Marine worms might seem like lowly, slow-witted creatures, but new gene mapping shows that we might share an ancient brainy ancestor with them.
Human cognition is largely rooted in the cerebral cortex, the part of the brain that enables consciousness, language and other higher-level functions. We share the basic evolutionary underpinnings of our big brains with other vertebrates, which have a structure known as the pallium.
Although lacking palliums, many invertebrates, such as insects, spiders and some worms, instead have what are know as mushroom bodies—sections of the brain so called because their shape resembles mushrooms. Mushroom bodies and vertebrate palliums are both responsible for some sensory integration and memory, and they have "long been noted and interpreted as convergent acquisitions," noted a team of researchers in a new study, published online September 2 in Cell. In other words, the thinking has been that these two kinds of brains evolved from independent paths.
The team, however, has proposed instead that these two brain structures do share a single common ancestor, one that likely lived some 600 million years ago. The group based their conclusions on new gene expression maps—"molecular fingerprints"— gathered from the mushroom bodies in developing marine ragworms (Platynereis dumerilii) that could be compared with gene expression patterns of developing vertebrate palliums.
These worms in particular have extra large mushroom bodies and have been shown to explore and learn. The team examined gene expression patterns in the brains of these worms in the first days of larval development using a new technique they devised called profiling by image registration (PrImR).
"Comparing the molecular fingerprints of the developing ragworms' mushroom bodies to existing information on the vertebrate pallium, it became clear that they are too similar to be of independent origin," Detlev Arendt, of the Developmental Biology Unit at the European Molecular Biology Laboratory in Heidelberg and coauthor of the new study, said in a prepared statement.
Based on the "gene-by-gene" comparison, the researchers found that the worms' mushroom bodies and vertebrates' pallium "develop from the same, molecularly defined subregion," the researchers explained in the paper. The overlap in shared transcription factors also supported the conclusion that the two structures "must share a common evolutionary precursor," as Arendt said.
Although no Einstein, the common evolutionary ancestor was probably able to use its basal brain structure to integrate sensory signals—most likely in service of finding food, the researchers noted.
Image of Platynereis brain with gene expression activity mapped courtesy of EMBL/R. Tomer