Whenever the warm interglacial periods ended and the climate cooled again, the Neanderthals already in central or northern Eurasia would perhaps have moved down to southern Eurasia in greater numbers in search of slightly warmer climes, putting pressure on the newly arrived Homo sapiens there. It is thus possible that the climate cycles and the presence of Neanderthals together are what scuppered the attempts of the first modern humans to colonize the rest of the world from Africa through the Levant.
