Xenacoelomorpha is the sister group to Nephrozoa

Animals with bilateral symmetry (bilaterians) form a well-defined phylogenetic group which contains the Deuterostomia (hemichordates, echinoderms, fish) and the Protostomia which is divided into Ecdysozoa (priapulids, nematodes, arthropods) and Spiralia (annelids, snails, nemerteans). But these animals are extremely diverse. And, for this reason, there’s a long-standing debate about how the last common ancestor of bilaterians looked like. Did it have segments? A brain? Eyes? Legs? An anus?

To begin answering these questions, we must resolve phylogenetic relationships of bilaterian groups whose positions remain uncertain in the animal tree. Xenacoelomorpha—an enigmatic lineage of bilaterally symmetric worms—is one of these groups.

Cannon et al. 2016 Bilateria trait evolution
Figure 4 from Cannon, J. T. et al. Xenacoelomorpha is the sister group to Nephrozoa. Nature 530, 89–93 (2016). https://doi.org/10.1038/nature16520

In our work, we used phylogenomic approaches to investigate the phylogenetic position of these worms. We found that they are the sister group to the remaining Bilateria (the three groups cited above). That means that a bilateral body organization evolved before a central nervous system, a through-gut and anus, and nephridia.