The ancestry of the long mound has long been a key focus in debates on the origins of monumental and megalithic architectures in western France. Typological schemes and absolute dates have alike been invoked in support of different models of monument development, but with limited success. Recent excavations at Prissé-la-Charrière, a 100-metre long mound in the Poitou-Charentes region, have emphasised the importance of internal structure and the complex process of modification and accretion by which many long mounds achieved their final form and dimensions. Excavations have revealed an early megalithic chamber in a dry-stone rotunda, that was progressively incorporated in a short long mound, then in the 100 m long mound we see today, which contains at least two further chamber tombs. The wide range of monument forms present in western and northern France during the 5th millennium BC suggests that the issue of monument origins must be viewed in a broad inter-regional perspective, within which a number of individual elements could be combined in a variety of different ways. Consideration of seven specific elements, including the shape of the mound, the position and accessibility of the chamber, and the significance of above-ground tomb chambers as opposed to graves or pits leads us to propose a polygenic model for the origins of the long mounds and related monuments of western France.