NEW YORK (GenomeWeb News) – A new study appearing online in PLoS Biology today suggests that animals with bilateral symmetry split off from other creatures in the metazoan tree much earlier than commonly believed.
A team of researchers from the US and Germany used morphological and genetic evidence to inform their phylogenetic analysis of dozens of animal taxa. The research suggests animals with bilateral symmetry branched off very early from other metazoans, with the humble Placozoans forming the base of the non-bilaterian branch. If so, that implies that complex biological systems — particularly the nervous system — evolved in parallel in the bilaterian and non-bilaterian branches of the tree.
"[O]ur analysis implies that the first major event in animal evolution split bilateral animals from all others, and our work firmly places Placozoa as the most primitive of the non-bilaterian animals," senior author Rob DeSalle, curator of the American Museum of Natural History's Sackler Institute for Comparative Genomics, said in a statement.
Interpreting phylogenic relationships relies on an accurate interpretation of early events in animal evolution, the authors noted. But determining these basal relationships has been challenging.
Traditionally, DeSalle told GenomeWeb Daily News, sponges have been placed at the bottom of the metazoan tree. But such basal relationships are extremely controversial and some have argued that Placozoans — simple organisms related to the disk-like Trichoplax adhaerens — belong at the root of the metazoan tree.
The positions of "higher" animals with bilateral symmetry have been slightly less contentious. In general, most biologists have proposed that bilaterian creatures evolved in a somewhat linear fashion from more primitive or "lower" life forms, DeSalle explained. "The idea that most animal biologists have is that bilateria are highly derived."
But if DeSalle and his team are right, that notion may be way off base. They assessed mitochondrial and nuclear gene sequences, as well as morphological data, in an effort to discern phylogenetic relationships between organisms from 24 different taxa. They also did a larger, supplementary study of 73 taxa.
The majority of the data used for these analyses came from existing databases, DeSalle explained, although the team had to sequence some of the more "bizarre," under-studied creatures.
They then did phylogenetic analyses of the taxa using parsimony, likelihood, and Bayesian inference. Based on those analyses, the researchers suggest that bilaterian animals actually belong to a sister group to non-bilaterian animals such as Trichoplax, sponges, comb jellies, and jellyfish.
If that is indeed the case, it means complex biological systems — particularly the nervous system — evolved independently through convergent evolution in animals with bilateral symmetry and those without, including jellyfish and related creatures.
"Some people might initially be shocked to see that nerve cells ... evolved independently," lead author Bernd Schierwater, director of the University of Veterinary Medicine Hannover's division of ecology and evolution, said in a statement. "But with this new phylogeny, we can take a closer look at the anatomy of these organisms — and we can see that their nervous systems are not all that similar at the morphological level after all."
The researchers also concluded that Placozoans were at the most basal position in the non-bilaterian arm of the new metazoan tree. That conflicts with some other recent reports, including an analysis done by researchers from the University of California at Berkeley, the US Department of Energy's Joint Genome Institute, and elsewhere who sequenced the Trichoplax genome last year.
That group suggested that sponges were the earliest diverging animals, with Placozoans forming a clade with Cnidarians (animals related to jellyfish) and Bilaterians. DeSalle attributes the different conclusion, in part, to differences in the number of taxa sampled. He argued that although those researchers had access to the entire Trichoplax genome, their analysis focused on fewer genes and species than the latest analysis from his team.
Still, DeSalle conceded that his team's paper has its own limitations. For instance, he noted that when he and his team did analyses with a wider range of taxa, the confidence in their conclusions went down. He also said more information could be gained by sampling more species and looking at additional genes, anatomical features, and developmental biology patterns.
While more research will be needed to flesh out this idea, DeSalle argues that there's already enough data to raise some doubts about the accepted history of bilaterian animals. He said biologists should be willing to at least entertain the notion that bilaterians did branch off earlier than previously thought, "I think you have to start to seriously consider it as a possibility."