Worm brain sheds light on the evolution of the cerebral cortex
The last time humans and the marine ragworm Platynereis dumerilii shared a common ancestor was roughly 600 million years ago, according to scientists from the European Molecular Biology Laboratory in Heidelberg, Germany (EMBL). That is, researcher have discovered a true counterpart of the cerebral cortex, also called the pallium, in this relative of the earthworm. This finding, explained the scientists, could be the key to unraveling the evolution of this important area of the human (and all vertebrates) brain.
“Two stunning conclusions emerge from this finding,” said Detlev Arendt, co-author of the study published yesterday in Cell, in a press release. “First, the pallium is much older than anyone would have assumed, probably as old as higher animals themselves. Second, we learn that it came out of ‘the blue’ – as an adaptation to early marine life in Precambrian oceans.”
The cerebral cortex is a sheet of neural tissue responsible for what is commonly called the “gray matter” of the brain; it plays a key role in memory, attention, perceptual awareness, thought, language and consciousness. According to the release, “our cerebral cortex … is a big part of what makes us human: art, literature and science would not exist had this most fascinating part of our brain not emerged in some less intelligent ancestor in prehistoric times.”
The discovery could provide scientists with a basic understanding of ancient brain centers—what they looked like and how they functioned—and provide insight into the development of the human cerebral cortex. Based on the ragworm’s brain structure, Arendt and colleagues found that the common ancestor’s brain structure was likely a group of densely packed cells which controlled olfactory signals and locomotion. In other words, it may have enabled the ancestors, which were crawling along the sea floor 600 million years ago, “to identify food sources, move towards them and integrate previous experiences into some sort of learning,” as stated in the release.
Previous to these findings, scientists believed that the vertebrate brain—as in humans—and invertebrate brain evolved separately along two different paths. That is, insects, spiders and other invertebrates typically have what are known as mushroom bodies—structures similarly responsible for sensory integration and memory. However, by comparing genes from the ragworm’s extra large mushroom body and the human cerebral cortex, the scientists suggested a common ancestor could have started the evolution of both brain structures.
“Most people thought that invertebrate mushroom-bodies and vertebrate pallium had arisen independently during the course of evolution, but we have proven this was most probably not the case,” said EMDL colleague Raju Tomer in the release. “The evolutionary history of our cerebral cortex has to be rewritten,” Arendt concluded.
Photo Credit: Natural History Museum