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The Kushan Empire, a collection of Yuezhi tribes, took control of the region in the first century CE and ruled until the 4th century CE during which time Buddhism, Nestorian Christianity, Zoroastrianism, and Manichaeism were all practiced in the region. Later the Hephthalite Empire, a collection of nomadic tribes, moved into the region and Arabs brought Islam in the early eighth century. Central Asia continued in its role as a commercial crossroads, linking China, the steppes to the north, and the Islamic heartland.

What empire brought Islam practices with them?
Answer: Hephthalite Empire,
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Around 1200 BC, the Dorians, another Greek-speaking people, followed from Epirus. Traditionally, historians have believed that the Dorian invasion caused the collapse of the Mycenaean civilization, but it is likely the main attack was made by seafaring raiders (sea peoples) who sailed into the eastern Mediterranean around 1180 BC. The Dorian invasion was followed by a poorly attested period of migrations, appropriately called the Greek Dark Ages, but by 800 BC the landscape of Archaic and Classical Greece was discernible.

What group of people came with others who left the geographical and historical region in southeastern Europe, now a mutual region between Greece and Albania ?
Answer: the Dorians
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A number of studies have reported associations between pathogen load in an area and human behavior. Higher pathogen load is associated with decreased size of ethnic and religious groups in an area. This may be due high pathogen load favoring avoidance of other groups, which may reduce pathogen transmission, or a high pathogen load preventing the creation of large settlements and armies that enforce a common culture. Higher pathogen load is also associated with more restricted sexual behavior, which may reduce pathogen transmission. It also associated with higher preferences for health and attractiveness in mates. Higher fertility rates and shorter or less parental care per child is another association that may be a compensation for the higher mortality rate. There is also an association with polygyny which may be due to higher pathogen load, making selecting males with a high genetic resistance increasingly important. Higher pathogen load is also associated with more collectivism and less individualism, which may limit contacts with outside groups and infections. There are alternative explanations for at least some of the associations although some of these explanations may in turn ultimately be due to pathogen load. Thus, polygny may also be due to a lower male:female ratio in these areas but this may ultimately be due to male infants having increased mortality from infectious diseases. Another example is that poor socioeconomic factors may ultimately in part be due to high pathogen load preventing economic development.

What does avoidance of other groups reduce?
Answer:
pathogen transmission