Modern Saimiri origin was the south-central Amazon, in Rondônia and Inambari centers. The Amazon River has limited squirrel monkey dispersal over the last 1 million years. Pleistocene river formation and climate fluctuations affected Saimiri speciation. Late Pleistocene glacial cycles and human intervention have caused secondary contact. Saimiri mitochondrial DNA phylogeny is discordant with Gothic vs. Roman division.