The High Middle Ages was a period of tremendous expansion of population. The estimated population of Europe grew from 35 to 80 million between 1000 and 1347, although the exact causes remain unclear: improved agricultural techniques, the decline of slaveholding, a more clement climate and the lack of invasion have all been suggested. As much as 90 per cent of the European population remained rural peasants. Many were no longer settled in isolated farms but had gathered into small communities, usually known as manors or villages. These peasants were often subject to noble overlords and owed them rents and other services, in a system known as manorialism. There remained a few free peasants throughout this period and beyond, with more of them in the regions of Southern Europe than in the north. The practice of assarting, or bringing new lands into production by offering incentives to the peasants who settled them, also contributed to the expansion of population.