Key Points:
- Scientists used single cell RNA-sequencing to study cells from the dorsolateral prefrontal cortex from four different kinds of primates, humans included.
- They found a type of microglia, an immune system cell, that was unique to the human brain sample.
- The scientists believe that the microglia evolved specifically in human brains as a response to the unique environment that humans live in.
A group of Yale University researchers have uncovered clues that may explain what makes the human brain unique from the brains of other animals. The team was particularly interested in distinguishing characteristics between the brains of humans and our closest primate cousins. The study, which appeared in Science, discovered species-specific characteristics after examining the prefrontal cortex of adult humans, chimpanzees, macaque, and marmoset monkeys.
The researchers collected hundreds of thousands of cells from the dorsolateral prefrontal cortex (dlPFC) of the primate brains. The dlPFC is an area of a primate’s frontal lobes related to higher cognitive functions, such as abstract reasoning and attention switching. They then applied single cell RNA-sequencing techniques to the cells, which allowed them to observe variations in gene expressions between each cell.
“Today, we view the dorsolateral prefrontal cortex as the core component of human identity, but still we don’t know what makes this unique in humans and distinguishes us from other primate species,” said Nenad Sestan, professor of neuroscience at Yale. “Now we have more clues.”
After a comparative analysis of the sequencing results, the researchers pinpointed 109 cells common to the primate group and five that were not. Among those five was a specific kind of microglia—resident immune cells in the brain—that was found only in the human sample.
The researchers hypothesize that this type of microglia is specific to the human brain due to it being highly responsive to external and environmental factors. Modern humans, for instance, live in habitats that are far removed, and vastly different, from the surroundings that other primates live in. It is reasonable to assume, therefore, that the microglia in human brains evolved as a reaction to that difference.
Additionally, the research identified that FOXP2, a protein coding gene involved in speech and language disorders, autism and epilepsy, was present only in human brains.
“FOXP2 has intrigued many scientists for decades, but still we had no idea of what makes it unique in humans versus other primate species,” said Shaojie Ma, one of the Yale researchers. “We are extremely excited about the FOXP2 findings because they open new directions in the study of language and diseases.”