Human population density and settlement patterns along the Dongola Reach changed dramatically in medieval times. A major factor was the introduction of the saqia water wheel, which along with changing farming practices and new multi-harvest crops contributed to the expansion of the population into areas that had been largely uninhabitable since the Kerma Period. The Kingdom of Nobadia controlled Lower and Middle Nubia between the Third and First Cataracts with a population in the order of 4000 divided into about 40 settlements, with perhaps as many as 20 people per square kilometre towards the south end of the

Batn-el-Hajar (Edwards 2004). The much better agricultural resources along the Dongola Reach will have sustained a much larger population. The Makuria Kingdom's main population centre lay at Old Dongola. Further north, settlements developed the west bank, where there had been limited human activity previously. This new settlement pattern was becoming more and more like the continuous settlement that exists along both banks today. There was also settlement along the western part of the Abu Hamed Reach above the Fourth Cataract. The occupation of the Nile banks expanded and by the early eighth century the two kingdoms unified under the King of Makuria (Edwards 2004).

From about 1500 years ago, humans were progressively creating conditions that were likely to facilitate expansion of mosquito populations along the length of the Dongola Reach, whereas for the previous 1000 years it's likely that any population would have been small and isolated, with little or no scope for recolonization from neighbouring human settlements or from outside the region. It would seem more likely that a species such as An. arabiensis, which is less well adapted to survival alongside humans than for example Culex quinquefasciatus Say, would have disappeared altogether. For perhaps the past 1700 years the only route into the Dongola Reach for mosquitoes was downriver and this remains the situation today, with the possible exception of passive transport facilitated by the railway to Wadi Halfa and Kareima, completed in the early twentieth century, and modern day transport.

In medieval times, the western Abu Hamed Reach was extensively occupied, which would have increased the likelihood of mosquito reinvasion downriver. Therefore whether or not An. arabiensis had disappeared previously, it was almost certainly present along the Dongola Reach for most of the last 1500 years. The biggest setbacks for both humans and mosquitoes during this time were mainly associated with floods and droughts, although major conflicts and social change were still to occur. The environmental history of the area is relatively poor for the first millennium AD, but there are records of several periods of a decade or so, when the Nile level was very low, other periods of high Nile (Adams 2001), and some years when ice formed on the Nile in Egypt, particularly in the ninth and tenth centuries. From 1300 to 1522 the Nile floods were generally good. After this and up to part way through the eighteenth century Nile levels and humidity were higher, but there were some droughts affecting the whole of Sudan in the late seventeenth century. In the eighteenth and nineteenth centuries the climate in general became more arid bringing more droughts, but interspersed with some heavy and destructive floods, with a very wet period in the 1790s (Nicholson 1978, Edwards 2004).

The data available over the last 500 years is quite detailed and could be used to make a more in-depth analysis of the likely impact on mosquito populations than the superficial account presented here. Nevertheless, it is clear that as with any mosquito at the edge of its species distribution, numbers will have fluctuated dramatically and the population structure will have been disrupted with successive contractions and expansions along parts of the Nile within Northern State, but it does not appear likely to have been eradicated from the area altogether at any point. The human population will have endured famine and conflict associated with major floods and droughts, and other major upheavals caused by political changes such as the expansion of the Funj Sultanate as far north as the Third Cataract and the expansion of the Ottoman Empire south (Edwards 2004). Nevertheless, the contiguous human settlement pattern along the Dongola Reach was maintained and became progressively more continuous, facilitating a more extended and presumably more homogenous mosquito population.

A more difficult question is the extent to which the Dongola Reach An. arabiensis population was isolated. The key is the Fourth Cataract and the western end of the Abu Hamed Reach, which was densely populated during the Kerma Period around 2500-1500

BC, and again in the post-Meroitic and medieval periods, between AD 350-1500. The population levels in the period in between are uncertain as work in the area is still underway. Continuity of occupation up to the present day is probable. The absence of human settlement would be a strong indication of a period of isolation. Human occupation does not however necessarily mean that mosquito migration was possible, since amongst other factors, the nature and size of the settlement and the prevailing wind and Nile level will all affect the likelihood. This again merits a more detailed analysis, but will have to await the results from the current Merowe Dam archaeological salvage project.

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