have been found in this area. Some settlements had massive defensive walls, while others were less fortified. In a now arid environment where arable land and pasturage were once at a premium, the population grew. Relatively large-scale political organizations emerged, leading to the development of military hierarchical aristocracies. The agro-pastoral society had a mixed farming economy of millet production combined with the rearing of livestock. They had learned how to work with copper. They traded in jewelry and semi-precious stones from distant parts of the Sahara and Sahel. They are believed to be the first to domesticate African rice. An archaeologist described their ancient, abandoned sites as representing "a great wealth of rather spectacular prehistoric ruins". A series of early cities and towns were created by Mande peoples, also related to the Soninke, along the middle Niger River in Mali, including at Dia, beginning from around 900 BC, and reaching its peak around 600 BC, and later at Djenné-Djenno, which was occupied from around 250 B.C to around 800 AD. Djenné-Djenno comprised an urban complex consisting of 40 mounds within a 4 kilometer radius. The site is believed to exceed 33 hectares (82 acres), and the town engaged in both local and long-distance trade During Djenné-Djenno's second phase (during the first millennium AD) the borders of the site expanded during (possibly covering 100,000 square meters or more), also coinciding with the development at the site of a kind of permanent mud brick architecture, including a city wall, probably built during the latter half of the first millennium AD using the cylindrical brick technology, "which was 3.7 meters wide at its base and ran almost two kilometers around the town". who are bards, storytellers, and oral historians. Religion Many of the Mandé speaking ethnic groups in the westernmost part of West Africa have been predominantly Muslim since as early as the 13th century. Others, such as the Bambara a Mandinka group, converted to Islam as late as the 19th century with some retaining their traditional beliefs. Muslim Mandinka also hold traditional beliefs, such as in the rituals of initiation groups like Chiwara, and Dwo, and beliefs in the power of nyama (a spiritual power existing in nature). Many smaller Mande speaking ethnic groups, such as the Bobo, retain pre-Islamic belief systems in their entirety. Many Mande-speaking groups in Sierra Leone and Liberia were also, for the most part, not islamized. According