Some researchers have attempted to explain this final stage as the arrival of new populations of Semitic origin (the future Akkadians), but there is no conclusive proof of this. In Lower Mesopotamia, the researchers identify this as the Jemdet Nasr period, which sees a shift to more concentrated habitation, undoubtedly accompanied by a reorganisation of power; in southwestern Iran, it is the Proto-Elamite period; Niniveh V in Upper Mesopotamia (which follows the Gawra culture); the "Scarlet Ware" culture in Diyala. In Lower Mesopotamia, the Early Dynastic Period begins around the start of the 3rd millennium BC, during which this region again exerts considerable influence over its neighbours. Lower Mesopotamia is the core of the Uruk period culture and the region seems to have been the cultural centre of the time, since this is where the principle monuments are found and the most obvious traces of an urban society with state institutions developing in the second half of the 4th millennium BC, the first system of writing, and it is the material and symbolic culture of this region which had the most influence on the rest of the Near East at this time. However, this region is not well-known archaeologically, since only the site of Uruk itself has provided traces of monumental architecture and administrative documents which justify seeing this region as the most dynamic and influential. At some other sites, construction from this period has been found, but they are usually known only as a result of soundages. In the current state of knowledge it remains impossible to determine whether the site of Uruk was actually unique in this region or if it is simply an accident of excavation that makes it seem more important than the others. This is the region of the Near East that was the most agriculturally productive, as a result of an irrigation system which developed in the 4th millennium BC and focussed on the cultivation of barley (along with the date palm and various other fruits and legumes) and the pasturing of sheep for their wool. Although it lacked mineral resources and was located in an arid area, it had undeniable geographic and environmental advantages: it consisted of a vast delta, a flat region transected by waterways, resulting in a potentially vast area of cultivatable land, over which communications by river or land were easy. It may also have become a highly populated and urbanised region in the 4th millennium BC, with a social hierarchy, artisanal activities, and long-distance commerce.