Published online 26 January 2009 | Nature | doi:10.1038/news.2009.55

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Placulan origin re-roots the tree of life.

Could organisms resembling this Trichoplax have been the common ancestor of humans and sponges? W. Jakob

Organ-less, slime-like creatures just a few millimetres wide could be the ancestors of both sponges and humans, according to new research. But the findings are proving contentious, not least because they suggest that the nervous system evolved twice.

Evolutionary biologists have debated how sponges and humans are related to one another for at least a century. Currently, most of the community think that the oldest common ancestor of all animals was a type of sponge, says zoologist Bernd Schierwater of the University of Veterinary Medicine Hannover in Germany.

Schierwater, however, thinks that humans and sponges evolved in parallel from a simpler group of organisms — the placula, the closest living relative of which is the placozoan Trichoplax adhaerens, an asexually reproducing film of slime tens of micrometres thick and a few millimetres in diameter.

Schierwater and his colleagues fed morphological data on the appearance of animals from 24 taxa together with genetic information into a computer program that assessed similarities and differences to generate a phylogenetic tree of life.

One line of the new tree goes to the placozoa, sponges and jellyfish and the other goes to the more complex animals, the researchers report in PLoS Biology1. And where these lines meet are the placula, the most ancient ancestor of all animals.

Hitting a nerve

If they're true, the findings have a difficult implication for many biologists: they suggest that the nervous system evolved twice in two separate systems. Placozoa and sponges don't have a nervous system, but the related cnidarians — which include jellyfish — do.

Many biologists remain unconvinced. "Given the large number of molecular similarities between the nervous systems of cnidarians, Drosophila and vertebrates, this is hard to swallow," says molecular geneticist David Miller of James Cook University in Townsville, Australia.

Schierwater, however, doesn't think this is a problem. "The placula already had all the genes necessary to make all the building blocks [of a nervous system], but it didn't have to make it because ecology didn't force it to do so."

For him, the idea that the nervous system arose twice is "not too complicated at all".

Moot method

Miller, however, is also wary of the way in which these trees are calculated. "Small alterations in the settings of some of these analysis tools can make major differences to the outcomes," he says. "This certainly isn't the last word on the scheme of animal evolution."

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Molecular phylogenist Hervé Phillippe, from the University of Montreal in Canada, however, has deeper concerns. He argues that the statistics don't support the conclusions and that "the taxon sampling is problematic".

Only one choanoflagellate — perhaps our nearest unicellular relative — was included in the study, he says, despite the fact that substantial data are available for two others.

And the value that can be added by considering morphological characteristics in this analysis is lost, he says, because they are swamped by the vastly larger number of genetic characteristics that are considered here. "I am tired of these molecular papers that don't make sufficient controls to check the reliability of the phylogenetic inferences."