The first Spaniards and Portuguese explorers in the Americas initially enslaved Amerindian populations. Sometimes this labor was available through existing Native American states that fell under the control of invading Europeans; in other cases, Native American states provided the labor force. In the case of the Portuguese, the weakness of the political systems of the Tupi-Guarani Amerindian groups they conquered on the Brazilian coastline, and the inexperience of these Amerindians with systematic peasant labor, made them easy to exploit through non-coercive labor arrangements. However, several factors prevented the system of Amerindian slavery from being sustained in Brazil. For example, Native American populations were not numerous or accessible enough to meet all demands of the settlers for labor. In many cases, exposure to European diseases caused high fatalities among the Amerindian population, to such an extent that workers became scarce. Historians estimate that about 30,000 Amerindians under the rule of the Portuguese died in a smallpox epidemic in the 1560s. The Iberian conquerors could not attract sufficient settlers from their own countries to the colonies and, after 1570, they began increasingly to bring enslaved people who had been kidnapped in Africa as a primary labor force.