Animal evolution. The end of the intermediate taxa?

Contrary to general belief, there has not been a reliable, global phylogeny of animals at hand within the past few decades. Recent progress in molecular phylogeny is rapidly changing the situation and has provided trees that constitute a reference frame for discussing the still controversial evolution of body plans. These trees, once purged of their possible artefacts, have already yielded confirmation of traditional, anatomically based, phylogenies as well as several new and quite significant results. Of these, one of the most striking is the disappearance of two superphyla (acoelomates such as flatworms, pseudocoelomates such as nematodes) previously thought to represent grades of intermediate complexity between diploblasts (organisms with two germ layers) and triploblasts (organisms with three germ layers). The overall image now emerging is of a fairly simple global tree of metazoans, comprising only a small number of major branches. The topology nicely accounts for the striking conservation of developmental genes in all bilaterians and suggests a new interpretation of the 'Cambrian explosion' of animal diversity.

[1]  E. Davidson,et al.  Set-aside cells in maximal indirect development: evolutionary and developmental significance. , 1997, BioEssays : news and reviews in molecular, cellular and developmental biology.

[2]  R. Raff,et al.  Molecular phylogeny of the animal kingdom. , 1988, Science.

[3]  F. Ayala,et al.  Origin of the metazoan phyla: molecular clocks confirm paleontological estimates. , 1998, Proceedings of the National Academy of Sciences of the United States of America.

[4]  J. M. Turbeville,et al.  Comparative Ultrastructure and the Evolution of Nemertines , 1985 .

[5]  G. Balavoine.  Are Platyhelminthes Coelomates without a Coelom? An Argument Based on the Evolution of Hox Genes' , 1998 .

[6]  J. Lake,et al.  Evidence from 18S ribosomal DNA that the lophophorates are protostome animals , 1995, Science.

[7]  D. Eernisse,et al.  Annelida and Arthropoda are Not Sister Taxa: A Phylogenetic Analysis of Spiralian Metazoan Morphology , 1992 .

[8]  R. Raff,et al.  Evidence for a clade of nematodes, arthropods and other moulting animals , 1997, Nature.

[9]  Axel Meyer,et al.  Limitations of Metazoan 18S rRNA Sequence Data: Implications for Reconstructing a Phylogeny of the Animal Kingdom and Inferring the Reality of the Cambrian Explosion , 1998, Journal of Molecular Evolution.

[10]  T. Cavalier-smith,et al.  Sponge phylogeny, animal monophyly, and the origin of the nervous system: 18S rRNA evidence , 1996 .

[11]  W. Gehring,et al.  Homeobox genes in the ribbonworm Lineus sanguineus: evolutionary implications. , 1998, Proceedings of the National Academy of Sciences of the United States of America.

[12]  E. Robertis,et al.  The ancestry of segmentation , 1997, Nature.

[13]  R. de Wachter,et al.  18S rRNA data indicate that Aschelminthes are polyphyletic in origin and consist of at least three distinct clades. , 1995, Molecular biology and evolution.

[14]  R. Raff,et al.  Phylogenetic position of phylum Nemertini, inferred from 18S rRNA sequences: molecular data as a test of morphological character homology. , 1992, Molecular biology and evolution.

[15]  A. Wilson,et al.  Generation time and genomic evolution in primates. , 1973, Science.

[16]  R. Christen,et al.  An analysis of the origin of metazoans, using comparisons of partial sequences of the 28S RNA, reveals an early emergence of triploblasts. , 1991, The EMBO journal.

[17]  G. Balavoine,et al.  One or Three Cambrian Radiations? , 1998, Science.

[18]  The Invertebrates: Protozoa Through Ctenophora , 1940 .

[19]  Timothy M. Collins,et al.  Deducing the pattern of arthropod phytogeny from mitochondrial DNA rearrangements , 1995, Nature.

[20]  H. Milne-Edwards,et al.  Le règne animal distribué d'après son organisation , 2009 .

[21]  A. H. Clark,et al.  Animal evolution , 1981 .

[22]  L. Salvini-Plawen.  2 – Early Evolution and the Primitive Groups , 1985 .

[23]  G. Balavoine.  The early emergence of platyhelminths is contradicted by the agreement between 18S rRNA and Hox genes data. , 1997, Comptes rendus de l'Academie des sciences. Serie III, Sciences de la vie.

[24]  W. Müller.  Origin of metazoan adhesion molecules and adhesion receptors as deduced from cDNA analyses in the marine sponge Geodia cydonium: a review , 1997, Cell and Tissue Research.

[25]  S. Carranza,et al.  Are the Platyhelminthes a monophyletic primitive group? An assessment using 18S rDNA sequences. , 1997, Molecular biology and evolution.

[26]  J. Felsenstein.  Cases in which Parsimony or Compatibility Methods will be Positively Misleading , 1978 .

[27]  Jeffrey S. Levinton,et al.  Molecular Evidence for Deep Precambrian Divergences Among Metazoan Phyla , 1996, Science.