“It has the most primitive form of nervous system of any bilateral animal,” intones the voiceover for the National Geographic video.
The “it” being referred to is an acorn worm, a little known kind of invertebrate that is actually relatively closely related to the vertebrates. Vertebrates belong to the chordate phylum, and acorn worms are hemichordates – literally, “half chordates.” Hemichordates are interesting in studies of chordate evolution (and thus, in a roundabout way, human evolution) because they hint at what features that very early chordates might have had. If a feature is shared by both hemichordates and chordates, that probably means it was present in the common ancestor of both.
One of the most unusual features of chordates is their dorsal nervous system. In most animals, the majority of the nervous system runs along the underside of the animal, but in chordates, it runs along the back. Hemichordates have a proverbial foot in both camps, with some neurons running dorsally and some ventrally. With such a strange organization, you might expect that the nervous system of hemichordates has been studied to death. But surprisingly, you’d be wrong.
This new paper by Nomaksteinsky and colleagues tries to answer a very basic question: do hemichordates have a central nervous system? While older studies agree that there are cords of neurons, they disagree over whether there are neuronal cell bodies in them, which most central nervous systems have.
The authors looked at Ptychodera flava (pictured). A big advantage that anatomists have now that classical anatomists didn’t is the ability to look for particular molecules using antibody labeling. Nomaksteinsky and colleagues were able to use a suite of labels that bind to molecules are found fairly specifically in particular kinds of neurons.
For instance, they found neuronal cell bodies with the neurochemical serotonin in the periphery, but not in a region called the collar. Taking the results for several different labels together, the overall pattern was not one of neurons sorted around higgledy-piggledy; rather, particular kinds of neurons were found in fairly specific locations, which is consistent with the sort of organization expected in a true central nervous system.
The authors do not venture an opinion as to whether the dorsal or ventral nerve cord in acorn worms is the evolutionary equivalent to the chordate dorsal cord, but clearly detailed anatomical work over development might help sort this out in future. One key point is that a central nervous system is a very old feature in the evolution leading to humans. Another key point is that, as with jellyfish, not to underestimate the complexity of nervous systems among the spineless.
Maybe that National Geographic video should get a new voiceover.
Nomaksteinsky, M., Röttinger, E., Dufour, H., Chettouh, Z., Lowe, C., Martindale, M., & Brunet, J. (2009). Centralization of the Deuterostome Nervous System Predates Chordates Current Biology DOI: 10.1016/j.cub.2009.05.063