Archaeological remains show that the first group of modern people to live in the British Isles were hunter-gatherers after the last ice age ended. The date is not known: perhaps as early as 8000 but certainly by 5000. They built mesolithic wood and stone monuments. Stonehenge was built between 3000 and 1600. Celtic tribes arrived from mainland Europe. Britain was a changing collection of tribal areas, with no overall leader. Julius Caesar tried to invade (take over) the island in 55 but was not able to do so. The Romans successfully invaded in 43.