Study Sheds Light on What Makes the Human Brain Unique

Scientists at the University of California San Diego School of Medicine have advanced our understanding of what differentiates the human brain by studying human-accelerated regions (HARs) of the genome. HARs are segments that have evolved faster than expected since humans diverged from chimpanzees roughly 5 million years ago, and they are thought to shape traits unique to humans. These regions also show connections to neurodevelopmental disorders such as autism. 

Among these genomic areas, one called HAR123 has attracted attention for its influence on brain development. Unlike a gene, HAR123 functions as a transcriptional enhancer—a regulatory element that controls when, where, and how much certain genes are activated during development. HAR123 specifically promotes growth of neural progenitor cells, which are precursors to the main brain cells: neurons and glial cells. 

Crucially, HAR123 affects the balance between neurons and glial cells produced from these progenitors. This balance is linked to cognitive flexibility, which is the human ability to discard previous knowledge and adapt to new situations—a key component of advanced thinking. The study, published in Science Advances, demonstrated that the human form of HAR123 operates differently than the chimpanzee counterpart in both stem cells and neuronal precursors grown in laboratory settings, providing a molecular basis for some of the evolutionary changes observed in the human brain.

While these findings illuminate how HAR123 contributes to brain development and function, researchers note that further research is necessary to fully decode the molecular mechanisms by which the human-specific version confers neural traits. Understanding the role of HAR123 in greater detail could reveal insights into neurological conditions, including autism, by clarifying how changes in these regulatory regions influence brain formation and behavior.