresolved. The acoel flatworms, traditionally classified as an order of the Platyhelminthes, are perhaps the simplest extant members of the Bilateria and have been viewed as either basal metazoans that evolved from ciliate protozoans ("syncytial or ciliate-acoel theory") (6) or a direct link between diploblasts and triploblasts ("planuloid-acoeloid theory") (1, 7). However, the lack of complexity has also been interpreted as a loss of derived features of more complex ancestors ("archicoelomate theory") (8).
The proposed metazoan phylogenetic trees that include acoels have shown them to branch after the diploblasts, indicating that they are considered primitive triploblastic animals (911). However, all 18S ribosomal DNAs (rDNAs) from acoels that have been sequenced so far show rates of nucleotide substitution that are three to five times the rates of most other Metazoa (10), resulting in the long-branch attraction effect in which rapidly evolving taxa cluster and branch together artifactually at the deepest base of the trees...