Gene Expression in Neurons Solves a Brain Evolution Puzzle

Researchers from Columbia University have used gene expression analysis to determine that the mammalian neocortex, which covers the brain and provides six layers of densely packed neurons, did not evolve from a structural feature shared by mammals and reptiles. Although reptile brains also have a three-layered functional structure called a dorsal ventricular ridge (DVR), the differences discovered in the gene expression in individual brain cells suggest that the DVR and neocortex are unrelated. It seems that mammals evolved the neocortex as an entirely new brain region, unlike the DVR, which was based on previously existing functions. Pyramidal neurons, which are the most abundant type in the neocortex, evolved as an innovation in mammals.
The report illustrates that during brain evolution, new parts are not the only new areas that are created. Even brain regions that may appear ancient are continuing to evolve, by being rewired with new types of cells. Researchers are rethinking the definition of some brain regions as a result of these findings, whilst reassessing the idea that some animals possess more complex brains than previously thought. The development of new techniques for gene expression analysis has enabled the researchers to refute older models of brain evolution; this analysis offered greater insight into the differentiation and evolution of neuronal types.
The outdated “triune brain” model, which suggested that the basal ganglia were a survival instinct common to advanced mammals, evolved from the limbic system of early mammals and the neocortex of humans, is thoroughly discredited. Scientists now believe that the brain evolves as a whole, modifying older parts to adapt to added functionality. The most prevalent theory for the evolution of new brain regions is that they are comprised of duplicated and modified preexisting structures and neural circuits. While some evolutionary biologists postulate that the neocortex and the reptilian DVR are homologous – that they both evolved from a structure passed down from an ancestor shared by mammals and reptiles, other research offers an alternative conclusion. In the development of mammals and reptiles, the neocortex and DVR supposedly took shape through completely different processes, casting doubt on their evolutionary relationship.



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