The specifics of the caste systems in Africa vary among the ethnic groups. Some societies have a rigid and strict caste system with embedded slavery, whereas others are more diffuse and complex. Countries in Africa that have societies with caste systems include Mali, Mauritania, Senegal, Gambia, Guinea, Ivory Coast, Niger, Burkina Faso, Cameroon, Liberia, Sudan, Sierra Leone, Algeria, Nigeria, Chad, Ethiopia, Somalia, Djibouti, Eritrea and others. While it is unclear when and how the caste systems developed in Africa, they are not ancient and likely developed sometime between the 9th century and 15th century in various ethnic groups, probably in conjunction with the institution of slavery.