"Probably one of the most interesting questions in neuroscience is, 'What makes us human?'" says Christopher Walsh of Harvard University and the Allen Discovery Center for Human Brain Evolution. "Specifically, what is it about the human brain that differentiates it from those of other closely related species? Looking at human accelerated regions provided us with a very targeted way to investigate that question from a genetic perspective."

To systematically identify which of the 3,171 previously identified HARs are most likely to be contributing to recent evolution of the human cerebral cortex, the researchers examined the role of these regions in regulating genes in studies of multiple human and mouse cell types and tissues.

"We knew going into this study that many HARs were likely to function as regulators of gene expression in the brain, but we knew very little about which cell types in the brain they worked in, where, or at what time in the human lifespan," explains Ellen DeGennaro, one of the study's first authors in the Walsh lab. "Our goal was to fill in these gaps of knowledge about which HARs had important roles in the brain, and how, so that we and other researchers could take the most important 'brain HARs' and perform deeper tests of their evolutionary function."

To overcome the limitations of earlier methods, Walsh and his colleagues developed an applied approach called CaptureMPRA. The new method leverages barcoded molecular inversion probes to capture target sequences that capture entire HAR elements and their surrounding DNA, overcoming some limitations of prior techniques. Using this approach, they looked for important differences in HAR enhancer function between humans and chimpanzees.