A 520-million-year-old fossilized arthropod with well-preserved brain structures has shown that complex brains have evolved much earlier than thought. According to University of Arizona neurobiologist Nicholas Strausfeld, who co-authored the study describing the specimen, the fossil is the earliest known to show a brain.
Embedded in mudstones deposited during the Cambrian period 520 million years ago in what today is the Yunnan Province in China, the approximately 3-inch-long fossil, which belongs to the species Fuxianhuia protensa, represents an extinct lineage of arthropods combining an advanced brain anatomy with a primitive body plan.
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 call their find "a transformative discovery" that could resolve a long-standing debate about how and when complex brains evolved.
"There has been a very long debate about the origin of insects," Strausfeld said, adding that until now, scientists have favored one of two scenarios.
Some 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.
Because 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 fossilized brain] matches that of a comparable sized modern malacostracan," the researchers said.
They argue the fossil supports the hypothesis that branchiopod brains evolved from a previously complex to a more simple architecture instead of the other way around.
Strausfeld traveled to the Yunnan Key Laboratory for Palaeobiology at Yunnan University in Kunming, China, to join his collaborator, Xiaoya Ma, a postdoctoral fellow at London's Natural History Museum, in studying the brain anatomies of various fossil specimens. In the institute's collection, they came across the fossil of Fuxianhuia protensa.
"I spent a frenetic five hours at the dissecting microscope, the last hours of my visit there, photographing, photographing, photographing. And I realized that this brain actually comprises three successive neuropils in the optic regions, which is a trait of malacostracans, not branchiopods," he said.
When Strausfeld traced the fossilized outlines of Fuxianhuia's brain, he realized 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. 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 fossil supports the idea that once a basic brain design had evolved, it changed little over time, he explained.
The discovery will be published in the journal Nature.