is similar to what happened in the Americas. The continued use of Stone Age tools in Australia has been much debated. Dispersal throughout Eurasia The population brought to South Asia by coastal migration appears to have remained there for some time, during roughly 60,000 to 50,000 years ago, before spreading further throughout Eurasia. This dispersal of early humans, at the beginning of the Upper Paleolithic, gave rise to the major population groups of the Old World and the Americas. Towards the West, Upper Paleolithic populations associated with mitochondrial haplogroup R and its derivatives, spread throughout Asia and Europe, with a back-migration of M1 to North Africa and the Horn of Africa several millennia ago. Presence in Europe is certain after 40,000 years ago, possibly as early as 43,000 years ago, rapidly replacing the Neanderthal population. Contemporary Europeans have Neanderthal ancestry, but it seems likely that substantial interbreeding with Neanderthals ceased before 47,000 years ago, i.e. took place before modern humans entered Europe. There is evidence from mitochondrial DNA that modern humans have passed through at least one genetic bottleneck, in which genome diversity was drastically reduced. Henry Harpending has proposed that humans spread from a geographically restricted area about 100,000 years ago, the passage through the geographic bottleneck and then with a dramatic growth amongst geographically dispersed populations about 50,000 years ago, beginning first in Africa and thence spreading elsewhere. Climatological and geological evidence suggests evidence for the bottleneck. The explosion of Toba, the largest volcanic eruption of the Quaternary, may have created a 1,000 year cold period, potentially reducing human populations to a few tropical refugia. It has been estimated that as few as 15,000 humans survived. In such circumstances genetic drift and founder effects may have been maximised. The greater diversity amongst African genomes may reflect the extent of African refugia during the Toba incident. However, a recent review highlights that the single-source hypothesis of non-African populations is less consistent with ancient DNA analysis than multiple sources with genetic mixing across Eurasia. Europe The recent expansion of anatomically modern humans reached Europe around 40,000 years ago from Central Asia and the Middle East, as a result of cultural adaption to big game hunting of sub-glacial steppe fauna. Neanderthals were present both in the Middle East and in Europe, and the arriving populations of anatomically modern humans (also known as "Cro-Magnon" or European early modern humans) interbred with Neanderthal