Sometimes, I confess, this whole common descent thing gets in the way and is really annoying. What we’ve learned over the years is that the evolution of life on earth is constrained by historical factors at every turn; every animal bears this wonderfully powerful toolbox of common developmental genes, inherited from pre-Cambrian ancestors, and it’s getting rather predictable that every time you open up some fundamental aspect of developmental pattern formation in a zebrafish, for instance, it is a modified echo of something we also see in a fruit fly. Sometimes you just want to see what evolution would do with a completely different starting point — if you could, as SJ Gould suggested, rewind the tape of life and let it play forward again, and see what novelties arose.
Take the worm. We take the generic worm for granted in biology: it’s a bilaterally symmetric muscular tube with a hydrostatic skeleton which propels itself through a medium with sinuous undulations, and with most of its sense organs concentrated in the forward end. The last common ancestor of all bilaterian animals was a worm, and we can see that ancestry in the organization of most animals today, even when it is obscured by odd little geegaws, like limbs and armor and regional specializations and various dangly spiky jointed bits. You’ll even see the argument made that that worm is the best of all possible simple forms, so it isn’t just an accident of history, it’s a morphological optimum.
But what if we could rewind the tape of life a little bit, to the first worms? Is it possible there are other ways such an animal could have been built? It seems nature may have carried out this little experiment for us, and we have an example of a reinvented worm, one not constructed by common descent from that initial triumphal exemplar in the pre-Cambrian — an alternative worm.
I’ve been waiting for this paper to come out since I first heard Peter Holland speak about at SICB. It’s a description of a worm, Buddenbrockia … but what a worm.
Buddenbrockia plumatellae is a parasite that lives inside bryozoans. It looks just like a tiny worm from the outside — it’s a slender tube — but on the inside and in its history it’s very different. Phylogenetic analyses show that this worm is most closely related to the Myxozoa.
Myxozoans are weird, weird, weird. Aquarists run into them all the time: they are responsible for whirling disease in fish, where this nasty parasitic myxozoan infests the brains of the animal and turns them into frantically spinning freaks until they die and release the spores of the parasite. They were thought to be protists for a long time, but then molecular work showed that they have various proteins associated only with metazoans, and they were then regarded as degenerate animals that had abandoned the joys of multicellularity to frolic as more simplified eukaryotes.
On the basis of the similarity of certain structures in the spores, the polar capsules, that resemble the nematocysts of cnidarians, they’re also thought to be, basically, greatly stripped down descendants of ancient jellyfish-like animals. This work from the Holland lab nails that down with a genomic analysis that aligns Buddenbrockia with the Myxozoa, and the Myxozoa with the Medusazoa. What this means is that a simplified lineage of cnidarians, radially symmetric diploblasts that do not look wormlike at all, which had degenerated into even greater simplicity to live as an aquatic parasite, has one descendant that has either retained a primitive structure or re-evolved some degree of multicellular complexity, and has independently evolved the worm. This animal is not descended from that bilaterian worm we take for granted, at any rate.
So it’s a worm of independent origin from all us other glorified worms. What’s different about it?
First, the aspects that are the same are the external elongate appearance, an internal fluid-filled space that acts as a hydrostatic skeleton, and its ability to locomote with sinuous undulations. Those are properties that make it a worm in the first place.
The way it does that is different, though. There is no gut at all. This is an animal that lives off the nutrients from its host, in which it is awash. It has no sense organs and no recognizable nervous system — sinuous locomotion requires muscular coordination, though, so I assume motor control is through waves of electrical activity through the muscles themselves, or through epidermal connections. It also has an usual mode of locomotion, coiling like a spring and straightening, which may be a useful way of escaping from its host (a cnidarian version of Spring Surprise). It is not bilaterally symmetric: it has two axes of symmetry and four longitudinal bands of muscle, so it’s actually a tetraradial worm.
Many of the attributes of the animal, such as the lack of a gut or sense organs, aren’t particularly titillating, since they’re most likely a product of the parasitic lifestyle of the creature. Tetraradial symmetry provokes the imagination, though — what if our ancestor, the ancestor of most metazoan clades, had been a tetraradian rather than a bilaterian? Try to imagine if our familiar animal life had been built on a body plan with four-fold, rather than two-fold, symmetry. Reinventing animal life with just a subtle tweak like that at the root of the tree would have dramatic effects on all the subsequent branches. This is a most unprepossessing worm, but our distant ancestors were similarly humble — but the patterns of their genes echo and re-echo through all of us, and those early choices have shaped and constrained our evolution every step of the way.
Jiménez-Guri E, Phillipe H, Okamura B, Holland PWH (2007) Buddenbrockia is a cnidarian worm. Science 317:116-118.