According to Nenad Sestan, senior author of the paper, Harvey & Kate Cushing Professor of Neurology and Professor of Comparative Medicine/Genetics/Psychiatry at Yale University:
Although we currently see the dorsolateral prefrontal cortex (dlPFC) as a central component of human identity, it remains unclear why it is unique to humans and distinguishes us from other primates.
The good news is that now we have more clues. To answer this question, the scientists first traversed whether any unique cell types also exist in humans or other non-human primate species analyzed.
After grouping cells with similar expression profiles, the researchers found 109 shared primate cell types, five of which were not shared by all species.
These include one type of microglia/brain-specific immune cell found only in humans, and another type of cell shared only by humans and chimpanzees.
A closer look allowed the identification of this human-specific microglial cell type, which is present throughout the species’ development and adulthood — suggesting that these cells play a role in maintaining the brain, rather than fighting disease.
Professor Nenad Sestan added that compared to other primates, humans live in a very different environment, with a unique lifestyle and glial cells.
Among them were microglia, which were very sensitive to these differences — meaning that this cell type may represent an immune response to the environment.
In addition, in analyzing the gene expression of microglia, the Yale scientists also found another human-specific surprise, the presence of the FOXP2 gene.
The finding is of great interest to researchers because variants of FOXP2 are associated with speech dyskinesia — a disorder that makes it difficult for patients to produce speech.
Other studies have linked FOXP2 to other neuropsychiatric disorders – such as autism, schizophrenia and epilepsy.
Sestan and colleagues show that the gene shows primate-specific expression in a subset of excitatory neurons, but human-specific expression in microglia.
Shaojie Ma, a postdoctoral associate in the lab and study co-author, concluded:
FOXP2 has intrigued scientists for decades, but we’ve been slow to understand how it gives humans this uniqueness that sets them apart from other primates.
We are extremely excited about the new discovery of FOXP2 as it opens up new directions in language and disease research.