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to afford remains connecting man with some extinct ape-like creature, have not as yet
been searched by geologists' (op. cit., p. 571).
Darwin's inference as to the close relationship between early hominids and early
African primates has been confirmed by modern genetic studies which indicate that
divergence between ancestral chimpanzees and ancestral hominids may have occurred
between 7 and 5Ma ago (Pilbeam,
1986
), and possibly even earlier. Caution is advised
before accepting such ages as immutably reliable, because we cannot assume that
mutation rates have been constant over time. We therefore need to bear in mind that
the 'molecular clock' pioneered by Sarich andWilson (
1967
) will always require calib-
ration against independently dated elements of the fossil record using the techniques
outlined in
Chapter 6
. The stunning continuing discoveries of very late Miocene,
Pliocene and younger hominid fossils in limestone caves in semi-arid southern Africa
(Dart,
1925
; Partridge et al.,
2003
;Walkeretal.,
2006
), at Olduvai Gorge in semi-arid
Tanzania (Blumenschine et al.,
2003
), around Lake Turkana in arid northern Kenya
(Spoor et al.,
2007
), from the Chad Basin (Brunet et al.,
1995
;Brunetetal.,
2002
;
Brunet et al.,
2005
) and from the Middle Awash Valley in the otherwise hyper-arid
Afar Desert (White et al.,
1994
; Alemseged et al.,
2006
; White et al.,
2006
; Wynn
et al.,
2006
) have fully vindicated Darwin's cautious prediction. Indeed, Pliocene
hominids appear to be unique to Africa, with later migrations from Africa to Eurasia
not occurring until well after the development in Africa of the first stone tools some
2.5 Ma ago (Corvinus,
1975
; Roche,
1980
; Semaw et al.,
1997
; Roche et al.,
1999
;
Ron and Levi,
2001
; Balter,
2002
).
The question has often been asked as to why Africa was such a good place for
Pliocene hominids to inhabit (see, for example, Bishop,
1978
). We should perhaps
widen the question and ask why certain parts of Africa were also good places in which
to die, to be preserved and to be found again millions of years later (Coppens et al.,
1976
;Hay,
1976
; Rapp and Vondra,
1981
). In the case of the Afar Desert and other
arid sectors of the East African Rift Valley, it can be argued that bone preservation
was aided by rapid burial in a dry, alkaline environment, with soft sediments protected
beneath younger basalt flows. Later exposure and discovery were made possible in
these tectonically active regions by uplift and faulting, which have been the object
of intensive study over the past forty years (Taıeb,
1974
; Pilger and Rosler,
1976
;
Accademia Nazionale dei Lincei,
1980
; Rapp and Vondra,
1981
; Popoff and Tiercelin,
1983
; Adamson and Williams,
1987
; Beyene and Abdelsalam,
2005
; Chorowicz,
2005
; Yirgu et al.,
2006
; Corti,
2009
).
The earliest hominid fossils presently known come from the Djurab Desert in the
northern Chad Basin near the Toros-Menalla fossil-rich site, and they are associated
with a savanna fauna (Brunet et al.,
1995
;Brunetetal.,
2002
;Brunetetal.,
2005
). Here,
Michel Brunet and his colleagues have recovered the remains of a very late Miocene
hominid they termed
Sahelanthropus tchadensis
, with a cranial capacity of about
360 cc. They considered the hominid to be around 7 Ma in age from the associated
