One of many open questions in evolution is the nature of bilaterian origins—when the first bilaterally symmetrical common ancestor (the Last Common Bilaterian, or LCB) to all of us mammals and insects and molluscs and polychaetes and so forth arose, and what it looked like. We know it had to have been small, soft, and wormlike, and that it lived over 600 million years ago, but unfortunately, it wasn't the kind of beast likely to be preserved in fossil deposits.
We do have a tool to help us get a glimpse of it, though: the analysis of extant organisms, searching for those common features that are likely to have been present in that first bilaterian; we're looking for the Last Common Bilaterian by finding the Least Common Denominators among living species. And one place to look is among the flatworms.
A recent paper by Bagun and Riutort examines one specific subgroup of the flatworms, the acoelomorphs (pronounced a-seel-o-morphs). These are tiny marine worms that have long been grouped under the platyhelminthes, but molecular work has been revealing that the platyhelminths have been a victim of our "worm is just a worm" bias, and are almost certainly polyphyletic. Bagun and Riutort argue that the acoels ought to be recognized as a separate phylum of their own, and further, that they represent basal bilaterians. They propose on the basis of molecular data that most of the Platyhelminthes belong in the protostome clade, and that they've secondarily lost characters common to most members of that group, such as segmentation. and that the acoelomorph flatworms are an early branch off the bilaterian root.
Continue reading "Acoelomorph flatworms and precambrian evolution" (on Pharyngula)