Way back in the early 19th century, Geoffroy St. Hilaire argued for a radical idea, that vertebrates and most invertebrates were inverted copies of each other. Vertebrates have a dorsal nerve cord and ventral heart, while an insect has a ventral nerve cord and dorsal heart. Could it be that there was a common plan, and that one difference is simply that one is upside down relative to the other? It was an interesting idea, but it didn’t hold up at the time; critics could just enumerate the multitude of differences observable between arthropods and vertebrates and drown out an apparent similarity in a flood of documented differences. Picking out a few superficial similarities and proposing that something just looks like it ought to be so is not a persuasive argument in science.
Something has changed in the almost 200 years since Geoffroy made his suggestion, though: there has been a new flood of molecular data that shows that Geoffroy was right. We’re finding that all animals seem to use the same early molecular signals to define the orientation of the body axis, and that the dorsal-ventral axis is defined by a molecule in the Bmp (Bone Morphogenetic Protein) family. In vertebrates, Bmp is high in concentration along the ventral side of the embryo, opposite the developing nervous system. In arthropods, Bmp (the homolog in insects is called decapentaplegic, or dpp) is high on the dorsal side, which is still opposite the nervous system. At this point, the question of whether the dorsal-ventral axis of the vertebrate and invertebrate body plans have a common origin and whether one is inverted relative to the other has been settled, and the answer is yes.
