Our knowledge of early Nigerien history is limited by the lack of written sources, though it is known that by at least the 5th century BC the territory of modern Niger had become an area of trans-Saharan trade. Led by Tuareg tribes from the north, camels were used as a well-adapted means of transportation through what was now an immense desert. This mobility, which would continue in waves for several centuries, was accompanied with further migration to the south and intermixing between sub-Saharan African and North African populations, as well as the gradual spread of Islam. It was also aided by the Arab invasion of North Africa at the end of the 7th century, which resulted in population movements to the south. Several empires and kingdoms flourished in the Sahel during this era. Their history does not fit easily within the modern boundaries of Niger, which were created during the period of European colonialism; the following adopts a roughly chronological account of the main empires.