Modern human settlement of what is now Rwanda dates from, at the latest, the last glacial period, either in the Neolithic period around 8000 BC, or in the long humid period which followed, up to around 3000 BC. Archaeological excavations have revealed evidence of sparse settlement by hunter-gatherers in the late Stone Age, followed by a larger population of early Iron Age settlers, who produced dimpled pottery and iron tools. These early inhabitants were the ancestors of the Twa, aboriginal pygmy hunter-gatherers who remain in Rwanda today. Between 700 BC and 1500 AD, a number of Bantu groups migrated into Rwanda, clearing forest land for agriculture. The forest-dwelling Twa lost much of their habitat and moved to the mountain slopes. Historians have several theories regarding the nature of the Bantu migrations; one theory is that the first settlers were Hutu, while the Tutsi migrated later to form a distinct racial group, possibly of Nilo-hamitic origin. An alternative theory is that the migration was slow and steady, with incoming groups integrating into rather than conquering the existing society. Under this theory, the Hutu and Tutsi distinction arose later and was a class distinction rather than a racial one.