The human brain is the result of a long evolution that has shaped its development and built its complexity. So are human synapses, the brain’s smallest subcellular units. The goal of this project is to understand the mechanisms underlying synaptic development and evolution, and determine how the humanization of synapses impacts neuronal plasticity. We have previously shown that the human-specific duplications of SRGAP2 (Slit-Robo Rho GTPase-Activating Protein 2), which occurred during the transition between Australopithecus and Homo, have contributed to the evolution of excitatory and inhibitory synapses by protracting their maturation and by increasing their density. These effects rely on the inhibition of the ancestral protein SRGAP2A by its human-specific copy SRGAP2C, a framework that enables for the first time to tackle evolutionary questions at the cellular and molecular levels. Here, we will use complementary approaches in mouse neurons and human neurons derived from induced-pluripotent stem cells to determine the role of SRGAP2 paralogs and the specificities of human neurons. We will first characterize SRGAP2A partners at synapses to identify molecular networks that have been modified in humans. We will then investigate the role of these networks in synaptic development and plasticity. In vivo single cell manipulations and live-cell super-resolution imaging will play a key role to dissect mechanisms in intact cortical circuits and access the chemistry of molecular interactions in synaptic machineries. In a context where human neurodevelopmental disorders are commonly studied using mouse models, the expected results will provide decisive and comparative insights into synaptic physiology and pave the way for future research on human brain evolution and diseases.