The essential infrastructure of vertebrate brains has been discovered in a distant and unlikely relative: a marine worm, saccoglossus kowalevskii. Inside, precise groups of cells that are indispensible to building complex brains have been identified in this humble animal that barely has a brain to speak of.
Now a major conundrum for evolutionary biologists working in neuroscience is finally coming into focus.
Even though this worm’s brain hardly compares to that of even the simplest vertebrate, it has three signaling centers which are virtually the same as those in the brains of vertebrate embryos.
Why do the closest relatives of vertebrates have brains made of standard bundles of neurons, yet even the simplest fish have the same brain organization scheme that all vertebrates have inherited?
The hemichordate acorn worm has three genetic programs known to underpin the development of brain anatomy in vertebrates:
- an anterior neural ridge which in vertebrate embryos develops into the forebrain.
- an isthmic organizer, which patterns the cerebellum and midbrain.
- a zone limitans intrathalamica, organizes the cerebrum from the thalamus, and provides the primary boundary in the vertebrate forebrain, called the cerebrum in humans. Organizers separate adjacent cell populations, creating different lineage zones. The boundaries prevent movement across the barrier of cells from different lineages.
Biologists can now be fairly certain that the most recent common ancestor of vertebrates would have had something similar to the brain of the acorn worm. In the distant past, some animal manifested the basic structure on which vertebrate neural complexity has been built.
When and why this occurred, along with hundreds of other questions, are now closer to being answered, although our new knowledge is merely the beginning of understanding why the structure originally mutated, what the ancestor organ was like, and why vertebrate and invertebrate brains evolved along such different paths.
Check out these laboratories that are pioneering evolutionary neuro-science.