Evolution Of Complex Brains
October 11, 2012

Fossil Find Suggests Complex Brains Developed Earlier Than Previously Believed

April Flowers for redOrbit.com - Your Universe Online

Anatomically complex brains evolved earlier than previously thought and have changed little over the course of time, according to a new study by University of Arizona neurobiologist Nicholas Strausfeld. The specimen described in the study, which will be published in the October 11 issue of Nature, is the earliest known fossil to show a brain.

The three inch long fossil was discovered embedded in mudstones deposited during the Cambrian period 520 million years ago in what is currently the Yunnan Province in China. It belongs to the species Fuxianhuia protensa, an extinct lineage of arthropods that combines advanced brain anatomy with a primitive body plan.

Arthropods are the taxonomic group that is comprised of crustaceans, arachnids and insects. This fossil represents a missing link that may shed light on the evolutionary history of the group. The research team thinks this fossil could resolve a long-standing debate about how and when complex brains evolved, calling the find a "transformative discovery."

“No one expected such an advanced brain would have evolved so early in the history of multicellular animals,” said Strausfeld, a Regents Professor in the UA department of neuroscience. Paleontologists and evolutionary biologists disagree on how arthropods evolved, especially, according to Strausfeld, about what the common ancestor looked like that gave rise to insects. Until now, the scientific community has favored one of two scenarios.

Some scientists believe that insects evolved from an ancestor that gave rise to malacostracans. Malacostracans are a group of crustaceans that include crabs and shrimp. Other scientists point to a lineage of less commonly known crustaceans called branchiopods, which include brine shrimp.

Branchiopod brain anatomy is much simpler than that of malacostracans, leading to the theory that the branchiopods were the more likely ancestors of the arthropod lineage. The discovery of a complex brain anatomy in an otherwise primitive organism like Fuxianhuia makes it unlikely that the branchiopods are the common ancestor.

“The shape [of the fossilized brain] matches that of a comparable sized modern malacostracan,” the authors write. The fossil supports the second hypothesis, which states that the branchiopod brains evolved from a previously complex to a more simple architecture instead of the other way around.

This second hypothesis arises from neurocladistics, a field pioneered by Strausfeld. Neurocladistics attempts to reconstruct the evolutionary relationships among organisms based on the anatomy of their nervous system. On the other hand, conventional cladistics usually look to an organism's overall morphology or molecular data such as DNA sequences to reconstruct the evolutionary relationships.

Strausfeld has cataloged approximately 140 character traits detailing the neural anatomies of almost 40 arthropod groups.

“There have been all sorts of implications why branchiopods shouldn´t be the ancestors of insects,” he said. “Many of us thought the proof in the pudding would be a fossil that would show a malacostracan-like brain in a creature that lived long before the origin of the branchiopods; and bingo! — this is what this is.”

Strausfeld collaborated with Xiaoya Ma, a postdoctoral fellow at London's Natural History Museum, in studying the brain anatomies of various fossil specimens at the Yunnan Key Laboratory for Palaeobiology at Yunnan University.

“I spent a frenetic five hours at the dissecting microscope, the last hours of my visit there, photographing, photographing, photographing,” he said. “And I realized that this brain actually comprises three successive neuropils in the optic regions, which is a trait of malacostracans, not branchiopods.”

The portions of the arthropod brain that serve particular functions — like collecting and processing input from sensory organs — are called neuropils. Scent receptors in the antennae are wired to the olfactory neuropils, for example, while the eyes connect to neuropils in the optic lobes.

By tracing the fossilized outlines of Fuxianhuia's brain, Strausfeld realized it had three optic neuropils on each side that once were probably connected by nerve fibers in crosswise patterns such that occurs in insects and malacostracans. The brain also has three fused segments, unlike branchiopods, which only have two fused segments.

“In branchiopods, there are always only two visual neuropils and they are not linked by crossing fibers,” Strausfeld said. “In principle, Fuxianhuia´s is a very modern brain in an ancient animal.”

"This fossil provides the most convincing, and certainly the oldest, description of nervous-system tissue in a fossil arthropod," Graham Budd of Uppsala University Earth Sciences Department wrote in a comment on the study according to the AFP news agency.

The study claims that this fossil supports the idea that once a basic brain design evolved, it changed little over time. Instead, peripheral components such as the eyes, antennae, and other appendages and sensory organs underwent great diversification. They specialized in different tasks, but all plugged into the same basic circuitry.

“It is remarkable how constant the ground pattern of the nervous system has remained for probably more than 550 million years,” Strausfeld added. “The basic organization of the computational circuitry that deals, say, with smelling, appears to be the same as the one that deals with vision, or mechanical sensation.”

"In principle, Fuxianhuia's is a very modern brain in an ancient animal," added Strausfeld.

"It is remarkable how constant the ground pattern of the nervous system has remained for probably more than 550 million years."