Gene expression in neurons solves puzzle of brain evolution.

The neocortex stops As a remarkable achievement of biological evolution. All mammals have this layer of tissue that covers their brains, and the six layers of dense neurons within it contain the associations that produce sophisticated calculations and cognitive abilities. Since no other animal except mammals has a neocortex, scientists have wondered how such a complex part of the brain developed.

Reptilian brains seem to give clues. Reptiles are not only the closest relatives of mammals, but their brains have a three-layered structure called the dorsal ventricular ridge, or DVR, similar to the neocortex. For more than 50 years, some evolutionary neuroscientists have argued that the neocortex and the DVR both derive from a common ancestor in mammals and reptiles.

But now, scientists have disproved this view by analyzing molecular details invisible to the human eye. Researchers at Columbia University looked at gene expression patterns in individual brain cells and found that despite anatomical similarities, the neocortex in mammals and the DVR in reptiles are unrelated. Instead, mammals appear to have evolved the neocortex as an entirely new brain region, one that developed without recognition of what preceded it. The neocortex consists of new neurons that appear to have no precursors in ancestral animals.

A paper describing this work, led by evolutionary and developmental biologist Maria Antonita Tosches, was published last September. Science.

This evolutionary process in the brain is not limited to the creation of new parts. Other works by Toshes and her colleagues in the same issue Science He showed that even seemingly ancient brain regions continue to improve by reprogramming with new types of cells. The discovery that gene expression shows these types of differences between neurons is prompting researchers to rethink how they define certain brain regions and to reassess that some animals may have more complex brains than previously thought.

Active genes in single neurons

In the year In the 1960s, the influential neuroscientist Paul McLean proposed a misconception about the evolution of the brain, but he still had a lasting impact on the field. He suggests that the basal ganglia, a cluster of structures near the base of the brain, is the “lizard brain” that evolved in animals and is responsible for survival instincts and behaviors. As early mammals evolved, they added the limbic system above the basal ganglia to control emotion. And when humans and other advanced mammals arose, according to McLean, they increased the neocortex. Like “intelligence” it sat at the top of the stack and gave a high level of awareness.

Carl Sagan This “trinitarian brain” model captured the public’s imagination after he wrote about it in his 1977 Pulitzer Prize-winning book. Dragons of Eden. Evolutionary neuroscientists were less impressed. Studies soon disproved the model, showing that brain regions do not transfer well to one another. Instead, as the brain evolves as a whole, old parts are being modified to adapt to the addition of new parts, explains cognitive neuroscientist Paul Cisek at the University of Montreal. “It’s not like upgrading your iPhone where you install a new app,” he said.

The most widely supported explanation for the origin of new brain regions is that they are largely modified by duplication and modification of existing structures and neural circuits. For many evolutionary biologists, such as Harvey Carton of the University of California, San Diego, the similarities between the mammalian neocortex and the reptilian DVR suggest that they are, in evolutionary terms, homologous—both descended from a single structure shared by a common ancestor of mammals and reptiles.

Other researchers, including Luis Pules of the University of Murcia in Spain, disagree. They saw indications that the neocortex and the DVR involved entirely different processes in the development of mammals and reptiles. This suggested that the neocortex and DVR evolved independently. If so, the similarity had nothing to do with morphology: it was probably a function of coincidences and constraints on structures.