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.
Continue reading "Reinventing the worm" (on Pharyngula)