A groundbreaking study published today in Current Biology reveals that Mollisonia symmetrica, an ancient creature from the Cambrian period, had a uniquely arranged 'backward' brain similar to modern spiders, suggesting that arachnids may have originated in the ocean rather than on land.

This distinctive brain organization likely provided evolutionary advantages such as improved motor control and hunting capabilities, which are critical for arachnids.

Researchers focused on the fossilized brain and central nervous system of Mollisonia, which lived approximately 500 million years ago, and found neural structures that align with those of modern arachnids.

Mollisonia possessed a segmented body with six pairs of appendages, indicative of its predatory nature, and lived around 515 to 480 million years ago, making it a key species in understanding arachnid evolution.

Statistical analysis of 115 anatomical traits positioned Mollisonia as a sister group to modern arachnids, supporting the theory of a shared lineage that led to today’s diverse arachnid species, including scorpions and sun spiders.

The study highlights Mollisonia's importance in understanding arachnid evolutionary history and emphasizes the role of brain organization in their development, encouraging further exploration.

Mollisonia's body structure, including a broad carapace and segmented trunk, resembles early chelicerates but indicates a more complex evolutionary status as an ancestor to arachnids.

The research suggests early arachnids may have adapted to terrestrial environments, potentially influencing the evolution of insects and their defense mechanisms, including flight.

The study's key finding is the backward organization of the arachnid brain, where the forebrain is situated above circuits controlling leg movement, supporting its connection to modern arachnids.

The research, published on June 22, 2025, by scientists including Nicholas Strausfeld, involved re-examining a 1925 fossil of Mollisonia collected in British Columbia, revealing neural structures previously overlooked.

Mollisonia lived during the Cambrian explosion, a period marked by rapid biodiversity and the emergence of many animal groups, making it a crucial species for understanding early arthropod evolution.

While the findings suggest Mollisonia was an early aquatic relative of spiders and scorpions, uncertainties remain due to the challenges inherent in fossil interpretation.

Paleontologist Paul Selden noted that although these findings are intriguing, further research is needed to fully understand the evolution of arachnids beyond spiders.