Brain structures directly related to the human brain have just been identified in a marine ragworm, according to a paper published in the latest issue of the journal Cell.
The discovery means that the origins of the human brain can now be traced back at least 600 million years, when we last shared a common ancestor with this species, Platynereis dumerilii, a relative of the common earthworm.
"This worm lives in self-made tubes, explores its environment actively for food, and shows signs of learning behavior," lead author Raju Tomer told Discovery News. "Therefore, we thought this ragworm would be the ideal candidate to look for the counterparts of vertebrate higher brain centers in invertebrates."
Tomer, a scientist at the European Molecular Biology Laboratory (EMBL), and his colleagues suspect that other invertebrates, such as insects, spiders, crustaceans and velvet worms likely also possess the brain structures, called "mushroom bodies," which correspond to our cerebral cortex. The cerebral cortex is a part of the human brain involved in memory, learning, thought, language, consciousness and more.
Tomer and his team used a new technique they developed, called "cellular profiling by image registration," to investigate a large number of genes in the marine ragworm's compact brain. The method enabled the scientists to determine each cell's molecular fingerprint, and to define cell types according to the genes they express, rather than just based on their shape and location, as was done before.
"The development and patterning mechanisms of annelid mushroom bodies and vertebrate brains are too similar to be explained by independent origins," Tomer said. "They must share a common evolutionary precursor, though less complex, which evolved in the last common ancestor more than 600 million years ago."
Co-author Detlev Arendt, also at EMBL, told Discovery News that the seafloor at that time must have been covered with various food sources. In order for organisms to explore these foods, it would have been "advantageous to evolve a brain center that was able to integrate the different smells and ultimately learn what is good and what is bad food."
This first pre-brain probably then consisted of a group of densely packed cells that received and processed very basic information about food and the environment. The structure may have enabled our ancestors crawling over the sea floor to identify food sources, move towards them, and then later to integrate previous experiences into learning.
When French biologist Felix Dujardin first observed the mushroom bodies in invertebrates in 1850, he proposed that these structures bestowed insects with a certain degree of free will control over their instinctive actions. Dujardin's theories have since been largely validated.
Subsequent research has established that the mushroom bodies, which look a bit like mushrooms, serve as a center for associative learning and memory formation, activities that are very similar to those of the cerebral cortex.
"Our cerebral cortex functions by associating sensory information, such as smell, sound and vision, with events, and by storing these associations as memories by modifying the connection strength of neurons," Tomer explained.
"These stored memories then form the basis for making right decisions in the future. Similar mechanisms are found in invertebrates as well, where mushroom bodies are known to be largely responsible for associative learning."
He doubts, however, that invertebrates think and feel just as we do, since their brains are small and lack the "immensely large number of neurons" present in the human brain.
In the future, the scientists hope to further investigate worm brains, and those of other invertebrates, to better determine how they work and to help figure out what the brain of the last common ancestor of vertebrates and these worms might have looked like.
"Our ultimate goal is to reconstruct and understand the evolution of brains in animals, to trace their neuronal composition and their function from the very beginning of animal evolution to something as complex as today's human brain," Arendt said.