Even though humans are closely related to chimps, the slight differences in human and chimp HAR123 suggest something unique in the human version. What is it that makes the human brain distinctly human? What makes it stand out from the brains of other creatures, and even our closest living primate relatives? The human and chimp version of HAR123 are orthologs—genes that are passed on to different species that arise from a common ancestor. Chimp HAR123 does not lead to the differentiation of as many cells as human HAR123.