I've just been demonstrating in a first-year undergrad practical exploring crayfish, earthworms, and tadpoles. Fairly early on, somebody declared that they were finished early. In desperation, I cast around for something interesting to keep them occupied - and what better than pointing out one of the great, eerie differences between the crayfish/earthworms on the one hand and something very peculiar about the tadpoles...
My mind was blown on several occasions at Oxford - including when somebody pointed out that, cladistically, we're sarcopterygian fish. But one glorious biological tidbit that really sent the neurones reeling was the radical theory of 'dorsoventral inversion': the crazy idea that, sometime before the origin of the chordates, our distant ancestor flipped upside down.
The mystery is this. In protostomes (one of the two great divisions of animal life - the branch of the evolutionary tree that hosts insects and spiders and squid and so forth), the nerve cord is on the underside of the animal. But in chordates (the group that we're in), it's on the upperside. In other words, it's ventral in the protostomes, but dorsal in us. Since we're descended from a common ancestor, something odd has gone on here.
During the French Enlightenment, the fabulous Etienne Geoffroy Saint-Hilaire pointed all this out. The solution later proposed was that there was a general flipping-over at some stage in evolution. Admittedly, this does sound a wee bit barking, and was unceremonially relegated to the waste-paper bin of evolutionary theories. But now - stunningly - 'dorsoventral inversion' is experiencing a shocking return to the limelight! The molecular evidence truly does look as though the cross-sections of protostomes and chordates are undeniably symmetrical (have a look at this very clear diagram - admittedly a Wikipedia offering, but that means I can reproduce it here without copyright...):
It really fired my imagination when I first encountered the idea, and it still does. It looks like somewhere in deep time, the lives of our ocean-dwelling ancestors were - literally - turned upside down. Which is why I'm sharing it briefly here. Hopefully the fast-paced undergrads in the practical thought that was suitably exciting...
Adventures of a
Dr Patrick Kennedy, Radford Lab, University of Bristol | Zoology