The human cerebral cortex controls complex cognitive behaviors. During mammalian evolution, the number of neurons increased in many lineages, requiring a larger cortical surface area to fit into a skull that did not scale proportionally. This space problem was solved by cortical folding, resulting in gyrencephalic (folded) brains. While several hypotheses have been proposed to explain cortical gyrification, we lack mechanistic insights to understand the process itself and in particular its underlying genomic changes, that lead to the appearance of cortical folds. In this issue of The EMBO Journal, de Juan Romero et al (2015) tackle the question using a transcriptomics approach to identify gene expression changes in the developing ferret brain prior to the onset of gyrification. During mammalian evolution, the size of the cerebral cortex increased leading to the formation of cortical folds. Large-scale transcriptome analysis provides new insight into the genetic regulation of cortical folding in the developing ferret brain. © 2015 The Authors.