The term used to denote the islands of Great Britain from the time of the earliest human presence, about 700,000 years ago, to the arrival of the Anglo-Saxons in the 5th and 6th centuries. During that time enormous changes took place. The most distinctive monument of the early Neolithic period is the long barrow, a rectangular mound of earth or rubble covering a burial place. These funerary monuments served entire communities. It is possible that they were associated with the worship of ancestors. By around 2500 B.C. new types of monuments had begun to emerge in Britain. Most important is the henge monument, a circular, banked earthwork constructed for ceremonial purposes. The best-known henges are Avebury and Stonehenge, but nearly 100 such sites have been found in Britain. The Celtic-speaking tribes and clans that occupied Britain during the Iron Age slowly gave way to kingdoms ruled by powerful dynasties that included queens as well as kings.