Ancient Fossils Illuminate the Origins of Animal Nervous Systems

An international consortium of researchers has unveiled a captivating piece of the evolutionary puzzle: the development of the ventral nerve cord, a fundamental element of the central nervous system, in ecdysozoan species, which encompass insects, nematodes, and priapulid worms.

Their results, detailed in a publication titled “Preservation and early evolution of scalidophoran ventral nerve cord” in Science Advances, offer significant insights into the genesis of these structures during the early Cambrian era.

This research group, including Dr. Deng Wang (Northwest University), Dr. Jean Vannier (Université de Lyon), Dr. Chema Martin-Durán (Queen Mary University of London), and Dr. María Herranz (Rey Juan Carlos University), examined exceptionally well-preserved fossils from important Cambrian formations. These fossils provide representations of the early-evolving Scalidophora, a subgroup of Ecdysozoa, granting a rare view into the nervous system framework of ancient organisms.

Ecdysozoans incorporate arthropods (such as insects and crabs), nematodes (roundworms), and smaller factions like kinorhynchs (“mud dragons”) and priapulids (“penis worms”). The structures of their central nervous systems, which consist of the brain and ventral nerve cord, have long fascinated researchers desiring to explore the evolutionary links among these classifications.

For instance, priapulids feature a singular ventral nerve cord, whereas loriciferans and kinorhynchs possess paired nerve cords, with kinorhynchs also showcasing paired ganglia. Did the original ecdysozoan have a singular or paired ventral nerve cord?

Additionally, while loriciferans and kinorhynchs demonstrate a similar nervous system architecture to arthropods, they are phylogenetically appointed distant. Are these resemblances the consequence of convergent evolution, or do they indicate a common evolutionary background?

Scalidophorans, consisting of priapulids, loriciferans, and kinorhynchs, first emerged in the early Cambrian. They constitute a vital lineage for examining the evolutionary path of the ventral nerve cord in ecdysozoans.

Through the investigation of fossils drawn from the Fortunian Kuanchuanpu Formation (e.g., Eopriapulites and Eokinorhynchus), the Chengjiang Biota (e.g., Xiaoheiqingella and Mafangscolex), and the Wuliuan Ottoia prolifica, the scholars detected elongated structures extending along the ventral aspect of these ancient creatures.

“These structures closely mimic the ventral nerve cords observed in contemporary priapulids,” stated Dr. Deng Wang and Dr. Jean Vannier. Their assessment suggests that these fossils preserve traces of singular ventral nerve cords, illuminating the probable ancestral condition for scalidophorans.

Phylogenetic evaluations bolster the theory that a single ventral nerve cord was the ancestral state for scalidophorans. Furthermore, the evolutionary association of nematoids and panarthropods (a clade encompassing arthropods, tardigrades, and onychophorans) implies that their common ancestor also likely possessed a single nerve cord.

“This prompts us to propose that the shared ancestor of all ecdysozoans had a single ventral nerve cord,” remarked Dr. Chema Martin-Durán. “The paired nerve cords identified in arthropods, loriciferans, and kinorhynchs probably evolved separately, presenting derived characteristics.”

The investigation also emphasizes a relationship between the evolution of paired ventral nerve cords, ganglia, and body segmentation. Loriciferans, kinorhynchs, and panarthropods display various levels of body segmentation, indicating that these structural transformations may have co-evolved with modifications to the nervous system.

Dr. María Herranz highlighted, “The advent of paired nerve cords likely enhanced coordination of movement, especially in segmented organisms. Throughout the Precambrian-Cambrian transition, alterations in the nervous and muscular systems were likely linked to the emergence of appendages, facilitating more advanced locomotion.”

This revelation enriches our comprehension of ecdysozoan evolution and underscores the significance of the fossil record in addressing pivotal questions regarding early animal development.

By connecting nervous system configurations to broader evolutionary patterns, the study enhances our understanding of how the diverse lineages of ecdysozoans emerged and adapted to their habitats.