The approximately 3-inch-long fossil belongs to the species Fuxianhuia protensa, and represents an extinct lineage of arthropods combining an advanced brain anatomy with a primitive body plan.
Embedded in mudstones deposited during the Cambrian period 520 million years ago in the Yunnan Province in China, the fossil is the earliest known to show a brain, according to University of Arizona neurobiologist Nicholas Strausfeld, who co-authored the study describing the specimen.
The fossil provides a "missing link" that sheds light on the evolutionary history of arthropods, the taxonomic group that comprises crustaceans, arachnids and insects.
The researchers called it "a transformative discovery" that could resolve a long-standing debate about how and when complex brains evolved.
"No one expected such an advanced brain would have evolved so early in the history of multicellular animals," Strausfeld said in a statement.
Evolutionary biologists have yet to agree on exactly how arthropods evolved, especially on what the common ancestor looked like that gave rise to insects.
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Some scientists believe that insects evolved from the an ancestor that gave rise to the malacostracans, a group of crustaceans that include crabs and shrimp, while others point to a lineage of less commonly known crustaceans called branchiopods, which include, for example, brine shrimp.
Since the brain anatomy of branchiopods is much simpler than that of malacostracans, they have been regarded as the more likely ancestors of the arthropod lineage that would give rise to insects.
However, the discovery of a complex brain anatomy in an otherwise primitive organism such as Fuxianhuia makes this scenario unlikely.
"The shape [of the fossilised brain] matches that of a comparable sized modern malacostracan," the authors wrote.
Strausfeld and colleagues traced the fossilised outlines of Fuxianhuia's brain and realised it had three optic neuropils on each side that once were probably connected by nerve fibers in crosswise pattern as occurs in insects and malacostracans.
Neuropils are portions of the arthropod brain that serve particular functions, such as collecting and processing input from sensory organs.
The brain was also composed of three fused segments, whereas in branchiopods only two segments are fused.
"In branchiopods, there are always only two visual neuropils and they are not linked by crossing fibers. In principle, Fuxianhuia's is a very modern brain in an ancient animal," Strausfeld said.
The discovery was published in the journal Nature.