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Mellars, 2006 ). The preferred hypothesis, although it is not without its critics (e.g.,
Thorne and Wolpoff, 1981 ; MacEachern, 2000 ; Dennell and Roebroeks, 2005 ), has
been named the 'Out of Africa' scenario, and is strongly supported by the evidence
from mitochondrial DNA (mtDNA), which is inherited through the mother (Cann
et al., 1987 ; Watson et al., 1997 ; Ingman et al., 2000 ), and Y chromosome data,
which is inherited via the father (Underhill et al., 2001 ). A comprehensive review by
Underhill et al. ( 2001 ), supplemented by other studies (Ke et al., 2001 ; Templeton,
2002 ; Mellars, 2006 ), points to multiple episodes of population expansion within
Africa and associated migrations of Homo sapiens out of Africa into Asia and on
to Australia by around 45 ka, as confirmed by the detailed study of an Aboriginal
genome (Rasmusson et al., 2011), western Europe by around 45 ka and the Americas
by around 13-11 ka. As noted earlier, one route was from north-east Africa and the
Sinai via the Levantine corridor (Derricourt, 2005 ), another was across the Sahara
following the last interglacial rivers to the Mediterranean coast (McKenzie, 1993 ;
Rohling et al., 2002 ; Osborne et al., 2008 ; Castaneda et al., 2009 ; Drake et al., 2011 ;
Coulthard et al., 2013 ). Some of these rivers flowed along the former courses of the
Neogene Sahabi rivers of the Sahara which flowed from northern Chad across Libya
to the Mediterranean Sea (Griffin, 2002 ; Griffin, 2006 ; Griffin, 2011 ). There is also
good evidence from dated lake deposits in the Murzuq Basin of southern Libya that
at least four large lakes were present in this now hyper-arid area between 500 and
100 ka, with U-series ages of around 415, 320-300, 285-205 and 138-128 ka (MIS
5e) (Geyh and Thiedig, 2008 ). These large lakes became progressively smaller during
each successive wet phase, indicating progressively less humid interglacial conditions
from MIS 11 onwards. Other Middle to Late Pleistocene lakes immediately west of
the Saharan Nile have been identified from satellite imagery and were fed in part
by overflow from the Nile (Maxwell et al., 2010 ), providing another possible well-
watered route across the eastern Sahara. Yet another potential route was across the
southern Red Sea at the Bab el-Mandeb Strait during times of low sea level between
short episodes of high interglacial sea levels (Bailey et al., 2007 ; Armitage et al.,
2011 ; Lambeck et al., 2011 ).
A recent exodus may have taken place about 70,000 years ago (Ambrose, 1998 ).
If correct, this would have been after the now precisely dated 74 ka huge eruption
from Toba volcano in Sumatra, which some workers consider to have been indirectly
responsible for a major drop in human population at that time (Ambrose, 1998 ;
Rampino and Ambrose, 2000 ). Views on the impact of the 74 ka Toba eruption
are polarised between those who claim little or no impact (Petraglia et al., 2007 )
and those who argue for a substantial and adverse impact (Williams et al., 2009a ).
Williams ( 2012b ; 2012c ) summarises this debate and suggests way to achieve better
progress for understanding the impact.
Not all movement was solely out of Africa. The mtDNA evidence shows that
people moved from the Levantine region of south-west Asia to both North Africa and
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