What was the earliest animal?

Genetic sequencing is finding new answers to ancient questions, writes ANDREW MASTERSON.

Just a couple of genes are responsible for one of the longest running arguments in evolutionary biology: which came first, the sponge or the jelly?

A new study, led by Antonis Rokas of Vanderbilt University in Tennessee, US, analysed thousands of genes sequenced from target organisms to try to resolve the issue.

Rokas and colleagues set out to determine whether marine sponges or ocean predators known as comb jellies represented the oldest branch of the animal family tree.

For the best part of a century, sponges were held to be the earliest form of animal life, based on the simplicity of their structures. In recent decades, however, genetic sequencing has thrown up comb jellies – or ctenophores – as a better candidate.

And thus the argument has raged. A 2008 study favoured the jellies. Earlier this year, another opted for the sponges.

For Rokas’s team, the object of the analysis was not only to try to resolve the matter but also to understand how the controversy arose in the first place. After all, in 95% of cases genetic sequencing answers questions of evolutionary precedence without ambiguity.

To do so, the researchers dug into the fine detail of genes shared across the two groups. “The trick is to examine the gene sequences from different organisms to figure out who they identify as their closest relatives,” explains Rokas. “When you look at a particular gene in an organism, let’s call it A, we ask if it is most closely related to its counterpart in organism B? Or to its counterpart in organism C? And by how much?”

By applying this method, the team determined that comb jellies diverged from a mutual ancestor first.

Phylogenetic data creates these occasional controversies, the researchers discovered, because a handful of “strongly opinionated” genes sometimes confound the statistical analyses employed to produce results. By correcting for the influence of these genes, Rokas and colleagues hope that future conflicts can be resolved quickly. The study was published in Nature Ecology and Evolution. (See page 52).