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onset of recurrent desert conditions in the Chad Basin began at least 7 Ma ago.
Further north, in the Hoggar, some elements of the Late Tertiary flora were already
physiologically well-adapted to aridity (Maley, 1980 ;Maley, 1981 ;Maley, 1996 ). If
we accept the sedimentological evidence of Servant ( 1973 ) and of Schuster et al.
( 2006 ) and the palynological evidence of Maley ( 1996 ), then it follows that the onset
of climatic desiccation and the ensuing disruption of the integrated Neogene Saharan
drainage network (Griffin, 2006 ) was a feature of the very late Cenozoic but long
pre-dated the arrival of Homo sapiens .
The late Miocene and early Pliocene climate in the Chad Basin fluctuated re-
peatedly, favouring animals adapted to highly varied ecosystems, including lake,
lake margin, riparian, woodland and savanna habitats (Griffin, 2006 ). It was within
this varied set of habitats that the late Miocene Toumaı hominid Sahelanthropus
tchadensis (TM 266) emerged (Brunet et al., 2005 ; see Chapter 17 ). As we saw in
Chapter 10 , McCauley et al. ( 1982 ; 1986 ) and McHugh et al. ( 1988 ; 1989 ) used
shuttle-imaging radar to identify a series of ancient river valleys in the eastern Sahara,
some of them former tributaries of the Nile. These valleys range in age fromMiocene
to Quaternary, with the younger channels associated with Acheulian and more recent
artefact assemblages ( Chapter 17 ). Small rivers occupied many of the Neogene river
valleys during wetter intervals in the Quaternary (Pachur and Altmann, 2006 ; Osborne
et al., 2008 ; Drake et al., 2011 ), but they were never as large as the Neogene and older
river systems, although they would have allowed the passage of plants, animals and
small bands of humans from central to northern Africa.
Williams et al. ( 1987 , p. 109) concluded that 'the origin of the Sahara as a con-
tinental desert ...may be said to stemfromtheMioceneAlpine orogeny and the
subsequent stripping of the Eocene deep weathering profile'. Sudano-Guinean wood-
land covered much of the Sahara during the Oligocene and early Miocene, having
replaced the equatorial rainforest of Palaeocene and Eocene times. During the late
Miocene and early Pliocene, a xeric flora, well-adapted to aridity, began to replace the
earlier woodland, so many elements of the present Saharan flora were already present
during the late Pliocene, when aridity became even more severe across the Sahara
and the Horn of Africa (Bonnefille, 1976 ; Bonnefille, 1980 ;Maley, 1980 ;Maley,
1981 ; Bonnefille, 1983 ). The combination of a reduction in plant cover and a trend
towards more erratic rainfall had a profound impact on the late Cenozoic rivers of the
Sahara (Griffin, 2006 ). Big rivers capable of carving large valleys became seasonal or
ephemeral. Integrated drainage systems became segmented and disorganised. Wind
mobilised the sandy alluvium into active dune fields. Dunes formed barriers across
river channels that were no longer competent enough to remove them. Dust storms
left the desert top-soils depleted in clay, silt and organic matter. The Sahara was now
a true wilderness, as the Arabic word implies.
The late Cenozoic desiccation which created the largest desert in the world was a
result of a number of factors. Northward drift of theAfrican plate ultimately helped dis-
rupt the warm Tethys Sea to the north, with its abundant supply of moist maritime air.
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