The neocortex stops A remarkable feat of natural evolution. All mammals have large amounts of tissue that cover their brains, and the six layers of neurons packed inside are responsible for the technical computations and associations that make up cognitive intelligence. Since no other animal except mammals has a neocortex, scientists wonder how this complex brain region evolved.
The reptilian brain seemed to provide information. Not only are reptiles the closest relatives of mammals, but their brains have three regions called the dorsal ventricular ridge, or DVR, which is similar to the neocortex. For more than 50 years, some biologists have argued that the neocortex and the DVR both evolved from a more primitive ancestor shared by animals and reptiles.
But now, by examining things that we cannot see, scientists are disputing this fact. By looking at gene expression in brain cells, Columbia University researchers have shown that despite their physical similarities, the neocortex in animals and the DVR in reptiles are not related. Instead, mammals seem to have evolved the neocortex into a new part of the brain, which was built independently of its predecessors. The neocortex is made up of new types of neurons that appear to have no ancestral animal origin.
A paper describing the work, led by evolutionary biologist Maria Antonietta Tosches, was published last September. Science.
The process of evolution in the brain is not limited to the creation of new parts. Another work of Tosches and his colleagues in the same issue of Science showed that even parts of the brain that appear to be old continue to evolve by connecting with new types of cells. The discovery that gene expression can reveal these kinds of important differences between neurons is prompting researchers to rethink how they describe certain parts of the brain and to examine whether other animals may have more complex brains than previously thought.
Genes Functioning in Single Neurons
In the 1960s, neuroscientist Paul MacLean proposed a theory of brain evolution that was flawed but had a lasting impact on the field. He said the basal ganglia, a group of structures near the brain stem, were remnants of the “lizard brain” that evolved into reptiles and had the ability to save genes and behaviors. Early mammals evolved, added the limbic system for emotional control above the basal ganglia. And when humans and other higher animals arose, according to MacLean, they added the neocortex. As a “thinking hat,” it sat on top of the tree and made a big impression.
This “triple brain” model gained public attention after Carl Sagan wrote about it in his 1977 Pulitzer Prize-winning book. Dragons of Eden. Evolutionary neuroscientists were not particularly interested. Research soon overturned that model by showing conclusively that brain regions do not transfer well one over the other. Instead, the brain changes as a whole, with old parts being replaced with the addition of new parts, explained Paul Cisek, a neuroscientist at the University of Montreal. “It’s not like upgrading your iPhone, where you download a new app,” he said.
The most plausible explanation for the emergence of new brain regions is that they evolved primarily by repeating and modifying pre-existing neural circuits. For many biologists, such as Harvey Karten of the University of California, San Diego, the similarities between the mammalian neocortex and the reptilian DVR suggest that, in evolutionary terms, they are the same—that both evolved from something else. common ancestor with mammals and reptiles.
But other researchers, including Luis Puelles of the University of Murcia in Spain, disagreed. In the development of animals and reptiles, they observed signs that the neocortex and DVR were formed in very different ways. This suggests that the neocortex and DVR evolved independently. If so, their similarities had nothing to do with homology: They were probably coincidences followed by the work and constraints of design.