neocortex stands Out as a marvelous achievement of biological evolution. All mammals have this band of tissue that covers the brain, and her six layers of densely packed neurons within it handle the sophisticated computations and associations that produce cognition. Since animals other than mammals lack a neocortex, scientists have wondered how such complex brain regions evolved.
A reptilian brain appeared to provide a clue. Reptiles are not only the closest extant relatives of mammals, but their brains have a three-layered structure called the dorsal ventricular ridge (DVR), which is functionally similar to the neocortex. For more than 50 years, some evolutionary neuroscientists have argued that both the neocortex and the DVR derive from more primitive features of an ancestor shared by mammals and reptiles.
But now scientists are refuting that view by analyzing molecular details invisible to the human eye. By examining gene expression patterns in individual brain cells, researchers at Columbia University have shown that the mammalian neocortex and her DVR in reptiles are unrelated, despite their anatomical similarities. . Instead, mammals seem to have evolved the neocortex as an entirely new brain region, with no previous traces. The neocortex is composed of new types of neurons that appear to be unheard of in ancestral animals.
paper Describe this study led by evolutionary and developmental biologists Maria Antonietta Toscheswas published last September chemistry.
This process of evolutionary innovation in the brain is not limited to creating new parts.Other works by Tosh and her colleagues in the same issue chemistry We showed that even seemingly ancient brain regions continue to evolve by being rewired with new types of cells. It’s prompting researchers to rethink how they define brain regions and reassess whether some animals may have more complex brains than they thought.
Active genes in single neurons
Back in the 1960s, influential neuroscientist Paul McLean proposed the wrong idea of brain evolution, yet it left a lasting impact on the field. He suggested that the basal ganglia, a group of structures near the base of the brain, are remnants from the ‘lizard brain’ that evolved in reptiles and was responsible for survival instincts and behavior. When early mammals evolved, a limbic system was added above the basal ganglia to control emotions. According to McLean, the neocortex was added when humans and other advanced mammals emerged. Like the “Thought Cap”, it sat at the top of the stack, giving it higher cognition.
This “Trinity Brain” model captivated the public imagination after Carl Sagan wrote it in his 1977 Pulitzer Prize-winning book. dragon of edenEvolutionary neuroscientists were less impressed. Research soon debunked the model by demonstrating conclusively that brain regions were not evolving well one after the other. Instead, the brain is described as evolving as a whole, with older parts being modified to accommodate the addition of new parts. Paul Sisek, a cognitive neuroscientist at the University of Montreal. “Loading a new app is not like upgrading an iPhone,” he said.
The best-supported explanation for the origin of new brain regions was that they evolved primarily by duplicating and modifying pre-existing structures and neural circuits. harvey curtain The similarities between mammalian neocortex and reptilian DVRs, researchers at the University of California, San Diego, suggest that, from an evolutionary point of view, the two are homologous, inherited from a shared ancestor between mammals and reptiles. suggested that it evolved from structure.
However, other researchers including Luis Puels A professor at the University of Murcia in Spain disagreed. In the development of mammals and reptiles, they saw indications that the neocortex and DVR formed through completely different processes, suggesting that the neocortex and her DVR evolved independently. . If so, their similarity has nothing to do with homology. They were likely coincidences dictated by functional and structural constraints.
