insect brain evolution 520 million years arthropod fossil

Fossil of Fuxianhuia courtesy of Xiaoya Ma

Your everyday cockroach might not seem terribly intelligent. But new fossil evidence from 520 million years ago suggests that this insidious insect might have had some surprisingly smart early ancestors.

Cockroaches and other insects belong to a group called the arthropods, which arose some 540 million years ago. A new Chinese fossil is yielding new insights into how the arthropod brain evolved and shows that within the first 20 million years of the group's emergence, the arthropod brain had already become surprisingly advanced. The new findings are based on a three-inch-long fossil arthropod known as Fuxianhuia protensa, found in what is now China's Yunnan Province and were described online October 10 in Nature (Scientific American is part of Nature Publishing Group).

"The fossil provides the most convincing, and certainly the oldest, description of nervous-system tissue in a fossil arthropod," Graham Budd, of Uppsala University's Department of Earth Sciences, wrote in an accompanying essay published in the same issue of Nature. The evidence for this brain material consists of "dark, iron-rich mineral traces," Budd noted, which can be interpreted as sections of a multi-part brain.

Fuxianhuia's body is understandably primitive, which is par for the prehistoric course, given that it lived some 290 million years before the dinosaurs emerged. But the brain architecture was a surprise. After examining the fossil under a dissecting microscope, the researchers found that this animal had three distinct, closely situated brain sections. It also has three optic neuropils, which are connected by nerve fibers. The well-preserved fossil even shows hints of linking fibers that connected these separate areas. "No one expected such an advanced brain would have evolved so early in the history of multicellular animals," Nicholas Strausfeld, a neurobiologist at the University of Arizona and co-author on the new paper, said in a prepared statement.

These findings might seem clear cut, but they have been made in "one of the most controversial and interesting arthropod species to boot, Fuxianhuia," Budd noted. Fuxianhuia has typically been classified as an early arthropod that was probably close to the common ancestor of all other hard-bodied invertebrates—of which there are more than 1.1 million described extant species. But the brain of this early creature bears a striking similarity to the brains of common bugs (the group that includes insects and arachnids) and malacostracans, (the group that includes crabs and lobsters), which have the three main brain sections and the connected optic neuropils. Another major group of arthropods, however, called the branchiopods (which includes brine shrimp and Daphnia water fleas) that emerged later, have much simpler brains with only two optic neuropils that are not connected like those in Fuxianhuia.

Strausfeld and his colleagues suggest that the new finding points to a malacostracan-type origin for modern day insects—rather than an ancestral primitive brain that more closely resembled that of Daphnia. If that is the case, "it is remarkable how constant the ground pattern of the nervous system has remained," Strausfeld said. This early complexity—especially in the optic area—could help to explain the stunning diversity of visual abilities in modern day relatives, such as compound eye vision.

Another possibility, albeit a controversial one, is that Fuxianhuia is, indeed, ancestral to all of these groups but the brains of those in the branchiopods actually simplified over time. "Either way," Budd noted, the findings "will prompt hasty reexamination of many old specimens, and quite possibly some recasting of recent theories."