Our understanding of the artistic accomplishments of this period derives from a limited number of stone remains and works in metal and terracotta. Entering the second half of the first millennium A.D., Aksum remaines a powerful Christian kingdom in northern Africa. When the kings convertes to Christianity in the fourth century, Aksum becomes linked to Byzantine Egypt. Trade extends to Alexandria to the north and beyond the Nile River to the south. By the close of the sixth century, Persian invaders have undermined Aksumite ascendancy; however, Christianity remains entrenched throughout the region. The shifting control of trade has a profound influence further south as well. Arab traders along the eastern coast of Africa learn local African languages and introduce Islam; the synthesis of African and Arab cultural elements produce the dynamic Swahili culture (Swahili comes from the Arabic sahel, meaning coast). Swahili trade extends from present-day Kenya and Tanzania at least as far south as Mapungubwe, contributing to the accumulation of wealth and the growth of city-states in what is now Zimbabwe. Also dating to this period, but at the far southern end of the continent, is a series of terracotta heads. These works are the earliest figurative sculpture on record from southern Africa.