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.
One reason this is important is that it changes our view of the earliest bilaterians. The cartoon below illustrates the point (with smily faces on critters!). Look at B first; this is one way we’ve been thinking about bilaterian evolution. An organism with radial symmetry, like a cnidarian polyp, acquired bilateral symmetry, and because modern deuterostomes and protostomes share complex characters like segmentation, anterior sensory structures, etc., the LCB had at least rudiments of that complexity. That’s a substantial leap. Not for the organisms, of course…that didn’t happen all at once, but the real problem is that the many intermediates left no trace and are invisible to us.
Figure A in the cartoon below shows the model that Bagun and Riutort are advancing: that there was a simpler LCB, and we can see its descendants in the acoelomorph flatworms.
Another useful part of their model is that they are arguing the LCB would have evolved from the planula larva of a cnidarian. Many radially symmetric cnidarians produce larvae that look like microscopic ciliated worms, such as the hydrozoan larva to the left. Now that is not such a large leap anymore, from an elongated planula to a gutless, blind, simple worm to a segmented worm. They also have a timeline based on the molecular data, suggesting that the transition from cnidarian to the LCB occurred 600-630 million years ago, that the transition from the LCB to the Eubilaterian occurred about 570 million years ago, and that the protostome-deuterostome split happened about 550 million years ago. I know the Intelligent Design gang looks at these numbers for pre-Cambrian and Cambrian events and thinks it means sudden appearance, but think about it: these transitions took place over thirty million years each. I always find it peculiar that people who find it reasonable to claim that life on earth has a history of only 6,000 years can simultaneously characterize events taking tens of millions of years and occurring half a billion years ago as abrupt.
On to some data…one particularly interesting part of this work is an examination of Hox gene distributions. The cnidarians typically only have two Hox genes. Most modern bilaterians have 7 or 8 or more Hox genes in a cluster, but the acoels have only 4 Hox paralogs, which are more similar to other bilaterian than cnidarian Hox genes, but are distinct in sequence from protostome (including platyhelminth) Hox genes. The diagram below shows our prior picture, in A, of the arrangement of genes in the Hox cluster, and how that maps onto a phylogenetic tree. B shows a similar tree, but with the new information from the Acoelomorpha added—they’re intermediate between the Radiata and the Eubilateria.
Bagun and Riutort also have a simple explanation for how the bilaterians could have evolved from the radiate cnidarians. Here’s a cartoon of the morphological transition from a free-swimming cnidarian larva to a crawling bilaterian:
The model really only involves two innovations: retention of planuloid larval form in an adult, and a symmetry-breaking event that leads to the formation of a mouth. And it leads to some testable predictions about developmental genes in cnidarians and flatworms!
Assuming [the figure above] as the most-likely scenario, such symmetry breaking would represent the key event in the evolution of the Bilateria as it produced at once a third layer and a new orthogonal body axis (likely the DV) with its concurrent bilateral symmetry. Moreover, an anterior concentration of nerve cells was already in place. To diversify the expanded AP pattern a novel set of central Hox genes (see below) needed to be added between the anterior and posterior Hox genes already present in cnidarians. Such ideas could be tested by studying the expression of Hox/ParaHox and other anterior (e.g. orthodenticle, empty spiracles, distaless, Pax 6), posterior (caudal, Brachyury, forkhead, wingless), dorsal (decapentaplegic/Bone Morphogenetic Protein), ventral (short gastrultion/chordin), and mesoendodermal (snail, twist, GATA factors) genes in embryos and adults of selected acoelomorph species. Results could then be compared to the expression data that are accumulating fromplanula larvae and polyps of several species of cnidarians. Interestingly, an impressive number of these genes, namely expressed at posterior regions in bilaterians, is expressed in a radial manner around the blastoporal region in planulae. This supports the homology between the aboral-oral axis of cnidarians and the AP axis of bilaterians, oral in planula corresponding to posterior in bilaterians.
We’re also getting an increasingly better focused image of what was going on in the pre-Cambrian and Cambrian. Funny, but it doesn’t seem to involve any ‘designers’.
Altogether, this reinforces the existence of a cryptic precambrian history of small, benthic, stem-group bilaterians, which left no trace fossils. The explosion of life forms during the Cambrian and thereafter would result from increasing body size and complexity linked to the emergence of indirectly developing larval forms in some lineages of stem deuterostomates, ecdysozoans and lophotrochozoans.
Bagun J, Riutort M (2004) The dawn of bilaterian animals: the case of acoelomorph flatworms. BioEssays 26:1046–1057.