Exploring the Mysteries of the Human Brain
- A recent study on gene expression in the human brain shows that instead of simply becoming more numerous, brain cells become increasingly specialized.
- The human brain is the organ most responsible for distinguishing us from other primates, with its remarkable size, complexity, and functionality surpassing that of any other species. Despite these significant differences, humans share over 95% of their genome with chimpanzees, our closest living relatives.
- UC Santa Barbara professor Soojin Yi, along with her doctoral student Dennis Joshy and collaborator Gabriel Santepere from the Hospital del Mar Medical Research Institute in Barcelona, explored how genes in different types of brain cells have evolved compared to chimpanzees.
- Their research revealed that while human genes produce nearly the same proteins as those of other apes, human genes tend to be far more productive. Their findings, published in the Proceedings of the National Academy of Sciences, highlight the crucial role of gene expression in the evolution and functionality of the human brain.
Gene Expression in the Brain: A New Understanding
- Every gene instructs cells to create specific molecules, but the process is mediated by messenger RNA (mRNA), which carries the genetic information to the cell's machinery. Scientists have long believed that the human genome could explain our unique traits, but a 2005 comparison between the human and chimpanzee genomes revealed a 99% similarity (a figure that has since been revised), suggesting minimal genetic differences.
- Now, researchers believe that differences in gene expression could explain these unique traits. Just as the monarch butterfly’s dramatic transformation from caterpillar to adult is driven by changes in gene expression, humans might experience similar differences that contribute to their distinctiveness.
A New Era in Brain Cell Study
- Until recently, research into gene expression primarily studied bulk tissue samples from various cells. However, recent advances allow for the analysis of individual cell nuclei, enabling a more detailed look at gene expression within specific cell types and even subtypes.
- Yi, Joshy, and Santepere used innovative technology to isolate individual nuclei from human, chimpanzee, and macaque brain cells and measure the amount of mRNA produced. They observed significant differences in gene expression between the species, with human cells showing more upregulated genes than chimpanzees, especially when analyzing subtypes of cells.
The Role of Glial Cells
- While the human brain's neural pathways are highly complex, Yi notes that the uniqueness of human intelligence cannot be attributed to neurons alone. Glial cells, which make up more than half of the cells in the human brain (much more than in chimpanzees), play a critical role. Among these cells, oligodendrocytes, which insulate neurons and enable efficient electrical signaling, showed the greatest differences in gene expression.
- Yi suggests that this enhanced neural plasticity and slow brain development are crucial factors in the unique cognitive abilities of humans. Furthermore, she believes that the evolution of the brain’s other cell types, such as glial cells, enabled the expansion and complexity of neurons and neural networks.
Future Directions
- Yi's team plans to continue investigating the mechanisms behind differential gene expression and how they relate to traits in the human brain. They aim to study gene expression in even more distantly related animals and explore how these differences might link to traits seen in other human species, like Neanderthals and Denisovans.
- This research sheds light on how gene expression, not just genetic changes, drives the evolution of human brains.
