Changes in rainfall over North Africa in the mid-Holocene had a drastic impact on life in the Sahara. The rock and cave paintings in the central Sahara, the best known of which are the 'Tassili Frescoes', depict a lost world of a land teeming with life and supporting vibrant pastoral economy.5 These images of antelope, elephant, giraffe, hippopotamus, ostrich, rhinoceros and pastoral lifestyle seem impossible when viewed against the surrounding superarid desert (Fig. 5.4). The climatic record is, however, unequivocal: for most of the time between around 14 kya and 5 kya the Sahara experienced a monsoonal climate (see Fig. 2.11). The region had considerably greater rainfall than now and much of the land had permanent vegetation. The suddenness of the changes, both in the onset of the wetter conditions during the Bolling/Allerod period, and similarly the rapid onset of drier conditions in the mid-Holocene, are a fascinating example of the non-linearity of the climate (see Section 2.11). They also provide an interesting example of the changes humans had to adapt to during the supposedly stable climate of the Holocene, especially in coming to terms with the desiccation of the Sahara.
Following the Younger Dryas, when the general cooling of the climate around the North Atlantic brought drier conditions to the Sahara, the summer monsoon spread much farther north. From the Horn of Africa in the east to Mauritania in the west vegetation zones moved northwards with the summer rains. Savannah woodland and open dry forests grew in regions that are now hyperarid desert.6 By 8 kya Lake Chad covered a vast central area of some 330 000 km2 (Fig. 5.5) To the east of the lake there were savannah grasslands that were periodically inundated. Rivers ran out of the central mountain areas of the Hoggar, Tibetsi and Darfur to fill this vast lake. Rivers also ran eastwards along what are now Wadi Howar and Wadi Melik to form tributaries of the Nile, and southwards into the Niger. Elsewhere large lakes developed, as can easily be seen on the ground by their characteristic sedimentary deposits and fossil shorelines. The
5 See the Tassili frescoes website http://www.paleologos.com/saharaan.htm.
6 The data presented here draw on the extensive work done by the University of Mannheim on palaeovegetation maps of Africa, details of which can be found on http://www. uni-mannheim.de/phygeo/palaeo.htm.
impressive river systems that drained into these lakes have been detected beneath the shifting sands by ground surveys and mapped by radar imagery from space (Fig. 5.6). These rivers not only drained eastwards into the Nile, but also flowed northwards and discharged directly into the Mediterranean.
It appears that the first incursions of nomads into the southern Sahara, which came from the south, did not take place until around 12kya. At around the same time migrants may also have entered the northern Sahara from the Mediterranean coast. There is evidence of their presence by 11.5 kya in the Acacus Mountains of Libyan Sahara. What is even more interesting is that between 9 and 8 kya these people had developed a hunting strategy that involved the capture, penning and feeding of Barbary sheep to manage their food supplies more efficiently (di Lernia, 2001). These early arrivals were reinforced by a new influx from the east around 8 kya. This wave of migration appears to have brought a pastoral economy to the region with the introduction of domesticated animals.
Attempts to use genetic mapping to shed light on migrations across the Sahara have focused on movements up and down the Nile Valley. These show that such movements have a complicated history. The basic observation is that the prevalence of a genetic marker associated with populations in sub-Saharan Africa (see Section 3.8)
figure 5.6. Satellite radar image of part of the Sahara showing the extensive drainage system that existed in the early Holocene, superimposed on a photographic image of the surrounding terrain, which is now covered by drifting sand.
falls from a high level in southern Sudan to a much lower level in Egypt (Krings et al., 1999). Furthermore, studies of the remains of ancient Nubians show that the proportion of this marker was the same then as it is for people now living in the area. This indicates that migrations had occurred in both directions along the Nile, which is consistent with historical evidence for long-term interactions between Egypt, Nubia and farther south. The bulk of these migrations probably occurred within recorded history and the flow from north to south was either earlier or less extensive than that from south to north.
There is no doubt about the sudden decline in rainfall across the Sahara. Around 5.5 kya deep-sea sediment records off the west coast of North Africa show a sudden increase in the amount of dust being transported in the winds from the east (see Fig. 2.11) (deMenocal et al., 2000). At the drop of a hat the climate shifted to a drier form and the desert began to take over. A survey of conditions around 5 kya shows a general drying, although the distribution and timing of these changes show interesting regional variations. In particular, the eastern Sahara aridification, which may have started as early as 6.5 kya, was already well advanced with the Libyan desert extending southwards into what is now Sudan. Only in favoured wadis did the savannah remain a feature of regions that are now desert.
Computer model studies of the early and mid-Holocene can produce simulations that reflect this pattern of changes. Temperatures were slightly lower than at present, in part because of increased cloud cover and more humid conditions. The models show increased monsoonal precipitation in the eastern Sahara and East Africa (Kutzbach & Liu, 1997; Bonfils et al., 2001). Simulations of vegetation cover show an upward shift of montane vegetation belts, fragmentation of drought-tolerant vegetation in southern Africa, and a major northward shift of the southern margin of the eastern Sahara. As noted above the existence of large lakes confirms the prediction of wetter conditions across northern Africa. The lake and pollen data also confirm the northward expansion of the monsoon in eastern Africa, but the model is unable to simulate the wet conditions in western Africa.
