Slavery is rare among hunter-gatherer populations, because it is developed as a system of social stratification. Slavery was known in the very first civilizations such as Sumer in Mesopotamia which dates back as far as 3500 BC.  The Byzantine–Ottoman wars and the Ottoman wars in Europe resulted in the taking of large numbers of Christian slaves. Slavery became common within much of Europe during the Dark Ages and it continued into the Middle Ages. The Dutch, French, Spanish, Portuguese, British, Arabs and a number of West African kingdoms played a prominent role in the Atlantic slave trade, especially after 1600. David P. Forsythe wrote: "The fact remained that at the beginning of the nineteenth century an estimated three-quarters of all people alive were trapped in bondage against their will either in some form of slavery or serfdom. " The Republic of Dubrovnik was the first European country to ban the slave trade in 1416, and in modern times Denmark-Norway in 1802.