Gene Expression Drives Evolution of Human Brain Complexity

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Source: https://neurosciencenews.com/gene-expression-brain-evolution-28301/


Gene Expression Drives Evolution of Human Brain Complexity

January 2, 2025

Summary:

While humans share over 95% of their genome with chimpanzees, our brains are far more complex due to differences in gene expression.

Research shows that human brain cells, particularly glial cells, exhibit higher levels of upregulated genes, enhancing neural plasticity and development.

Oligodendrocytes, a glial cell type, play a key role by insulating neurons for faster and more efficient signaling. This study underscores that the evolution of human intelligence likely involved coordinated changes across all brain cell types, not just neurons.

Key Facts:

Gene Expression: Human brain cells show higher levels of gene activity compared to chimpanzees.

Glial Cell Role: Differences in oligodendrocyte gene expression may enhance neural plasticity and efficiency.

Evolutionary Insight: Human brain complexity evolved through specialized gene expression across cell types.

Our brain is arguably the organ that most distinguishes humans from other primates. Its exceptional size, complexity and capabilities far exceed those of any other species on Earth. Yet humans share upwards of 95% of our genome with chimpanzees, our closest living relatives.

UC Santa Barbara professor Soojin Yi, in the Department of Ecology, Evolution, and Marine Biology, her doctoral student Dennis Joshy and collaborator Gabriel Santepere, at Hospital del Mar Medical Research Institute in Barcelona, aimed to determine how genes in different types of brain cells have evolved compared to those in chimpanzees.

Human glial cells account for more than half of the cells in our brains, a much larger percentage than in even chimpanzees.

Yi, Joshy and Santepere used datasets generated from a device with a very narrow channel to separate each nucleus into its own chamber in an array. Then they grouped the cells by type before performing statistical analysis

The team measured gene expression by observing the amount of mRNA a specific gene produced in humans, chimpanzees and macaques. An upregulated gene produces more mRNA in a given species compared to the others, while a downregulated gene produces less.

Comparing chimpanzees and humans to macaques enabled the researchers to tell when differences between the two apes were due to changes in chimpanzees, changes in humans, or both.

The authors recorded differences in the expression of about 5-10% of the 25,000 genes in the study. In general, human cells had more upregulated genes compared to chimpanzees.

This is a much larger percentage than researchers found when they couldn’t break down the analysis by cell type. And the percentage grew to 12-15% when the authors began to consider cell subtypes.

“Now we can see that individual cell types have their own evolutionary path, becoming really specialized,” Yi said.

Not just neurons:

The intricacy of our neural pathways is unrivaled in the animal kingdom, however Yi suspects that our unique intellect isn’t a result of this on its own.

Human glial cells account for more than half of the cells in our brains, a much larger percentage than in even chimpanzees.

Among glial cells, oligodendrocytes showed the greatest differences in gene expression. These cells create the insulation that coats neurons, enabling their electrical signals to travel much more quickly and efficiently.

In a collaborative study published the previous year, the team observed that humans have a higher ratio of precursor versus mature oligodendrocytes compared with chimpanzees.

Yi suspects this may relate to the amazing neural plasticity and slow development of human brains.

“The increased complexity of our neural network probably didn’t evolve alone,” Yi said.

“It could not come to existence unless all these other cell types also evolved and enabled the expansion of the neuron diversity, the number of neurons and the complexity of the networks.”

This study only considered cells from a few regions of the brain; however, the cells in one area of the brain may differ from their counterparts in other areas. Yi plans to study the mechanisms behind differences in gene expression and how genes map to different traits.

She also plans to trace differential gene expression even earlier in our evolutionary history by incorporating baselines from even more distantly related animals. And she’s interested in studying genomic differences between us and other archaic humans, like Neanderthals and Denisovans.

Evolution is about more than merely changing genes. “Differential gene expression is really how human brains evolved,” Yi said.