By: Gene Yang ‘19

The question of whether the central nervous system evolved once or multiple times is a subject of much study and debate. Humans and other animals with bilateral symmetry, all of which possess central nervous systems, are known to have descended from a common ancestor. In the past, it was believed that the central nervous system evolved just once in our bilateral ancestor. A central finding that supports this conservation of evolution is the fact that many genes, belonging to the homeobox gene family, are expressed in a similar order in the nerve cord during development in flies and vertebrates.

Collaboration between researchers from the University of Bergen, Natural History Museum of Denmark, and Swedish Museum of Natural History has shown that this story may be more complex. Xenacoelomorphs, a group of ancient bilaterians, are small animals with morphologically diverse nervous systems. When researchers studied four Xenacoelomorphs species, they found patterns of homeobox gene expression that were more diverse and not consistent with other bilaterians. For example, the transcription factor nkx6 did not co-localize with the neuron marker ChAT, even though this was previously expected to be conserved.

In addition to the four Xenacoelomorphs, these researchers also studied five other species representative of different groups of organisms. Most notably, one of the species studied was the ringed worm Owenia fusiformis. This species is the only ringed worm known to have a nervous system trunk, a structure that mirrors those found in vertebrates and flies. However, the study found that this ringed worm possessed a distinctly different gene expression pattern along the trunk that did not correlate with the neuronal markers found in vertebrates or flies. Collectively, these molecular differences point to a complicated view of nervous system evolution, and suggest that independent evolution occurred multiple times throughout the bilaterian lineage.

References

1. J. Martín-Durán, et al., Convergent evolution of bilaterian nerve cords. Nature 553, 45-50 (2018). doi: 10.1038/nature25030.
2. Image retrieved from: https://commons.wikimedia.org/wiki/File:NettleJelly.JPG