3.8 million year old remains of Australopithecus afarensis—Louis Leakey had found what he thought was a baboon tooth at Laetoli in 1935 (which was not identified as afarensis until 1979), a fragment of hominid jaw with three teeth was found there by Kohl-Larsen in 1938–39, and in 1974–75 Mary Leakey recovered 42 teeth and several jawbones from the site. Middle Stone Age Mumba Cave in northern Tanzania includes a Middle Stone Age (MSA) to Later Stone Age (LSA) archaeological sequence. The MSA represents the time period in Africa during which many archaeologists see the origins of modern human behavior. Later Stone Age and Pastoral Neolithic Reaching back approximately 10,000 years in the Later Stone Age, Tanzania is believed to have been populated by hunter-gatherer communities, probably Khoisan-speaking people. Between approximately 4,000 to 3,000 years ago, during a time period known as the Pastoral Neolithic, pastoralists who relied on cattle, sheep, goats, and donkeys came into Tanzania from the north. Two archaeological cultures are known from this time period, the Savanna Pastoral Neolithic (whose peoples may have spoken a Southern Cushitic language) and the Elmenteitan (whose peoples may have spoken a Southern Nilotic language). Luxmanda is the largest and southernmost-known Pastoral Neolithic site in Tanzania. Iron Age Approximately 2000 years ago, Bantu-speaking people began to arrive from western Africa in a series of migrations collectively referred to as the Bantu expansion. These groups brought and developed ironworking skills, agriculture, and new ideas of social and political organization. They absorbed many of the Cushitic peoples who had preceded them, as well as most of the remaining Khoisan-speaking inhabitants. Later, Nilotic pastoralists arrived, and continued to immigrate into the area through to the eighteenth century. One of Tanzania's most important Iron Age archeological sites is Engaruka in the Great Rift Valley, which includes an irrigation and cultivation system. Early coastal history Travellers and merchants from the Persian Gulf and Western India have visited the East African coast since early in the first millennium CE. Greek texts such as the Periplus of the Erythraean Sea and Ptolemy's Geography list a string of market places (emporia) along the coast. Finds of Roman-era coins along the coast confirm the existence of trade, and Ptolomey's Geography refers to a town of Rhapta as "metropolis" of a political entity called Azania. Archaeologists have not yet succeeded in identifying the location of Rhapta, although many believe it lies deeply