Fossils found in China dating to 535 million years ago are some of the oldest examples of a nervous system. They reveal how the first nervous systems evolved in early relatives of worms and insects.

The new analysis, published in Science Advances, sheds light on the origins of ventral nerve cords – a major structure in the central nervous systems of invertebrates.

Ventral nerve cords allow insects and other invertebrates to transmit sensory input from various appendages, and it is crucial to locomotion.

Ecdysozoan worms from the Kuanchuanpu Formation in southern China were compared with other fossils from Chengjiang, also in southern China, and the Burgess Shale in Canada.

All the worms date to the early Cambrian period (541–485 million years ago).

Ecdysozoans today include arthropods (insects, spiders, crabs etc.), nematodes, and smaller groups like kinorhynchs (“mud dragons”) and priapulids (“penis worms”). Their central nervous systems include a brain and ventral nerve cord.

Some groups have single ventral nerve cords, while others have paired cords. Some groups which share similarities in central nervous system have quite different body plans.

Scientists have long wondered if these simple nervous systems could help explain the evolutionary relationship between these groups.

Advanced three-dimensional imaging of the fossils revealed impressions left behind by single ventral nerve cords.

“These structures closely resemble the ventral nerve cords seen in modern priapulids,” explain Deng Wang and Jean Vannier from the Northwest University, USA, and Université de Lyon, France, respectively.

“This leads us to propose that the common ancestor of all ecdysozoans possessed a single central nerve cord,” says Chema Martin-Durán. “The paired nerve cords observed in arthropods, loriciferans, and kinorhynchs likely evolved independently, representing derived traits.”