Trends in Genetics
Volume 15, Issue 3, 1 March 1999, Pages 104-108
Journal home page for Trends in Genetics

Review
Animal evolution: the end of the intermediate taxa?

https://doi.org/10.1016/S0168-9525(98)01671-0Get rights and content

Abstract

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.

Section snippets

A short history of metazoan molecular phylogeny

Metazoan molecular phylogeny is a relatively recent exercise whose beginnings can be dated rather precisely with the paper by Field et al.5 in 1988. There, the first broad phylogeny of animals, based on partial 18S rRNA sequences of 20 taxa belonging to 10 phyla, was proposed. This was an era of naive optimism and great expectations.

The first molecular trees displayed expected and surprising results. Diploblastic organisms were deeply split from triploblastic ones, to the point that the

Developmental and paleontological implications

The new phylogeny has several profound implications for understanding animal evolution. First, it suggests that the hope of finding extant ‘intermediates’ in the bilaterian lineage could well be doomed. We should consider the possibility that the tree might lead from diploblastic organisms to the coelomate bilaterians without any of the long-cherished intermediate phyla. This makes it more difficult to picture the origin of the bilaterian body plan but, at the same time, accounts much better

Acknowledgements

Parts of this paper were written while we were participating in the Embryology course at the Marine Biological Laboratory in Woods Hole. We thank the course and its organizers, S. Fraser and M. Bronner-Fraser, for the hospitality and facilities made available. We also thank H. Philippe and C. Nielsen for numerous discussions over phylogeny, J. Venutti and E. Davidson for many helpful comments. Finally, we thank D. Tautz for encouraging us to write this review. The work in the Orsay lab is

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