<P> 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 Lake Toba created a 1,000 year cold period, as a result of the largest volcanic eruption of the Quaternary, 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 be in part due to the greater prevalence of African refugia during the Toba incident . However, a recent review highlights that the single - source hypothesis of non-African populations is less supported by ancient DNA analysis than multiple sources plus genetic mixing across Eurasia . </P> <P> 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) have interbred with Neanderthal populations to a limited degree . Populations of modern humans and Neanderthal overlapped in various regions such as in Iberian peninsula and in the Middle East . Interbreeding may have contributed Neanderthal genes to palaeolithic and ultimately modern Eurasians and Oceanians . </P> <P> An important difference between Europe and other parts of the inhabited world was the northern latitude . Archaeological evidence suggests humans, whether Neanderthal or Cro - Magnon, reached sites in Arctic Russia by 40,000 years ago . </P> <P> Cro - Magnon are considered the first anatomically modern humans in Europe . They entered Eurasia by the Zagros Mountains (near present - day Iran and eastern Turkey) around 50,000 years ago, with one group rapidly settling coastal areas around the Indian Ocean and one group migrating north to steppes of Central Asia . Modern human remains dating to 43 - 45,000 years ago have been discovered in Italy and Britain, with the remains found of those that reached the European Russian Arctic 40,000 years ago . </P>

What continent is to the left of africa