Geoscience Reference
In-Depth Information
summaries of the complex field of hominid taxonomy, and no doubt amended versions
will follow. However, we can be reasonably confident that the brains of
H. erectus
and its variably named successors (
H. antecessor
,
H. rhodesiensis
(0.7-0.3 Ma),
H. heidelbergensis
(0.5-0.3 Ma) and
H. neanderthalensis
(around 300 to 35 ka))
became progressively larger and more complex, attaining a cranial capacity of about
1,400 cc by around 0.3 Ma ago (Stringer and McKie,
1996
). The as yet poorly
dated Denisovans may have arisen before 300 ka ago and appear to have interbred
with both Neanderthals and
H. sapiens
(Cooper and Stringer,
2013
). Anatomically,
modern humans (
H. sapiens
) are known from the lower Omo Valley in southern
Ethiopia in sediments dated between 195 and 104 ka, with the earlier date more
likely (McDougall et al.,
2005
; Fleagle et al.,
2008
; McDougall et al.,
2008
), and
from South Africa in cave deposits at Border Cave and Klasies River Mouth dated
to at least 100 ka (Grun and Stringer,
1991
; Stringer and McKie,
1996
;Brauer et al.,
1997
).
The hominid discoveries are of interest in their own right, and learning about them
appeals to our atavistic desire to understand more about our own past. However, in
regard to our main theme - climatic change in deserts - the hominid fossils alone
are of secondary interest. What is of interest for our present purpose is the huge
volume of cognate research devoted to unravelling the environmental context of the
fossil discoveries (Taıeb,
1974
;Hay,
1976
; Pickford,
1994
; WoldeGabriel et al.,
1994
;
Kalb,
1995
; Barboni et al.,
1999
; WoldeGabriel et al.,
2001
; Quade et al.,
2004
;
Wynn et al.,
2006
). For example, pollen analysis of the late Pliocene fossil-bearing
deposits in the Middle Awash Valley has revealed a mosaic vegetation pattern of
woodland, grassland and dense riparian forest (Bonnefille et al.,
2004
), which has
been confirmed by analysis of the stable oxygen and carbon isotopic composition of
pedogenic carbonate nodules within the fossil soils associated with the hominid fossil
discoveries (WoldeGabriel et al.,
2009
).
One tantalising question relating to the origin of the African hominids concerns
the extent to which their evolution was influenced or even determined by climatic
changes, especially late Cenozoic cooling and desiccation which led to an expansion
of savanna grassland at the expense of forest and woodland (Brain,
1981a
; Brain,
1987
;Vrbaetal.,
1995
; deMenocal,
2004
; Derricourt,
2005
; Behrensmeyer,
2006
;
Trauth et al.,
2007
;Trauthetal.,
2010
; Cerling et al.,
2011
; deMenocal,
2011
; Maslin
and Christensen,
2007
). Changes in African faunal assemblages, especially bovids,
indicative of more open habitats at 2.9-2.6 Ma and between 1.9 and 1.6 Ma appear to
coincide with step-function changes in hominid evolution and the emergence of the
genus
Homo
(deMenocal,
2004
; Maslin and Christensen,
2007
; deMenocal,
2011
).
Maslin and Christensen (
2007
) and Trauth et al. (
2007
;
2010
) emphasised the extreme
climatic variability at 2.7-2.5, 1.9-1.7 and 1.1-0.7 Ma, and they suggested a causal
link between these interludes of high environmental variability and speciation and
dispersal episodes among hominids and other mammals in East Africa.
