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The human brain is the organ that most sets us apart from other primates. Its substantial size, intricacy, and abilities surpass those of any other species inhabiting our planet.
Nonetheless, we share upwards of 95% of our genetic material with chimpanzees, our nearest living relatives. How, then, can such genetically similar species demonstrate such pronounced disparities in brain function?
According to a study spearheaded by Professor Soojin Yi at the University of California, Santa Barbara, specific types of brain cells might hold the answer. The research was carried out in collaboration with partners at the Hospital del Mar Medical Research Institute in Barcelona.
The results indicate that although humans and chimpanzees have many identical genes, variations in gene activity could explain the unique characteristics of the human brain.
Focusing on gene expression
This is quantified by measuring messenger RNA (mRNA), the molecule responsible for conveying a gene’s instructions to the cell’s machinery.
Previously, when comparing the human genome to that of chimpanzees, researchers believed differences were located within the genes themselves.
However, given over 90% genetic similarity, that rationale seemed inadequate. Instead, gene expression—the level of mRNA produced—appears to be the pivotal element in clarifying how the human brain distinctly diverges from its primate relatives.
Gene expression and the evolution of the brain
Emerging evidence suggests that even minor changes in gene regulation can result in substantial alterations in structure and behavior. “Differential gene expression is essentially how human brains evolved,” Yi noted.
The researchers aimed to pinpoint exactly which genes in various brain cell types might show significant differences in expression between humans, chimpanzees, and other primates.
Historically, scientists analyzed gene expression by obtaining tissue samples containing mixed cell types, which complicated identifying unique activity patterns across different categories of brain cells.
In the current study, the team addressed this issue by employing modern techniques capable of analyzing individual cell nuclei. By isolating and sequencing the genetic material from individual brain cells, the researchers could classify them according to cell type before evaluating gene expression levels.
Diverse cell types among primates
This targeted approach on specific cell populations yielded extensive insights into how gene activity in distinct regions and cell types varies among primates.
Data from humans, chimpanzees, and macaques indicated that many genes exhibit common expression patterns, but humans frequently showed enhanced expression levels.
Specific cell types, particularly those associated with essential cognitive functions, demonstrated notably stronger upregulation in humans compared to other primates.
Glial cells and the expansion of the brain
Neurons often receive most of the focus in discussions regarding the capabilities of the human brain. However, the study emphasizes that glial cells—non-neuronal cells providing structural support, insulation, and other essential functions—may also significantly contribute to distinguishing the human brain from its primate relatives.
Human brains typically have a higher ratio of glial cells to neurons compared to chimpanzee brains, suggesting these support cells could enhance advanced cognitive functions.
One particularly remarkable subset of glia, known as oligodendrocytes, appears to exhibit notable differences in gene expression. These cells form myelin sheaths that safeguard neurons and accelerate signal transmission.
The researchers found that humans have a greater proportion of precursor oligodendrocytes than chimpanzees, a distinction that might account for prolonged developmental periods and increased plasticity within the human brain.
Directions for future research
Although the study focused solely on certain brain regions, its findings imply that gene expression in additional brain areas may also vary significantly among primates.
Yi and her team plan to explore these queries further and investigate the molecular processes that drive these evolutionary changes.
The experts also aim to expand their research to include more distantly related species to reconstruct a broader evolutionary narrative and evaluate how archaic humans, like Neanderthals and Denisovans, fit into this context.
In a broader context, gene expression research offers potential explanations for how relatively minor genomic variations can lead to substantial functional differences. It reveals that evolution may occur through subtle modifications in the regulation of existing genes rather than necessitating the complete creation of new genes.
By enhancing our understanding of these regulatory networks in the brain, scientists are advancing toward explaining how humans attained their exceptional cognitive skills.
The study has been published in the journal Proceedings of the National Academy of Sciences,
Image Credit: Matt Perko
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