The finding shows that the human brain is more similar to a macaque than a chimpanzee brain.
It is still unknown how the other great apes lost the cells, but the researchers theorize that the genetic alterations that affect the migration of cells to different parts of the brain, differentiation or survival could have led to the loss.
Sestan explained that, as a city, the brain is a highly organized arrangement of discrete units connected by transport and communication systems. In the brain, cells are found between units, migratory pathways are roads or paths, and various electrical or chemical signals, including dopamine, are communication systems.
Similar to the real estate industry of a city, the three most important things are location, location, location.
"Very few neurons born in the developing brain reside in the same place in the adult," Sestan said. "Instead, they are born, migrate to a new location, establish a functionality and then, eventually, die, as expected, many things can go wrong, a cell could not be born or could die prematurely, could migrate to error, location, or could badume or acquire a different functionality. "
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These events could help explain why there are brain differences between humans and chimpanzees, with whom we share up to 98 percent of the same DNA. "In principle," Sestan said, "small changes in the wiring of the brain can lead to profound and specific functional changes."
Interneurons of the human brain express the enzymes tyrosine hydroxylase (TH) and DOPA (3,4-dihydroxyphenylalanine) decarboxylase (DDC). The two proteins are involved in the biosynthesis of dopamine.
While the ancestors of chimpanzees and gorillas lost the ability to express these enzymes in the neocortex, it is likely that a human ancestor has recovered it. Scientists do not know what human ancestor recovered this ability, or when.
Since dopamine in the mesencephalon plays many roles in the central nervous system linked to cognition and behavior, humans seem to have won the evolutionary prize of the brain. The definition of intelligence is subjective, but our work memory, reflective exploratory behavior and other cognitive abilities seem to improve in a unique way compared to these abilities in other animals.
"After all, to the best of our knowledge, we are the only living species that tries to understand how our brain works and what makes our brain different from the brain of other species," said Sousa.
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On the other hand, there appear to be drawbacks badociated with the structure and organization of the human brain.
"In general, the additional brain size and connectivity of the human brain compared to the chimpanzee or macaque, together with the prolonged period of time during which human neurodevelopment occurs, means that there are many more problems than they can arise and a longer period of time during which these problems can occur, "Sestan explained.
Previous research, for example, determined that dopamine-producing neurons throughout the brain are damaged in Parkinson's disease. In fact, Parkinson's patients often receive L-DOPA, an amino acid produced by TH. DDC can then produce dopamine using L-DOPA as a substrate.
Therefore, comparisons of primate brains may reveal differences that may be relevant to the therapeutics of the disease, as well as obtain a better understanding of human cognition and behavior and what makes us unique among other primates.
Sestan admits that he and his international team began the experiments thinking that they might not observe significant findings. "That's the nature of basic research," he said. "You do not know what's really important until you look at it, and many times you have to waste time just learning where to look first."
Currently, researchers are studying the function of several genes with specific expression patterns for humans. Sousa said: "The function of most of these genes in the brain has not yet been discovered."
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