Sensory stimuli drive the maturation and function of the mammalian nervous 
system in part through the activation of gene expression networks that regulate 
synapse development and plasticity. These networks have primarily been studied 
in mice, and it is not known whether there are species- or clade-specific 
activity-regulated genes that control features of brain development and 
function. Here we use transcriptional profiling of human fetal brain cultures to 
identify an activity-dependent secreted factor, Osteocrin (OSTN), that is 
induced by membrane depolarization of human but not mouse neurons. We find that 
OSTN has been repurposed in primates through the evolutionary acquisition of DNA 
regulatory elements that bind the activity-regulated transcription factor MEF2. 
In addition, we demonstrate that OSTN is expressed in primate neocortex and 
restricts activity-dependent dendritic growth in human neurons. These findings 
suggest that, in response to sensory input, OSTN regulates features of neuronal 
structure and function that are unique to primates.