The greatest coexistence between Neanderthals and early modern Homo sapiens likely occurred in Europe, particularly in regions like the Iberian Peninsula and the Balkans. This extended period of interaction, which lasted for thousands of years, might have contributed to the eventual extinction of Neanderthals.
As both groups shared similar environments and resources, there could have been competition for food and territory, leading to increased pressure on Neanderthals. Additionally, there's evidence of possible interbreeding between Neanderthals and early Homo sapiens, which could have resulted in hybridization, potentially impacting the viability of Neanderthal populations.