Marking 25 years in metazoan phylogeny- happy birthday Darwin

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Twenty-five years ago today, on the 12th February 1988, a landmark article was published in Science describing a phylogenetic tree of the metazoans, derived from molecular data. The article, by Katherine Field and colleagues (widely referred to as ‘Field et al.’), represented the first use of small subunit ribosomal RNA to establish the metazoan phylogeny. To mark the anniversary of its publication, and coincidentally Darwin’s 204th birthday, a new commentary is published today in EvoDevo evaluating the impacts of this article.

In this commentary, Max Telford (UCL) critically assesses the techniques and methods used by Field et al., and reconstitutes their dataset, to establish a metazoan phylogeny roughly in line with that accepted today. Despite producing a revolutionary phylogenetic analysis, several errors in the original article have since been identified. Telford addresses the reasons behind the two main errors made; the polyphyletic origins of Metazoa, and the placement of the platyhleminths outside of the Lophotrochozoa. Recapitulating their phylogenetic analysis, he demonstrates that using a Maximum Likelihood method of the era, rather than the distance methods used in the original article, it is possible to correct at least the first of these errors. This technique was available at the time, however, Telford suggests that it would have taken many months or even years to compute using 1980s computer technology.

Visible lophophores in a bryozoa colony. Image credit: Daniel Stoupin

Although discrepancies can now be seen in the tree published by Field et al., in comparison with the currently accepted metazoan phylogeny, this article remains a hugely significant and much cited article in evolutionary biology. Telford concludes by discussing two of their revolutionary conclusions, which have since been shown to be correct. Firstly, the convergent evolution of the lophophore (indicated by the placing of brachiopods closer to annelids and molluscs than to the lophophorate hemichordates), and secondly the separation of arthropods and annelids, which introduced one of the most hotly contested on-going debates in evolutionary biology: the evolution of body segmentation.

Telford concludes that the “25th anniversary of Field et al. deserves to be marked and its coincidence with Darwin’s birthday is both remarkable and fitting”.