As for human activities, the early decline in rainfall in the eastern Sahara may have led to the oldest-known astronomical alignment of megaliths in the world (Wendorf & Schild, 1998). Consisting of a set of huge stone slabs in the desert of southern Egypt, known as Nabta, it forms a stone circle, a series of flat, tomb-like stone structures and five lines of standing and toppled megaliths, and is dated at around 6.5 to 6 kya. The stone slabs, some of which are 3 m high, were dragged more than a kilometre to the site. They lie on the shoreline of an ancient lake, which was used by nomads for grazing their cattle during the summer rainy season. To judge from carbon dating of charcoal and ostrich shells, occupation may have started as early as 12 kya. Initially the settlements at Nabta were small seasonal camps of cattle-herding and ceramic-using people. These cattle are regarded as the first example of the African pattern of herding. Cattle served as a 'walking larder' and provided milk and blood, rather than meat, as they do to this day for the Masai in Kenya. The site was intensively exploited by around 9 kya. This included the digging of walk-in wells that suggest the site could have been occupied throughout the year. Nomads used the area until about 5.6 kya, at which time it became hyperarid and uninhabitable.
Nabta is close to the Tropic of Cancer, and so the noon sun is vertically overhead around the summer solstice. The vertical sighting stones in the circle form a calender system, when combined with an east-west alignment between one megalithic structure and two stone megaliths over a kilometre distant, and two other geometric lines involving about a dozen additional stone monuments that lead both northeast and southeast from the same megalith. The timing of the construction of the array coincides with the onset of increasingly erratic rainfall patterns. It is possible that the Nabta people were reacting to these changes by using the array to measure the summer solstice and predict the onset of the rainy season. The megalith may have been part of a variety of ritual activities as there were a number of tumuli containing the bones of cattle on the site.
There may be a link between people at Nabta and the Neolithic people living along the Nile. This could have contributed to the rise of social complexity in ancient Egypt. Clearly the people at Nabta had a structured society that enabled them to plan the arrangement of their villages, the excavation of large, deep wells, and the construction of complex stone structures made of large shaped and unshaped stones. Other Nabta features, which subsequently appear suddenly and without evident local antecedents in the late Predynastic and early Old Kingdom in the Nile Valley, are the role of cattle to express differences of wealth, power and authority, the emphasis on cattle in religious beliefs, and the use of astronomical knowledge and devices to predict solar events. Many of these had been well developed for a long time at Nabta.
It may be that as the desert became more and more inhospitable the crocodile-infested swamps of the Nile grew more attractive. Indeed, as the monsoon ceased to spread so far north the flow of the lower Nile may have abated somewhat. Whatever the changes, it is an interesting fact that the earliest records of Egyptian history relate to the period around 5.15 kya (3150 BC) that is only a century or two after the Sahara started its descent into the formidable desert we now know.7 So the legend of the Scorpion King in Egyptian prehistory may be a tribal memory of an earlier pastoral savannah existence.
More relevant here, these changes are a natural bridge between the impact of climate change on prehistory and the recorded histories of the ancient civilisations. Saharan desiccation could have led to the rise of Dynastic civilisation in Egypt. Perhaps the nomads, who built the megalithic structure at Nabta, brought their more complex cosmology to enrich the culture of the Nile. More directly, population pressures in the south, and the collapse of the pastoralism, could have led to a move to agriculture as people that had previously practised seasonal migration settled permanently in the Nile Valley. This move around 5.5 kya may have stimulated both technological innovation and social stratification in Predynastic Upper Egypt. In time, this may have been the stimulus for the unification of Upper and Lower Egypt around 5 kya and the creation of the Old Kingdom.
In the central and western Sahara the tale of desiccation emerges somewhat later. The record from the deposits formed in the Kajemarum Oasis, in the Manga grasslands of northeastern Nigeria, spanning the last 5.5 kya, reveals the episodic deterioration of the climate in the region to the south of the Sahara (the Sahel) (Street-
7 Up to this point all dates have been expressed in units of kya (thousand years ago), but once we get to dates that are more commonly expressed in terms of BC and AD, it is easier for the reader either to have both forms presented or whichever form appears to be the most appropriate.
Perrott et al., 2000). Desert-dust deposition began to increase around 4.7 kya. Rainfall during the summer-monsoon season declined permanently after 4.1 kya. While the presence of groundwater maintained vegetation in some wadis for a considerable period of time, the inexorable desiccation changed the landscape. Farther west, however, there is evidence that the moister conditions continued longer with only a slow transition towards current conditions.
These changes in the Sahara did not mean that all human activities came to an abrupt end. For example, the Wadi Tanezzuft, which originates in the Tassili and Acacus mountains before disappearing 300 km to the north beneath the dunes of the Ubari erg, had abundant water and fed several lakes along its course. Pastoral communities intensively occupied it during the mid-Holocene. With the coming of the drought, the local consequence was a decline in the wet areas. Lakes and swamps between the sand dunes of the ergs dried up quickly, but the main course of the Wadi was still active between 3 and 2 kya. The increasing aridity is, however, evident from the progressive retreat of archaeological sites to the south.
The onset of the present superarid conditions did not occur until around 1.5 kya. Before this final decline came the blossoming of the kingdom of the Garamantes that supplied salt to West Africa (Cremaschi & di Lernia, 1999). This kingdom in the oasis of the Wadi Tanezzuft appears to have benefited from a brief period of increased rainfall between 2.45 and 2.25 kya. Information about precipitation changes has been obtained from a dendroclimatological study of wood from trees of the Cypress family (Cupressus dupreziana) which had been imported from the Wadi Tanezzuft and used to make house doors in the old cities of Ghat and Barkat, Libya.
A progressive decline of rainfall from 2.25 kya corresponds to a period when the Garamantian kingdom tried to react against increased aridity by intensifying horticulture, establishing walled citadels and developing the long-distance trade across the Sahara using camels. This delayed the final collapse of the kingdom until around 1.6 kya. The onset of hyperarid conditions coincided with a sharp decrease in tree-ring widths in the dendroclimatological measurements. Thereafter, the only use of Wadi Tanezzuft was as part of the main caravan route connecting the Mediterranean coast to the central Africa.
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