The immediate origins of the blackhouse are unclear as few pre-eighteenth century examples have ever been excavated. One reason for this is that, unlike their later counterparts, the early examples may have been made of turf and thatch and quickly returned to the earth once abandoned. As one of the most primitive forms of the North Atlantic longhouse tradition it is very probable that the roots of the blackhouse, in which cattle and humans shared the same roof, is well over 1000 years old. The Lewis examples have clearly been modified to survive in the tough environment of the Outer Hebrides. Low rounded roofs, elaborately roped, were developed to resist the strong Atlantic winds and thick walls to provide insulation and to support the sideways forces of the short driftwood roof timbers.