Zoologists have generally placed acoels on the earliest branch of the bilaterians — before the split between protostomes and deuterostomes — because the worms lack so many key features such as a separate mouth and anus, a central nervous system and organs to filter waste.(From Nature News).
Click for larger version.
After analysing sequences from nuclear DNA, the group made a separate evolutionary tree based on genes in mitochondria. They also studied microRNAs, which regulate gene expression but do not code for proteins. According to co-author Kevin Peterson, a palaeontologist at Dartmouth College in Hanover, New Hampshire, microRNAs are particularly useful for studying deep evolutionary relationships. The team found that acoels have a type of microRNA known to be specific to deuterostomes, suggesting that they are related. [Emphasis added.]Really? This microRNA evidence weighs more than the fact that Acoels have no anus, etc.? The researchers did not only use microRNA:
Here we assemble three independent data sets—mitochondrial genes, a phylogenomic data set of 38,330 amino-acid positions and new microRNA (miRNA) complements—and show that the position of Acoelomorpha is strongly affected by a long-branch attraction (LBA) artefact. When we minimize LBA we find consistent support for a position of both acoelomorphs and Xenoturbella within the deuterostomes.Okay, but since I don't give much credence to microRNA compared to presence/absence of separate mouth/anus and central nervous system, what would happen to the inference if the microRNA data were taken out?
Unlike some other researchers who like very much that we have a living form ancestral to all bilaterians (organisms with bilateral symmetry), my problem here is that we are asked to believe that an ancestor of Acoels had separate mouth and anus, etc., but then lost those later on. The hell?! One biology lab manual I used once had the text "the critical invention of the anus", and I think this is apt. What happens to a lineage that makes it lose it's anus, and instead start barfing out excrements?
Philippe, H., Brinkmann, H., Copley, R., Moroz, L., Nakano, H., Poustka, A., Wallberg, A., Peterson, K., & Telford, M. (2011). Acoelomorph flatworms are deuterostomes related to Xenoturbella Nature, 470 (7333), 255-258 DOI: 10.1038/nature09676