The neocortex, the largest part of our brain, enables us to speak, dream and think. During human evolution, the size of the neocortex has increased dramatically. To understand what caused this expansion, researchers at the Max Planck Institute of Molecular Cell Biology and Genetics (MPI-CBG) in Dresden have focused in previous studies on a gene called ARHGAP11B that is only found in humans and triggers an increased production of brain stem cells – a prerequisite for a larger brain. Now, studying different strains of mice, the same group has identified another, very different cause of neocortex expansion: the longer the gestation period (the time between conception and birth) of a species, the longer is the period during which neocortical neurons are generated (the neurogenic period), and the greater is the number of the so-called upper-layer neurons, a hallmark of neocortex expansion. The researchers also found that during pregnancy, the maternal environment has a dominant influence on the length of the neurogenic period and hence on the numbers of upper-layer neurons produced. They have thus uncovered a hitherto unknown link between the maternal environment and embryonic cortical neurogenesis during pregnancy.