Scientists have discovered the earliest known complete nervous system exquisitely preserved in the fossilized remains of a never-before described creature that crawled or swam in the ocean 520 million years ago. The find solves a long-standing debate as to when the ancestors of chelicerates — spiders and their kin — made their first appearance and provides evidence that their biting mouthparts evolved from the claw-like appendages of a long-extinct group known as megacheirans.